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The cytological and genetical mechanisms of plant domestication exemplified by four crop models.

II. Introduction

Plant domestication and natural evolution show similarities as well as differences (van Raamsdonk, 1993). The most important common aspects are the mechanisms such as mutation, selection, genetic drift, hybridization, and polyploidization. Changes in natural evolution involve the development of locally adapted populations (anagenesis, adaptation), and occasionally it results in new, reproductively isolated populations (cladogenesis, speciation; Rensch, 1959; Stuessy, 1990). De Wet (1981: 178-179) describes plant domestication as "changes in adaptation that insure total fitness in habitats especially prepared by man for his cultigens" (see Table I for definitions). The term "crop plant" will be used throughout this article as a more general term to include all cultigens. What should be added to this definition is the fact that a considerable number of crop plants are dependent on man for establishing new generations due to non-dehiscence, non-shattering, and absence of seed dormancy (Simmonds, 1979). Domestication can thus be defined as a process resulting in characteristics beneficial to humans but generally unprofitable for plants in natural habitats and in a decrease or total lack of the capability of disseminating viable offspring. The survival of these crops, then, depends on humans providing special growing conditions and reproduction strategies. Hammer (1984), based on Schwanitz (1967), included in his domestication syndrome characteristics such as loss of natural dispersal mechanisms, larger propagules, loss of mechanical means of protection, loss of toxic or repellent chemicals, colour changes in fruit or seeds, simultaneous ripening, and even and rapid seed germination. These characteristics apply mainly to seed crops; a much larger list of breeding objectives is given by Simmonds (1979). It can be concluded that domestication and evolution are related processes with respect to the mechanisms involved but that these two processes are directed by entirely different objectives. This paper emphasizes the several mechanisms of plant domestication. Modern plant breeding will be included as far as genetical mechanisms are involved.

Table I

Definition of terms used

process: an activity leading to a defined result

entity: a resultant of a process, or a parameter in or a character of a model

modifier: a promoter or inhibitor of a process

causal relationship: the logical connection between two processes or a process and an entity

stochastical relationship: the (not necessarily existing) link between two processes or a process and an entity

evolutionary species: an ancestral-descendent sequence of populations with its own unique evolutionary role and tendencies, evolving separately from others

cultigen: result of a domestication process, i.e., governed by humans and not found in the wild

crop: any set of plants cultivated by humans and used for a special purpose; includes all cultigens (but not exclusively)

weed: as defined here, close relative of a crop, originated after hybridization, after escape from cultivation, or being the ancestor of the crop

horizontal gene transfer: the transfer of genetic material between unrelated organisms by means of a vector which does not belong to the nature of these organisms

Specific crop situations will be arranged here into four main domestication models by means of a descriptive method and terminology. The currently developed models focus primarily on the mechanisms rather than on the resulting traits. The rationale of the method is discussed by van Raamsdonk (1993). The models consist of processes such as mutation and hybridization, modifiers regulating the processes such as selection and genetic drift, entities resulting from these processes (i.e., isolation barriers), and both causal and stochastical relationships between processes. The various types of elements of the models are depicted by distinct symbols; see Table I and Figure 1 for definitions and legends, respectively. Simmonds (1979: 21) presented comparable models, but types of actual plant material has been used as entities instead of parameters. The final representations in the figures may not be complete, but they are meant as an example of the use of graphic modeling of dynamic systems.

A different approach of modeling the position of crop plants with respect to the extent of reproductive isolation barriers is the gene pool concept (Harlan & de Wet, 1971). The concept is used primarily for informal classification of cereals (Harlan & de Wet, 1971) and grain legumes (Smartt, 1984; Muehlbauer et al., 1994), and occasionally for fruits (citrus, this paper) and ornamentals (tulip, van Raamsdonk et al., 1995). The primary gene pool (GP-1) consists of the crop and all taxa that are easily crossable with the crop making it equivalent to the biological species. GP-1A denotes domesticated races and GP-1B wild races. Taxa belonging to the secondary gene pool (GP-2) hybridize with difficulty with the taxon to which the crop belongs. Gene transfer between species located in the tertiary gene pool (GP-3) and GP-1 is possible only by artificial means (Harlan & de Wet, 1971; Hancock, 1992). The possibility of distinguishing between the domesticated and wild forms, GP-1A and GP-1B, respectively, depends largely on the absence of weedy intermediates, which implies the impossibility of development and maintenance of hybrid swarms. The boundaries between the primary, secondary, and tertiary gene pools are vague but can be expressed by means of a crossability coefficient. This coefficient is calculated by a formula including percentage of seed set, seed germinability, and either pollen fertility or bivalent formation in meiosis. The result is a number (distance) between zero (interfertile) and one (intersterile). A matrix with cells containing a distance coefficient for each cross combination can be used for cluster analysis (van Raamsdonk, 1990, 1992; van Raamsdonk et al., 1992, 1995). The longer the branches in the resulting dendrogram, the more difficult species hybridization will be. The different gene pools can easily be delimited in the dendrogram. The link between the gene pool concept and the models proposed here is reproductive isolation. The domestication models describe the mechanisms leading to a particular set of reproductive barriers, while the extent of the barriers is used to distinguish between the different gene pools.

In this paper, the structure and setup of crop domestication models is presented and the main models will be illustrated with several crop examples. This formalized approach of domestication models and crossability dendrograms leads to better insights and to new conclusions in several crop examples. Besides main aspects of domestication scenarios, each crop possesses a unique domestication history. The included entities can be fine tuned to specific crop situations.

III. Four Crop Models

The four crop models are distinguished two by two by the presence or absence of polyploidy in the domestication process itself. On the diploid level a distinction will be made between autogamous and allogamous crops, the latter in several examples known as crop-weed complexes.

Model 1 describes the domestication of crops in which polyploidization plays an important role during domestication. Because of ploidy differences between the crop and wild relatives, domestication in this model leads directly to a more or less reproductively isolated gene pool, creating a new evolutionary lineage [ILLUSTRATION FOR FIGURE 1 OMITTED]. This model will be referred to as the "bread wheat model."

Polyploidization prior to domestication is the major factor in model 2. The main effects are twofold. The crop and its wild polyploid ancestor are largely reproductively isolated from the diploid relatives of the genus, and the genetic effects of selection and inbreeding in polyploids and diploids are different [ILLUSTRATION FOR FIGURE 3 OMITTED]. This model will be referred to as the "cotton model."

The most important processes in model 3 are selection and drift, leading to a straightforward domestication at the diploid level of the crop out of the gene pool of the wild ancestor. Introgression in some stage of this process may occur. The crops are predominantly autogamous and/or selection against intermediate or weedy forms prohibit the existence of important "hybrid" swarms. Only some reproductive isolation barriers are important, especially those facilitating autogamy [ILLUSTRATION FOR FIGURE 4 OMITTED]. According to this model, it might be possible to distinguish between the gene pools GP-1A and GP-1B. This model will be referred to as the "soybean model."

