Genotypic variation and clonal structure in coral populations with different disturbance histories.
Factors that may affect genotypic diversity within populations include the relative levels of production, recruitment success, and survival of sexual versus asexual propagules (Hughes 1989). The influences of these factors may vary both among and within species, and will be dependent on the various environmental regimes and physical disturbances experienced by populations. Physical disturbances cause mortality among sessile individuals, opening new space for recruitment from both sexual and asexual sources. Disturbances may also accelerate the production of asexual propagules, particularly in organisms that reproduce by fragmentation such as sponges (Wulff 1985), gorgonians (Lasker 1990), corals (Tunnicliffe 1981; Highsmith 1982), asteroids (Crump and Barker 1985), and ophiuroids (Mladenov and Emson 1990).
For most clonal species, it may be expected that new or highly disturbed habitats will be colonized initially by sexually derived recruits. |Important exceptions may include species with parthenogenetically or asexually produced larvae, for example, the reef coral Pocillopora damicornis (Stoddart 1983, 1988).~ Genotypic diversity will remain (maximally) high with continued recruitment of sexually derived propagules until either (1) established genotypes begin producing asexual propagules, or (2) competitive interactions among established clones exclude or eliminate less-fit genotypes. In stable or mature habitats, locally adapted clones may propagate asexually and become dominant through competitive advantage, thereby minimizing the availability of unoccupied substratum and limiting opportunities for recruitment of new genotypes. Populations such as these are expected to be almost wholly structured by the processes of clonal replication and competition, with a resultant decrease in genotypic diversity. In situations where periodic disturbances create new areas of bare substratum, opportunities may occur for both new sexual recruits and asexually produced propagules to become established in the population. Genotypic diversity in these habitats should then depend on the frequency and intensity of disturbance as factors in the creation of available space, and the relative success of sexual and asexual recruitment.
The effects of disturbance on genotypic diversity within populations of species with both sexual and asexual modes of reproduction have been modeled by Sebens and Thorne (1985). Their models predict that intermediate disturbance or nonequilibrium situations will promote the highest levels of clonal diversity under a variety of conditions (Sebens and Thorne 1985, p. 387). Population projections for two clonal species showed increasing genotypic diversity with higher levels of disturbance. Direct analysis of clonal diversity in populations at different successional stages or under various environmental regimes can test these model predictions and provide a more thorough description of the overall spatial and/or temporal characteristics of clonal population structure within a species (Ellstrand and Roose 1987). In the present study, the relationship between genotypic diversity and disturbance history was investigated in populations of the scleractinian coral, Porites compressa.
MATERIALS AND METHODS
Porites compressa is one of the most abundant scleractinian corals in Hawaii and occurs in a variety of habitats on all islands in the Hawaiian chain. This endemic species often forms extensive monospecific stands, with total cover approaching 100% on the periphery and slope of reefs in Kaneohe Bay, Oahu and the west coast of the island of Hawaii (Maragos 1972; Dollar 1982). P. compressa is gonochoric, reproducing sexually by broadcast spawning of eggs and sperm at full moon during summer (Hunter 1988). Asexual propagules are produced continuously throughout the year by fragmentation of branches from parent colonies (Hunter and R. Kinzie unpubl. data).
Sampling was conducted within six sites, five on Oahu and one on the island of Hawaii. These sites were chosen a priori to represent a range of disturbance regimes of various recency and intensity. Criteria used to classify successional age and habitat stability in these populations are presented in table 1. Colony size was measured at and perpendicular to the maximum diameter of each colony sampled. At the three sites within Kaneohe Bay (NKB, SKB, and CI), colonies displayed compact and discrete growth forms; however, colonies at the KBO, LAN, and HAW sites were thicketlike, with live tissue restricted mainly to distal branch tips. Although colony borders could not always be accurately determined at these sites, colony sizes were estimated as ranging from 0.5 to 1.0 m in maximum diameters. The percentage of unoccupied substratum (sand, bare rock, algal turf, or calcareous algae) was visually estimated. Disturbance histories for each site were compiled from the available literature, personal observations, and interviews with local residents. Because various types of disturbances differ in their impacts on fragment production and the opening of new space, an index was developed that incorporated both disturbance intensity and recency into a single descriptor for each site.