Crops in model 4 are accompanied by a more or less abundant weedy gene pool. Besides selection and drift, hybridization is an important factor [ILLUSTRATION FOR FIGURE 6 OMITTED]. Gene exchange is unrestricted, and hybridization between any pair of the three components of a crop-weed complex - i.e., crop, wild relatives, and weedy intermediates - may occasionally lead to new weedy or cultivated populations. This model will be referred to as the "chili pepper model."

The four models are each named after a major economically important crop. Reviews of all important crops have been presented by Simmonds (1976a), Hancock (1992), Smith (1995), and Smartt & Simmonds (1995).

Artificial selection is an important modifier in every stage of domestication. In each generation, individuals will be selected as parents for the next generation (Harlan et al., 1973). Selection, whether conscious or unconscious, (Heiser, 1988; van Raamsdonk, 1993) is a mechanism in each of the four models. Additionally, choosing only a few individuals for establishing the next generation or a new population automatically leads to genetic drift, or, when establishing a new population, to the so-called founder effect. This effect has a great impact on domestication and is, for instance, a logical effect of polyploidization when one or only a few hybrids possess a doubled genome constitution (Ladizinsky, 1985).

The differences between the soybean model and the chili pepper model are based on the possibility of spontaneous hybridization between wild and domesticated races and on the existence of stabilizing circumstances for weedy derivatives. Chances for hybridization or escape are promoted when domestication takes place in the original geographic area (Capsicum, Beta, Zea, and others), or when cultivation was started in the same habitat as the natural relative used to grow (Daucus carota and Brassica oleracea in the New World). It is important to realize that reproductive barriers can act on every level between complete and weak isolation and that the entire range between autogamy and allogamy can be found. So intermediate domestication scenarios, especially between the soybean model and the chili pepper model, will occur. Disruptive selection against weedy intermediates may lead to differences in flowering season and non-adaptation of intermediates (Harlan et al., 1973) and, hence, to slight reproductive barriers between wild and domesticated races. This effect is comparable to the Wallace effect in nature (Grant, 1981). This situation is indicated by the box with isolation barriers [ILLUSTRATION FOR FIGURE 4 OMITTED], which is absent in Figure 6.

A. THE BREAD WHEAT MODEL

Triticum aestivum (bread wheat), Solanum tuberosum (potato), and Musa acuminata (banana) will be treated as examples to illustrate the differences in importance of reproductive isolation barriers resulting from polyploidy under artificial circumstances [ILLUSTRATION FOR FIGURE 1 OMITTED].

In Triticum 2x, 4x, and 6x ploidy levels exist. The cycle hybridization-polyploidization took place twice [ILLUSTRATION FOR FIGURE 1 OMITTED]. Both wild and domesticated taxa occur at the lower ploidy levels, while hexaploids appear to be exclusively domesticated. The diploid species are generally well-isolated reproductively through hybrid sterility, except for closely related taxa (boxes in [ILLUSTRATION FOR FIGURE 2B OMITTED]; Feldman et al., 1995). At the polyploid levels, strongly reduced seedset has been found after hybridization; when produced, the hybrids are sterile to a large extent [see female fertility data in, e.g., Kihara & Mishiyama (1930) and Kihara & Lilienfeld (1934)]. Notwithstanding the hybrid sterility, the homologous chromosome pairing during meiosis has been analysed and the ancestry of the domesticated wheats is almost completely known. Triticum turgidum (emmer, 4x: AABB) and T. aestivum (6x: AABBDD) share their A genome with T. monococcum (einkorn). The D genome of bread wheat is found on the diploid level in Aegilops squarrosa (= A. tauschii: Mac Key, 1988). Because of several lines of evidence, the B genome is supposed to be a modified S genome of A. speltoides or some related species (Hancock, 1992; Di Terlizzi et al., 1993; Feldman et al., 1995). The domestication of bread wheat (6x) can be discussed in terms of the bread wheat model, but those of Triticum monococcum (2x) and T. turgidum (4x) best fit the soybean model and the cotton model, respectively. At least three GP-1's can be recognized in Triticum [ILLUSTRATION FOR FIGURE 2 OMITTED], one for each ploidy level: diploid are T. monococcum (domesticated) and T. boeoticum (wild); tetraploid are T. dicoccum, T. turgidum, T. durum (domesticated), and T. dicoccoides (wild); and hexaploid T. aestivum (exclusively domesticated). A presumed fourth GP-1 with species locally grown in Georgia and Transcaucasia (4x: AAGG) should consist of T. timopheevi (domesticated) and T. araraticum (wild) (Harlan & de Wet, 1971; Feldman et al., 1995), but the separation of this gene pool is not supported by the crossability data (Kihara & Lilienfeld, 1934) as shown by the calculated dendrogram ([ILLUSTRATION FOR FIGURE 2 OMITTED]; Kihara & Nishiyama, 1930; Kihara & Lilienfeld, 1932, 1934; Kihara, 1954; Matsumura, 1954; Riley et al., 1958; Fedak, 1977; Jouve & Montalvo, 1977). The G genome may also be considered a modified representative of the S genome family, since a spontaneous T. timopheevi-like (AAGG) somatic mutant was found in tillers of a T. dicoccoides (AABB) plant (Kushnir & Halloran, 1983). T. araraticum is even suggested to be the progenitor of T. dicoccum (Smith, 1995). A fifth GP-1 can be recognized containing the octoploid Triticale hybrids [ILLUSTRATION FOR FIGURE 2 OMITTED]. A common GP-2 consists of Aegilops and Secale, among others. The absence of a wild hexaploid wheat points to an origin under domestication. The wild diploid Ae. squarrosa and the wild tetraploid T. dicoccoides together form a hexaploid after hybridization and chromosome doubling that does not match T. aestivum; moreover, these wild species are allopatric (Zohary, 1969).

One of the progenitors of the domesticated tetraploid Solanum tuberosum ssp. andigena is most likely the domesticated diploid S. stenotomum (Hancock, 1992), which points to the bread wheat model as the domestication scenario. The other parent is presumed to be S. sparsipilum. Several other polyploid species are in cultivation. The triploid S. chauca is assumed to have been originated from a cross between S. stenotomum and Solanum tuberosum ssp. andigena, while the latter also may have given rise to S. curtilobum (5x) after a cross with S. juzepczukii (3x; Hawkes, 1988; Grun, 1990). The first introduction of S. tuberosum in Europe presumably concerned representatives of the short-day-adapted ssp. andigena in Spain around 1570. Within a few decades, potato spread through Europe as a curiosity plant (England in 1596; Germany in 1601). Solanum tuberosum was not of major importance until long-day races were selected late in the eighteenth century. Solanum tuberosum ssp. tuberosum could have been the result of selection from the European gene pool of ssp. andigena (Simmonds, 1976b, 1995a; Matsubayashi, 1991) or the result of hybridization between South American representatives of ssp. andigena and some unknown progenitor (Hawkes, 1988; Grun, 1990). Imported Chilean long-day races, indicated by Smith (1995) as the first introduction of true ssp. tuberosum, have been involved in further breeding after the serious epidemic of late blight (Phytophtora infestans) in the 1840s which killed a lot of European races (Simmonds, 1995a). Species of Solanum are generally outcrossing, and ecologic-geographic reproductive isolation played an important role in evolution [ILLUSTRATION FOR FIGURE 1 OMITTED]. The reproductive isolation barriers are not completely effective: The success after crossing species from different ploidy levels has been clarified by postulating the Endosperm Balance Number hypothesis (Hawkes & Jackson, 1992; van Raamsdonk, 1993). Hybridization, whether or not intended, and transportation are still part of agricultural practice in the Andean region (Quiros et al., 1992).

Most modern cultivars of banana are triploids originating either from Musa acuminata (genome composition AA) or from hybridization between this species and M. balbisiana (BB). These triploid cultivars consist of at least one A genome. It is generally believed that the B genome was not able to form edible genotypes on its own. Rare edible hybrid diploids (AB) and tetraploids (AAAB, AABB, ABBB) do occur (Simmonds, 1976c, 1995b; Howell et al., 1994).

Other examples of crops domesticated according to this model include Coffea arabica (coffee: Berthou et al., 1983; Orozco-Castillo et al., 1994) and several species of Dioscorea that are simultaneously domesticated in Asia, Africa, and tropical America (yam: Coursey, 1976; Hahn, 1995).

In contrast to the previous discussion, Harlan et al. (1973) considered Triticum aestivum the sole example of speciation in the course of domestication. Ladizinsky (1985) listed Triticum aestivum and T. zhukovskyi (AAGGDD), Gossypium hirsutum, Nicotiana tabacum and N. rustica, and Brassica napus as polyploid species evolved under domestication. The crops belonging to the genera Gossypium and Nicotiana are treated as examples of the cotton model, while aspects of several different models are involved in the domestication of the crops belonging to the genus Brassica.

B. THE COTTON MODEL

Some of the different ways to prove natural polyploidization prior to domestication will be shown in Gossypium (cotton, cytogeographical data) and Nicotiana (tobacco, genetical data). In addition, the example of Cucumis sativus (cucumber) will be treated as an example of dysploidy.

The genus Gossypium consists of about 50 species with a pantropical distribution. Two diploids (the African G. herbaceum and the Indian G. arboreum) and two tetraploids (the American G. barbadense and G. hirsutum) are cultivated (Phillips, 1976; Wendel, 1995). Seven basic genomes have been designated (Endrizzi, 1991; Wendel & Albert, 1992). The A genome is present in both cultivated diploids, and it is also one of the constituent genomes in the tetraploids [ILLUSTRATION FOR FIGURE 3 OMITTED]. However, it is not known to occur at the diploid level in the New World. Since cotton is known to be cultivated in the Americas as early as 3500 B.C. (see Appendix) and a tetraploid species occurs on the Hawaiian islands, a recent introduction by man of the A genome in the New World (Ladizinsky, 1985) is not likely. Natural transpacific or transatlantic dispersal of the A genome to America has been proposed (Wendel, 1989, 1995; Endrizzi, 1991; Voytas, 1992). In both tetraploid species, truly wild plants were found (Pickersgill, 1977). Ultimately, four independent domestication events may have occurred in this genus.

The two domesticated species of the genus Nicotiana are amphidiploids with 2n = 4x = 48. Nicotiana tabacum, which is the most economically important, is considered to be a doubled hybrid between N. sylvestris and N. otophora (Goodspeed, 1954; Heiser, 1987), N. tomentosiformis (Getstel, 1976; Reed, 1991; Gerstel & Sisson, 1995), or a hybrid between the latter two species (Kenton et al., 1993). Nicotiana rustica might be a doubled derivative from a cross between N. paniculata and N. undulata (Reed, 1991). Both tetraploid species are presumably not growing wild, although escapes have been found. This could point to an artificial origin, but cultivation of the diploid ancestors of N. tabacum is not to be expected, since a dominant allele of a "converter" gene in the leaves of the diploids demethylates the active alkaloid nicotine into the inactive nornicotine (Gerstel & Sisson, 1995). Only homozygous recessive plants could contain higher levels of nicotine and could be of some interest to be cultivated. It is, in fact, not proven how the domesticated tetraploid tobacco crops originated. The difference between the domestication of T. aestivum and the Nicotiana crops is that at least the tetraploid progenitor of T. aestivum is proven to be domesticated. This makes an origin resulting from domestication of T. aestivum most likely, while the ancestral diploid Nicotiana species are of limited use. Triticum aestivum consists exclusively of non-brittle races, which is a general feature in seed-propagated domesticates. In the cultivated tobacco species, dehiscent as well as non-dehiscent races are known (Goodspeed, 1954). Contrary to the assumption of Ladizinsky (1985), Nicotiana tabacum and N. rustica are treated here as examples of the cotton model. Purseglove (1965) also indicated the crops of Nicotiana, as well as those of Gossypium, as examples of polyploids that reached the higher ploidy level prior to domestication.

The genus Cucumis consists of two subgenera. The subgenus Melo (x = 12; di- and polyploids) from Africa contains more than 20 species, C. melo among them (Jeffrey, 1980). Cucumis sativus belongs to the subgenus Cucumis (x = 7; diploids), found in southeastern Asia. The remaining taxa of subgenus Cucumis, C. hardwickii and C. sikkimensis, are sometimes treated at the specific level (Kuriachan & Beevy, 1992; Kirkbride, 1994) but hardly deserve this recognition. The well-defined separation of the subgenera is supported by molecular data (Perl-Treves & Galun, 1985). Representatives of both subgenera are completely intersterile because of dysploidy (van Raamsdonk et al., 1989). It is not likely that the two subgenera diverged during the separation of Africa and Asia, since the genus is absent from Madagascar. More likely, progenitors of subgenus Cucumis migrated in more recent times from Africa through the Middle East to the Asian continent, but cucumber was a natural inhabitant by the time humans started cultivation on the Indian subcontinent. Cucumis hardwickii, as progenitor of C. sativus, was assumed to be endemic to the southern border of the Himalaya, but recently it was found in southern India. At least two independent domestication events have been postulated based on this disjunct occurrence in India. The hot weather and rainy season cultivars of C. sativus should have originated in southern India, and cultivars adapted to temperate climate and C. sikkimensis originated in northern India (Kuriachan & Beevy, 1992).