The three sites located within Kaneohe Bay, Oahu, are protected from storms and swells by an extensive barrier reef system. Age of patch reefs within the bay has been estimated at 8000 to 10,000 years (Roy 1970). The North Kaneohe Bay site (NKB) is nearest to the mouth of the bay, and subject to occasional gentle swells. Because of its depth and distance from shore, however, this site has not been affected by subaerial exposure, freshwater flooding events, or pollution characteristic of near-shore fringing reefs in the bay (Banner 1968; Jokiel et al. in press). The reef at the South Kaneohe Bay site (SKB) was dredged to a depth of 3.3 m in the early 1940s for a seaplane runway (Roy 1970). The south end of the bay was also severely affected by a major sewage outfall from the 1940s until 1978, when the outfall was diverted offshore. At that time, there were almost no corals living in the south basin because of high turbidity and algal overgrowth (Smith et al. 1981), but a gradual recovery of the coral community began as eutrophication diminished (Maragos et al. 1985). A nearby site (CI) consisted of colonies growing on the vertical surface of a 30-m section of concrete seawall built 20 to 25 years ago. This site was selected to represent a population in which all colonies were assumed to be of sexual (planular) origin because of the difficulty of an asexual fragment attaching to the smooth vertical face of the wall. This assumption was supported by the lack of colonies greater than 25 cm in diameter on the wall, probably because the weight of larger colonies exceeds the strength of their basal attachments.
A fourth site (KBO) was located on the outer slope of the barrier reef across the mouth of Kaneohe Bay. This reef slope is exposed to swells generated by prevailing tradewinds and occasional winter storms. The LAN site was located in the bay immediately south of Kaneohe, on a fringing reef that extends 1.25 km from shore. This area typically has calm sea conditions close TABULAR DATA OMITTED to shore but has been subjected to considerable sand movement with continuous scour and burial of corals. Periodic storms have caused extensive damage to the monospecific stands of P. compressa characteristic of the HAW reef slope site, on the island of Hawaii, with approximate intervals of 4 ("intermediate" intensity) to 40 ("severe" intensity) years (Dollar 1982). All sites have been subject to chronic, low intensity disturbances from human impacts, primarily boat anchoring and fishing, as well as natural bioerosion and collisions from fish and turtles that result in fragmentation of colonies.
Samples of 45 colonies each from the NKB, SKB, KBO, and HAW populations were collected following a nested quadrat sampling design modified from Greig Smith (1979). This sampling regime was chosen to enable an analysis of both fine-scale and large-scale genetic variability in clonal populations, as well as to estimate the spatial extent of individual genets (clones). Nested-quadrat sampling eliminates bias that would result from fixed-size quadrats that otherwise define the spatial scale over which clones can be detected.
Random numbers were used to plot point locations of colonies to be sampled. Five random points were drawn from each of the first three nested quadrats (1 |m.sup.2~, 4 |m.sup.2~, 16 |m.sup.2~), and 15 points each from the fourth and fifth quadrats (64 |m.sup.2~ and 256 |m.sup.2~). An area of 256 |m.sup.2~ within each field population was marked by stakes and TABULAR DATA OMITTED the plotted points were located within the resultant grid. Colonies under or closest to each predetermined point were sampled. Because coral cover at the LAN site was patchy, 45 colonies were haphazardly sampled from a 2-m-wide belt transect extending approximately 20 m to 110 m perpendicular to shore. CI samples were collected from each of the first 45 colonies (|is greater than~ 10 cm minimum diameter) encountered along a 30-m section of seawall. The NKB, SKB, and HAW populations were sampled in 1986. All six populations were sampled in 1988.
Six to 10 branch tips were removed from each colony sampled. As freezing affected activity of some enzymes, half of the branches were maintained alive in flowing sea water while the remainder were frozen at -80 |degrees~ C until assayed.
Electrophoresis. -- Multilocus genotypes were determined electrophoretically for all corals sampled. A total of 133 colonies from the three populations sampled in 1986 was analyzed for five enzyme systems. The 269 colonies from six populations sampled in 1988 were scored for seven loci. Tissue extraction methods followed those of Stoddart (1983), and buffers and stains were prepared following methods of Selander et al. (1971), Redfield and Salini (1980), and D. Potts (unpubl. data). Horizontal gels utilized Electrostarch (11.4% wt/vol) for 1986 samples and Sigma starch (12.0% wt/vol) for 1988 samples. Descriptions of enzyme assays used for P. compressa samples are given in table 2. The 1986 samples were scored for GTDH, LTY-1. LTY-2, LGG, and EST-B; 1988 samples were scored for GPI, PGM, MDH-P, GTDH, MDH, VL, and PGDH. EST-B, LTY, and LGG stains gave smeared or inconsistent bands in the 1988 assays, possibly because of the change of starch used in the gels, and could not be reliably scored.