Other examples of crops domesticated according to this model are Cocos nucifera (coconut: Harries, 1995), Ipomoea batatas (sweet potato: Hancock, 1992; Bohac et al., 1995), Medicago sativa (alfalfa or lucerne: Lesins & Lesins, 1979; Small, 1984; Langer, 1995), Allium porrum (leek: Hanelt et al., 1992), and Arachis hypogaea (ground nut: Smartt, 1984, 1990; Singh et al., 1994).

C. THE SOYBEAN MODEL

Reproductive isolation caused by internal reproductive isolation barriers, post-zygotic barriers, external reproductive barriers, and apomixis at the diploid level will be exemplified by the crops of Glycine (soybean), Lactuca (lettuce), and Citrus (orange, citron, etc.), respectively. Autogamy is especially important in this model.

The genus Glycine is represented by two subgenera. One of them consists of about twelve perennial species. The other subgenus includes two annual species: the domesticated soybean G. max and its presumed wild progenitor, G. soja. The absence of other close relatives indicate the exclusive position of G. soja [ILLUSTRATION FOR FIGURE 4 OMITTED]. The possibility of introgression of genetic material from other sources may be indicated by extensive hybridization studies (Grant et al., 1984, 1986; Singh & Hymowitz, 1985a, 1985b; Singh et al., 1987, 1988). The results are illustrated in a dendrogram calculated from crossability coefficients for each species pair [ILLUSTRATION FOR FIGURE 5B OMITTED]. A crossability coefficient between 0 and 0.1 indicates the possibility of (semi-)fertile hybrid progeny, in which the level of bivalent formation can be investigated. Crossability distances between 0.1 and 0.5 indicate seedling sterility or inviability. The production of non-germinable seeds optimally produces distances [greater than or equal to]0.5. Soybean and G. soja together may form a GP-1. Glycine max forma gracilis is thought to be a hybrid between G. max and G. soja, but G. max f. gracilis differs in one mutation from G. soja, while four (other) mutations were found between G. max and its progenitor (Close et al., 1989; [ILLUSTRATION FOR FIGURE 5A OMITTED]). This racial difference in G. max was also found after principal component analysis of variation in nuclear DNA (Keim et al., 1989). The visualization by means of a dendrogram proves the absence of a GP-2 of soybean. Introgression as indicated in Figure 4 is unlikely. This is illustrated by hybridization experiments between G. soja and G. clandestina, which resulted in some sterile hybrids after immature seed culture (Singh et al., 1987). The species of the other subgenus together belong to GP-3 of soybean [ILLUSTRATION FOR FIGURE 5 OMITTED]. A phylogenetic tree based on cpDNA largely reflects the crossability relationships ([ILLUSTRATION FOR FIGURE 5A OMITTED]; Doyle et al., 1990). The main differences between the dendrograms are the positions of the species G. tomentella (D genome) and G. falcata (F genome), which both have the A type of cytoplasm. Notwithstanding the slightly separated position in the crossability dendrogram, hybrids between G. tomentella and G. argyrea show a high number of bivalents in meiosis (Grant et al., 1986), which may point to the latter species as cytoplasm donor of G. tomentella. Both species are sympatric in Queensland (Doyle et al., 1990). Differences between nDNA and cpDNA variation can indicate unilateral introgression.

In Lactuca sativa, some variation on the molecular level was traced which was not found until now in L. serriola, its putative ancestor. Lactuca sativa originated either as a form selected out of the gene pool of L. serriola with simultaneous introgression from another species or as independently selected species from a large ancestral gene pool. The detected additional variation originates from the introgressant in the first scenario (Kesseli et al., 1991), while in the second scenario ancestral polymorphism (phylogenetic sorting: Soltis et al., 1992) clarifies the differences between the two sibling lineages L. sativa and L. serriola. Only one L. sativa population resembled L. serriola significantly (de Vries & van Raamsdonk, 1994). The floral structure of Lactuca, with a stigma growing through the anther tube with ripe pollen, ensures a high level of self-pollination. With proper emasculation, L. sativa crosses easily with L. serriola and, with greater difficulty, with the other relatives L. saligna and L. virosa (de Vries, 1990). The results of the pilot study on morphological variation by Frietema et al. (1994) do not match those of the more comprehensive study by de Vries & van Raamsdonk (1994), and at this moment there is no reason to consider L. sativa and L. serriola as conspecific.

The species of the genus Citrus are facultative or obligate apomicts. The usual reduced embryo sac degenerates and is replaced by an unreduced somatic embryo, a type of apomixis called nucellar embryony (den Nijs & van Dijk, 1993). Citrus media (citron), C. grandis (syn. C. maxima; pummelo), and C. reticulata (mandarin) are primarily sexual, and C. aurantium (sour orange), C. sinensis (sweet orange), and C. paradisi (grapefruit) are highly apomictic. The species are capable of self-pollination, although self-incompatibility and inbreeding depression exist (Cameron & Soost, 1976; Hancock, 1992; Roose et al., 1995). The oranges may have originated from hybrids between the sexual species C. grandis and C. reticulata (Barrett & Rhodes, 1976) with C. grandis as maternal parent, according to the cpDNA patterns (Green et al., 1986). Citrus limon (lemon; partly apomictic) may be a hybrid of C. media and C. aurantifolia (lime; partly apomictic). Barrett and Rhodes (1976) considered the three sexual taxa C. media, C. grandis, and C. reticulata to be the only true species (see also Roose et al., 1995). Together they form one GP-1. The other species each possess a GP-1 of their own because they are apomictic to a large extent. All the species together can be regarded as a GP-2, since interspecific hybridization between sexual and apomictic forms is possible to a certain degree (Grant, 1981). Wild species are not known (Cameron & Soost, 1976).

Other examples of crops domesticated according to this model are Pisum sativum (pea: Palmer et al., 1985; Muehlbauer, 1993), Phaseolus vulgaris, P. coccineus, P acutifolius, and P. lunatus (common bean, runner bean, tepary bean, and lima bean, respectively: van Schoonhoven & Voysest, 1991; Khairallah et al., 1992; Llaca et al., 1994; Jacob et al., 1995; Debouck & Smartt, 1995), Cicer arietinum (chickpea: Ladizinsky & Adler, 1976a, 1976b; Muehlbauer, 1993), Lens culinaris (lentil: Zohary, 1972; Muehlbauer, 1993), and Vigna unguiculata (cowpea: Smartt, 1990; Ng, 1995). The soybean model applies to most legume crops, since these crops are predominantly autogamous (Smartt, 1984; van der Maesen & Somaatmadja, 1989). Disruptive selection and the resulting absence of weeds are reported for cereals (Harlan et al., 1973), sorghum, and cassava (Jennings, 1976). Chloroplast DNA analysis in Manihot esculenta (cassava: Fregene et al., 1994) did not provide evidence for introgression. Unilateral incongruity in Phaseolus under cultivation is reported (Evans, 1976), and domestication has led to reproductive isolation between Vicia faba (field bean) and the other species of the genus Vicia (Hanelt, 1986). Both mechanisms are part of the soybean model.

D. THE CHILI PEPPER MODEL

Crop-weed complexes in which genetic information is exchanged more or less freely between the different constituent parts of the complex can be recognized in Capsicum crops (chili pepper), Oryza (rice), Zea mays (maize), and Beta (beet), among others.

Five cultivated species originating from at least three different domestication events are known in the genus Capsicum (McLeod et al., 1983). The most important one is C. annuum, consisting of a wild and a domesticated variety. The same systematic situation (two varieties) applies to C. baccatum. Of the two very closely related species C. frutescens and C. chinense, only the former consists of wild representatives. As with C. chinense, C. pubescens is not known wild, and C. eximium or some other related species is assumed to be the progenitor. In all cases, the weedy representatives appear to be intermediate between the wild and domesticated races. In the minimum spanning tree the wild accessions take a central position, but those of C. baccatum appear to be located laterally (Pickersgill et al., 1979). The domesticated representatives of the species are difficult to cross with each other; the wild parts of the species are less reproductively isolated (Pickersgill, 1991).

The perennial Oryza rufipogon is the basal progenitor of O. sativa with the annual O. nivara as immediate progenitor. The domestication started in southern China, most likely in the valley of the Yangtze river, but the date of earliest domestication is debated (Smith, 1995). An entirely parallel situation occurs in Africa. The perennial O. longistaminata is the progenitor of the annual O. barthii, from which the domestication of O. glaberrima most likely started. A range of weedy intermediate "spontanea" forms exists between O. sativa and the wild relatives (Chang, 1976, 1995; Kaneda, 1993). This straightforward domestication is debated and four different models have been proposed (Chang, 1995). The immediate ancestry, whether perennial or annual, is not clear, according to some authors (Oka, 1974). The genetic base of the domesticated rice species may even consist of parts from several sources due to repeated introgression (Hancock, 1992). Although introgressive hybridization predominantly resulted in gene flow from domesticated to weedy and wild races (Langevin et al., 1990; Gregorius & Steiner, 1993; Chang, 1995), some reproductive isolation between O. sativa, "spontanea" forms, and O. nivara/O. rufipogon exists. Sterility and low fitness of hybrids are the main barriers. There is also some hybrid sterility between races of the three cultivar groups of O. sativa, Indica, Javanica, and Japonica (Chang, 1976, 1995; McIntyre et al., 1992; Ishi et al., 1993). Notwithstanding the existence of hybrid sterility at low levels, which is an aspect of the soybean model, the domestication of the crops of Oryza is treated here being in accordance with the chili pepper model.

Zea mays ssp. mays (maize) belongs to a small genus with four annual and perennial species. The most closely related taxa are ssp. mexicana and ssp. parviglumis (annual teosintes: Doebley, 1990a). The genus Zea has a basic chromosome number x = 10, and the related genus Tripsacum has x = 9. Three theories have been postulated for the origin of maize: 1) maize, teosinte, and Tripsacum all originated from one common ancestor; 2) teosinte gave rise to maize by means of selection (weedy ancestor); 3) teosinte originated from maize (domesticated ancestor: Goodman, 1988). The so-called tripartite hypothesis consists of three parts: first, a cross between ancestral Zea and Tripsacum resulting in teosinte; second, primitive pod corn leading to more modern cultivars of maize; and third, intensive introgression of new characteristics from teosinte into maize (Mangelsdorf, 1974). This theory, first proposed in 1939, was largely abandoned in the 1970s (Goodman, 1988), but evidence was found that introgression did play a role in maize domestication (Doebley, 1990a, 1990b; Table II). The Z. mays ssp. mays male sterile line with S-cytoplasm shows the cpDNA type A which is also found in some locally occurring populations of ssp. mexicana, while the other male sterile lines (C-cms and T-cms) show the typical cpDNA of their own subspecies. Introgression was postulated as most likely (Doebley & Sisco, 1989). The two most frequently found cpDNA types in Z. mays ssp. mays, the types B and E, are also found in ssp. parviglumis and ssp. mexicana, while the most frequent type of these two teosinte subspecies occurs in ssp. mays (Table II). Assuming one domestication event (Doebley, 1990b), these data indicate bidirectional introgression. Allozyme distribution also supports introgression (Doebley, 1990b). Most maize researchers support the theory that teosinte is ancestral to maize (Doebley, 1990a; Hancock, 1992). This theory does not violate the sexual transmutation theory, which postulates the development of the maize ear from male lateral branch tassels (Benz & Iltis, 1992). The use of a formal descriptive model as presented in Figure 6 enables a clearer recognition of the status of domestication of maize, which is rarely discussed in terms of a crop-weed complex.
Table II


Number of accessions of three subspecies of Zea mays possessing a
specific type of cpDNA (data abstracted from Doebley 1990a, 1990b).


cpDNA type


Subspecies A B C D
E


mexicana 3 4 22 -
-
parviglumis - 1 15 12
3
mays 1 36 8 - 35


The genus Beta has a geographical distribution along the coasts of Europe and Asia minor. The crops fodder beet and sugar beet and their immediate wild relative ssp. maritima are all considered subspecies of the diploid B. vulgaris (Letschert & Freese, 1993). The crops are outcrossing, and extensive hybridization between wild and domesticated forms exists. Populations of weed beets are abundant (Evans & Weir, 1981; Ford-Lloyd & Hawkes, 1986; Gliddon, 1994), for instance, in France. Study of the cpDNA type and of life cycle variation revealed that these French weed beets originated after pollen flow from ssp. maritima to sugar beet seed production fields in S France and subsequent spread as hybrid seeds from beet production fields (Boudry et al., 1993). Escapes of domesticated forms have been found, but naturalized populations are rare. Due to the production of unreduced gametes, tetraploid types exist. Triploid cultivars resulting from recent breeding attempts are commercially successful (Bosemark, 1993). The basic state of domestication is best described according to the chili pepper model, notwithstanding the existence of triploid cultivars, since no apparent reproductive isolation results from the rise in ploidy level. Natural polyploidy is also found in the rest of the genus.

Other examples of crops domesticated according to this model are Hordeum vulgare (barley: von Bothmer et al., 1991; Harlan, 1995), Sorghum bicolor ssp. bicolor (sorghum: de Wet, 1978; Doggett & Prasada Rao, 1995), Daucus carota (carrot: Small, 1978, 1984; Brandenburg, 1981; Wijnheimer et al., 1989), Vitis vinifera (grape: Alleweldt et al., 1991; Olmo, 1995), Chenopodium quinoa (quinoa: van Raamsdonk, 1986; Wilson & Manhart, 1993), Raphanus sativus (radish: Klinger et al., 1991; Warwick & Black, 1991), Prunus salicina, P. cerasifera, P. domestica, and P. americana (plums: Ramming & Cociu, 1991; Watkins, 1995), and Pennisetum glaucum (syn. P. americanum) and Setaria italica (pearl millet and foxtail millet, respectively: de Wet et al., 1979; Till-Bottraud et al., 1992; de Wet, 1995a, 1995b; Li et al., 1995). The crops belonging to the genera Beta, Capsicum, Chenopodium, Daucus, Hordeum, Raphanus, Sorghum, and Zea were listed by Pickersgill (1981) as examples of crop-weed complexes. In regions where Daucus carota (carrot) is introduced, it can escape and form weedy races that are able to hybridize with the cultivars (Small, 1984). Daucus carota can therefore be regarded as a crop-weed complex. Introgression between Sorghum bicolor ssp. arundinaceum (wild sorghum) and Sorghum bicolor ssp. bicolor (domesticated sorghum) has been proved by means of morphological and DNA data (Harlan, 1992; Duvall & Doebley, 1990; Aldrich & Doebley, 1992), notwithstanding the existence of disruptive selection between domesticated and wild subspecies (Doggett & Majisu, 1968; Doggett & Prasada Rao, 1995) and clear morphological distinction between them (de Wet, 1978).

IV. Mixed Model Situations

The state of domestication of the crops discussed is changeable. Breeding programs utilizing new techniques and using different genetic resources may alter the situation of domestication of a crop. In some specific crop situations, such as Lycopersicon (tomato), Saccharum (sugar cane), Arena (oat), Medicago (alfalfa), and Brassica (cabbages, turnip, rapes, mustards), more than one model may be applied.

Although Hancock (1992) indicated Lycopersicon esculentum as predominantly self-pollinating, this statement applies primarily to modern western cultivars. The cultivars occurring in the center of origin are capable of cross-pollination due to their different floral structure. The wild and weedy forms of L. esculentum var. cerasiforme are self-pollinators, but some related species are cross-pollinators (Rick, 1988; Warnock, 1988; Philouze & Hedde, 1993). The original status of domestication of L. esculentum can be described according to the chili pepper model, but achievements of modern plant breeding resulted in inclusion of the autogamous aspect of the soybean model when considering the modern cultivars. A comparable shift from allogamy to autogamy was achieved in Vitis (grape): Wild species and some primitive cultivars (dioecious) are cross-pollinators, whereas more modern cultivars (hermaphrodite) are self-pollinating (Olmo, 1995).

Aneuploidy is a common feature in Saccharum. Chromosome numbers have been reported from 2n = 40 to 2n = 128 (Roach, 1995). In the wild S. spontaneum (Asia, Africa) a polyploid series occurs with x = 8 but with a number of aneuploids in between, which are less frequent than the euploids. Saccharum robustum (predominantly 2n = 60, 80) is the other wild species occurring on some Indonesian islands. This wild species forms an evolutionary complex with the domesticated "noble canes" S. officinarum (2n = 80). The domesticated S. sinense (2n = 82-124), found predominantly in India and adjacent areas, is considered to consist of lines selected primarily from S. spontaneum, although introgression took place at least in recent times. In modern plant breeding, hybridization between S. officinarum and S. spontaneum has been performed frequently, leading to new aneuploid chromosome numbers, since S. spontaneum produces unreduced gameres (Whalen, 1991). The different chromosome numbers in Saccharum evolved partly under domestication as described in the bread wheat model. Still, reproductive isolation barriers as a main part of this model are not effective, which is comparable to the chili pepper model.

In the genus Arena, a complex of species on the hexaploid level consists of the ancestral wild species A. sterilis, the cultivated species A. sativa, A. byzantina, and A. nuda, and the agressive weed A. fatua, which is assumed to be either derived from weedy types of A. sterilis or an escape from A. sativa. Wild as well as domesticated representatives are also found on the diploid and the tetraploid levels (Hancock, 1992; Thomas, 1995). Consequently, the cultivated Arena species are part of a crop-weed complex (chili pepper model), domestication of which started after naturally occurring differences in chromosome number, which in itself is according to the cotton model. The Medicago sativa complex (alfalfa) is presented in a previous section as an example of the cotton model (q.v.). Although introgression between wild and domesticated representatives is likely, as indicated by the existence of hybrid swarms, detailed indications of the level of gene flow are not available (van Raamsdonk, in press). Introgression from wild yellow-flowering to domesticated purple-flowering populations was not traced in Turkey after checking flower color on numerous occasions (Small, 1984). Intermediate forms between domesticated ssp. sativa and wild ssp. falcata, including M. x varia, show reduced fertility (Stace, 1975). On the other hand, equal pollen fertility and seed set has been found in M. sativa ssp. falcata and in M. x varia (Ohlendorf, 1960). A more final decision on the model to be applied to the domestication of M. sativa awaits further study.

The genus Brassica includes a large range of different crops and a variety of mechanisms, of which only an outline is presented in Figure 7. The relationship between three diploid and three tetraploid species was elucidated by U (1935). He was able to prove the derivation of each of the tetraploids from a hybridization/polyploidization event between two of the three diploids. This triangle of U is confirmed by means of information from a range of different sources. The maternal parent could be indicated with molecular data ([ILLUSTRATION FOR FIGURE 7 OMITTED]; Palmer et al., 1983; Song et al., 1988, 1990; Warwick & Black, 1991). Brassica napus results from hybridization between B. rapa and B. oleracea with subsequent polyploidization. It is unclear whether the hybridization-polyploidization cycle occurred naturally in recent times or unconsciously but artificially in medieval gardens (McNaughton, 1995). On one hand, artificial crosses between B. rapa and B. oleracea appear to be very difficult, which may point to a natural origin; but on the other hand, records of B. napus in the wild presumably concern escapes. Therefore, the domestication of B. napus is an example either of the cotton model or of the bread wheat model. Recent breeding efforts include hybridization with B. rapa and introgression of the B. rapa A genome (Renard et al., 1993; [ILLUSTRATION FOR FIGURE 7 OMITTED]). The most important diploid species, B. rapa and B. oleracea, include a variety of crops due to disruptive selection of various aspects of natural variation (Helm, 1963; Nieuwhof, 1993). Whether or not introgression from related wild species took place depends on the species concept used (von Bothmer et al., 1995). Most related species will be incorporated in the variability of B. rapa and B. oleracea when using a lumping strategy (Oost, 1986; Oost & Toxopeus, 1986). In parts of the world where the crops are not indigenous, weedy forms can run wild (viz. Americas). Since the Brassica species are predominantly outbreeding, gene flow between wild, weedy, and domesticated forms can be expected (Kerlan et al., 1992; Timmons et al., 1994). At least certain aspects of the chili pepper model can be recognized in the domestication history and present situation of B. rapa and B. oleracea.

V. Breeding Strategies

The setup of breeding plans is based partly on knowledge of systematic and evolutionary position, breeding system, and flower and pollination biology (Simmonds, 1979: 30). A graphical representation of the evolutionary pathway of the crop under consideration will give insights into the need for further analyses of these aspects, and it may give hints for future breeding strategies, such as crossing and back-crossing, use of inbred lines, pre- and post-fertilization improvement techniques, or genetic transformation. A crossability dendrogram can clarify the background of the chosen method. Either actual or potential crossability can be presented. For example, naturally occurring crossability between annual species of Helianthus and especially with H. annuus (Heiser, 1969; Arias & Rieseberg, 1994) indicates the ease with which artificial hybrids can be obtained. In contrast, even potential crossability has not been found between Cucumis sativus and representatives of the other subgenus after extensive hybridization studies. In this case, the difference in chromosome number presents a serious impediment to hybridization breeding with other species (van Raamsdonk et al., 1989). The cotton model implies that improvement of cucumber by means of biotechnological methods is a logical choice.

Mutation breeding is an important way of introducing new variation in specific crops (Micke & Donini, 1993). Mutations can be induced in any situation; i.e., they are evenly important for each of the four presented crop models and for every stage.

The incorporation of a model and a crossability study in actual breeding programs of Allium cepa (onion) and Lesquerella densipila (an oil seed crop) will be used to exemplify the strategy setup.

The most important desired traits for breeding of Allium cepa (sec. Cepa) are found in A. fistulosum (van der Valk et al., 1991) and in A. roylei (Kofoet et al., 1990; de Vries et al., 1992). The evolutionary position of onion can be described merely by the soybean model [ILLUSTRATION FOR FIGURE 4 OMITTED]: It is a cultigen, presumably domesticated from its close relative A. vavilovii, while weedy intermediates have not been reported. The center of domestication of onion is presumed to be in the Middle East, although its relatives are native to the mountains of Central Asia (Hanelt, 1986). Of the two most closely related species, A. vavilovii is completely interfertile and A. oschaninii completely intersterile with A. cepa (van Raamsdonk & de Vries, 1992; van Raamsdonk et al., 1992). Crosses between A. cepa and A. fistulosum have been attempted since the 1930s, but no useful results were obtained (Peffley & Mangum, 1990). Strong prezygotic barriers between A. cepa and A. fistulosum have been found (van der Valk et al., 1991). More promising results were obtained between A. cepa and several related species with postzygotic techniques (Gonzalez & Ford-Lloyd, 1987). Moreover, crossing of A. cepa with the less-related A. roylei (sec. Rhizirideum) resulted in relatively low seed set (van Raamsdonk et al., 1992), but introgression of A. roylei traits in an A. cepa background appeared to be possible (van der Meer & de Vries, 1990). As stated, the determination of the soybean model obliges extensive crossability studies, and in the case of A. cepa such a study was carried out with success. Chloroplast DNA variability (Havey, 1992) shows a high degree of similarity with levels of interfertility, including that of A. roylei (van Raamsdonk & Sandbrink, 1995). The use of a crop domestication model provides a foundation for the conclusion that breeding by hybridization with the mentioned and other species is justified, while study of chloroplast DNA variation may play an important role in indicating the level of interfertility in this crop.

Seed oils of several Lesquerella species (Cruciferae) contain high levels of usable hydroxy acids, i.e., lesquerolic acid (14-hydroxy, cis-11-eicosenoic acid, C20:1) in L. fendleri, L. grandiflora (2n = 18), and relatives, and densipolic acid (12-hydroxy, cis-9, cis-15-octadecadienoic acid, C18:2) in L. densipila (2n = 16) and relatives (Mikolajczak et al., 1962; Kleiman, pers. comm.). Especially the cultural practice and fatty acid optimalization, e.g., of L. fendleri, have been studied (Roseberg, 1993; Hayes & Kleiman, 1993). Other important breeding objectives are synchronous seed germination, yield, and optimalization of harvesting methods. Stabilized natural and artificial hybrid and back-cross populations between L. densipila and L. lescurii and between L. densipila and L. stonensis have been reported (Rollins, 1954, 1955, 1957). Papers on hybridization experiments in the context of a breeding program were not found. Prospects for breeding research are illustrated in Figure 8. Some accessions of L. lescurii and L. stonensis contain even greater amounts of densipolic acid than the amount found in L. densipila (Mikolajczak et al., 1962; Kleiman, pers. comm.). Other species can be considered depending on the breeding objective. Prezygotic external reproductive barriers within species groups are predominant in the genus; species are self-incompatible and outcrossing (Rollins, 1954, 1955; Fig. 8). Crosses between L. grandiflora and L. densipila and relatives failed (Rollins, 1957), because these species belong to different parts of the genus. Prezygotic barriers are easily avoided in breeding programs. The combination of different types of information in the domestication model of Lesquerella justify hybridization studies as part of a breeding strategy.

VI. Domestication and Evolution

The relationship between domestication and evolution is much discussed (van Raamsdonk, 1993). Only some recent developments will be briefly mentioned here.

Changes in the genetic constitution of crops under domestication predominantly involve the replacement of wild type alleles rather than the addition of entirely new loci (Regal, 1994). Several characteristics as included in the mentioned domestication syndrome are examples of allometric changes. Also important in plant breeding are addition lines, which are comparable to aneuploids in nature. Artificial polyploidy has its direct naturally occurring parallel. Bearing this in mind, no principal difference exists between the classical mechanisms of domestication and evolution except for the completely different objective. The counterparts of the two main aspects of evolution, speciation and adaptation, are also recognizable in the process of domestication. Several crops domesticated according to the bread wheat model can be regarded as species in an evolutionary sense (van Raamsdonk, 1995). A range of other crops are taxonomically treated as species but do not have an independent evolutionary role. These crops usually are closely related with their wild relative and together they form a primary gene pool. The culton concept is proposed as a basis for the classification of crops (Hetterscheid & Brandenburg, 1995), to distinguish between crops and naturally occurring species to which the taxon concept applies. Cucumis sativus and Cocos nucifera are examples of crops with hardly any close wild relatives. They are clearly domesticated, but, due to the absence of a naturally occurring part of the GP-1, they also have an evolutionary role. So these crops could be treated as taxa as well. The close relationship between most other crops and their wild relatives makes it necessary to circumscribe the boundaries of each crop within which can be applied a practical classification system of domesticated plants based on the culton concept (Hetterscheid & Brandenburg, 1995). Every crop forms at its highest level of classification one entity in practice, regardless of the number of domestication events. This highest level of classification can be assigned in a botanical classification to either the generic, specific, subspecific, or other level. The crops sugar cane and grape consist primarily of hybrid cultivars, and they should be asssigned as Saccharum cultivars and Vitis cultivars, respectively. Some crops are recognized at the subspecific level, e.g., cauliflower, Brassica oleracea ssp. botrytis. A majority of crops can be assigned to the specific level. I will advocate the use of a practical classification system of domesticated plants at infracultigenic levels and the use of a Latin name for the crop as entity, which clarifies the connection between the two systems of classification.

The situation that taxa can be monophyletic, paraphyletic, or polyphyletic (e.g., Rieseberg & Brouillet, 1994) can be taken as a parallel to the recognition of a single domestication event vs. multiple domestication events. The strict monophyletic background of taxa, which is stated to be one of the differences between taxa and culta (Hetterscheid & Brandenburg, 1995), is much disputed. For instance, when accepting both the monocotyledons and dicotyledons as classes, i.e., on the same level, the dicotyledons are paraphyletic (Brummitt, 1994). In fact, a lot of angiosperm groups appeared to be paraphyletic depending on the evidence used, e.g., after analyzing sequence data of the rbcL gene (Chase et al., 1993).

Adaptation - or, in other words, the achievement of characteristics as indicated in the domestication syndrome - is the exclusive aspect of domestication by the absence of speciation. On the other hand, the international registration of new cultivars involves the norm of distinction from existing cultivars and the uniformity and stability of the new cultivar (DUS-norm). Registration includes the establishment of a cultivar description. The stability aspect of the DUS-norm implies the absence of adaptation within a cultivar (van Raamsdonk, 1993).

The introduction of biotechnological techniques in plant breeding, especially biotransformation, resulted in several important differences between traditional and modern plant breeding and, hence, between domestication and evolution. Horizontal gene transfer (see Table I for definition), which is the mechanism used in biotransformation, is so far not found to happen in nature (van Raamsdonk, 1995). The scattered distribution of, for instance, transposons can be clarified by assuming horizontal gene transfer, but other explanations can be proposed as well (Cummings, 1994; Prins & Zadoks, 1994). The result of biotransformation is an addition of one or more genes to the recipient genome and not a simple replacement of alleles (Regal, 1994). The source of genes can theoretically be the entire range of living creatures, sometimes referred to as the quaternary gene pool (Zeven, pers. comm.). So the new genetic environment of the replaced gene could result in unexpected pleiotropic and epistatic effects (Regal, 1994). The broader and more frequent use of certain resistance mechanisms can also result in a higher rate of co-evolution of host and parasite (Gould, 1988).

VII. Concluding Remarks

Only minor correlation is found between ancientry and domestication model for the 20 economically most important crops. The economically leading polyploid crops (bread wheat, potato) range among the earliest domesticated ones (Appendix). Beside these two and sweet potato, sugar cane, and oat, the other polyploid crops are domesticated much more recently. The crops listed in Table III are arranged according to six main gene centers. Three centers and three non-centers are used here as defined by Harlan (1971). In the centers (Middle East, China, and Mexico), domestication is assumed to have taken place only once per crop, while in the non-centers (Africa, South East Asia, and South America) agriculture was introduced from its center, and parallel domestication events of local crops may be assumed (see also Zeven & de Wet, 1982). There is a notable parallel between the centers and non-centers in the core of crops cultivated. In almost all six centers, fruit parts of domesticated representatives of the families Gramineae, Leguminosae and Cucurbitaceae are being used for human consumption. There are only two exceptions: One is Saccharum (sugar cane), which is cultivated for its stems, and the second includes the grasses originating from the South American center which are used almost exclusively as fodder crops (van Raamsdonk, 1995). Differences concern the distinction between floral and non-floral agriculture, depending on whether or not interest was paid to the cultivation of ornamentals (Anderson, 1952), and the interest in medicinal and drug use: The majority of the solanaceous crops originate from South America, and members of this family have a dominant occurrence among the crops of this center (Table III).

As discussed, the domestication models can add considerably to breeding strategies as far as genetical mechanisms are concerned. Polyploidy, either natural (cotton model) or artificially induced (bread wheat model), often leads to a higher level of genetic isolation from the diploid progenitors. If any possibility for backcrossing exists, the resulting B[C.sub.1] hybrids are triploids and highly sterile in most cases. On the other hand, artificial chromosome doubling of hybrids results in higher levels of fertility and hence in greater possibilities of further breeding by crossing with species possessing the same chromosome number. Selection in a (auto)tetraploid may be more difficult due to tetrasomic inheritance (Hayward & Breese, 1993; van Tuyl, 1993). Genetic isolation by incongruity, which is one of the differences between the soybean model and the chili pepper model, needs much research on the extent of the intra- and interspecific variation of crossability. The existence of disruptive selection (soybean model) may predict a reduced fitness of the hybrids, which could result from future breeding programs. In autogamous crops with a physical mechanism promoting self-pollination (Lactuca, Lycopersicon modern cultivars), simple emasculation can be used prior to artificial hybridization. In general, prezygotic reproductive isolation forces the use of fertilization [TABULAR DATA FOR TABLE III OMITTED] techniques (mentor pollen, cut style method, in vitro fertilization), whereas postzygotic barriers necessitate the use of rescue techniques (ovary culture, embryo rescue, seed culture; van Tuyl et al., 1988, 1991; Pickersgill, 1993).

The descriptive models as presented here succinctly depict the domestication history, present status, and future prospects of plant breeding in specific cases. The visualization of genetical mechanisms by means of the models indicates clearly the biology and nature of breeding strategies.

VIII. Acknowledgments

For their valuable contributions and support the author thanks his colleagues W. A. Brandenburg, H. Limburg, A. P. M. den Nijs, and L. M. van Soest (CPRO-DLO); L. J. G. van der Maesen (French translation of resume); R. G. van den Berg, S. Nourse and P. Stam (Wageningen Agricultural University); P. Hanelt and K. Hammer (IPK, Gatersleben); and G. Kimber (University of Missouri, Columbia).

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X. Appendix

Major crops arranged according to their approximate age and model of domestication. The twenty most important crops have been listed (FAO, 1991; fresh production), complemented with other crops in italics. Data for pre-Christian millennia are taken mainly from Hancock (1992) and Smartt & Simmonds (1995). A question mark indicates a presumed date of origin.

[TABULAR DATA OMITTED]
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Title Annotation:Interpreting Botanical Progress
Author:Raamsdonk, L.W.D. van
Publication:The Botanical Review
Date:Oct 1, 1995
Words:14505
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