Analysis of Electrophoretic Data. -- Allelic and genotypic frequencies were determined for each population. An estimation of the power of the seven loci to differentiate unique genotypes was done in two ways, both based on genotypic data from the CI population in which each colony was assumed to be the result of settlement of sexually produced planula larvae and therefore genetically unique. First, diversity was calculated as a function of the mean number of unique genotypes that were resolved by each locus individually (Hoffmann 1986). The sequential addition of the total number of genotypes resolved with all possible combinations of two, three, four, five, six, and seven loci provided a plot of observed diversity versus cumulative number of loci assayed. Second, the ratio of repeated genotypes to total sample size for the CI population provided an index of the probability of assigning genetically different individuals to the same multilocus genotype.
The magnitude of clonal replication (number of ramets per genet) within each population was assessed by comparing the observed genotypic diversity with that expected for a population reproducing solely by sexual means. Clonal membership was defined by any colonies that shared the same seven-locus genotype. For a population in which every colony is unique (e.g., no clonal replication has occurred), genotypic frequencies should conform to Hardy-Weinberg equilibria under the assumptions of random mating, recombination, and no selection. Observed genotypic diversity was calculated as
|G.sub.o~ = 1 / |summation of~ |f.sub.x~|center dot~|(x/N).sup.2~ where x=0 to N
where |f.sub.x~ is the number of genotypes observed x times and N is the total sample size of each population (Stoddart and Taylor 1988). It is influenced by departure from Hardy-Weinberg equilibrium and linkage disequilibrium. Observed genotypic diversity (|G.sub.o~) was compared to the genotypic diversity expected under Hardy-Weinberg equilibrium |Mathematical Expression Omitted~, calculated from a simple binomial expansion of the frequencies of genotype classes using a program provided by J. Stoddart to P. Mladenov (pers. comm. 1987). Degree of departure of |G.sub.o~:|G.sub.e~ from unity provided an index of deviation from both H-W equilibria and multilocus linkage equilibria. A population dominated by a few clonal types will have low |G.sub.o~:|G.sub.e~ ratios, and |G.sub.o~:|G.sub.e~ approaches unity as genotypic individuality is maximized.
To facilitate comparisons with genetic studies of other plant and animal populations (e.g., Parker 1979; Hoffmann 1986; Ellstrand and Roose 1987), diversity was also calculated three additional ways: (1) as D, the complement of Simpson's Index (SI) (Simpson 1949):
D = 1 - SI = 1 - |summation of~ |n.sub.i~(|n.sub.i~ - 1)/N(N - 1),
where |n.sub.i~ is the number of individuals belonging to the ith clone, and N is the total number of individuals sampled per population: (2) as D* = the number of unique genotypes/N; and (3) as E, a measure that allows scaled comparisons among populations with different clone sizes and sample sizes (Parker 1979), calculated as
E = (|D.sub.obs~ - |D.sub.min~)/(|D.sub.max~ - |D.sub.min~)
Both D* and D vary between 0 and 1, with 1 indicating a population where all colonies are unique. E, which estimates evenness, also ranges from 0 (|D.sub.obs~ = |D.sub.min~, complete unevenness) to 1 (|D.sub.obs~ = |D.sub.max~, complete evenness).
Resolution of Genetic Diversity
Interpretations of banding patterns for Porites compressa were consistent with multiple allele systems described for other diploid species (Harris and Hopkinson 1976; Hillis and Moritz 1990). Most gels produced distinct and clear bands for all assays with the exception of those stained for GTDH and MDH. Interpretations of these gels were made with caution, and assays for each coral sample were repeated until consistent results were attained.
GTDH has been variously interpreted among different taxa as a dimer (Harris and Hopkinson 1976), a tetramer (Hillis and Moritz 1990), and a hexamer (D. Ayre pers. comm. 1986). Based on molecular weight and SDS-PAGE mobility of the purified GTDH enzyme from the staghorn coral Acropora formosa. Catmull et al. (1987) determined that the subunit structure is probably tetrameric. For P. compressa. diffuse bands intermediate in migration to those interpreted as representing the two homozygous states were scored as heterozygotes but were not resolved sufficiently to determine subunit structure. Similarly, resolution of MDH, reported to be dimeric in other plant and animal species examined (Harris and Hopkinson 1976; Hillis and Moritz 1990) was poor for many samples of P. compressa; two (often smeared) bands were interpreted in this study as the heterozygous condition, with the (normally) third band presumably masked by the band staining for either the A or B allele. Pocillopora damicornis has shown a similar anomalous banding pattern for heterozygotes at the MDH locus (Stoddart 1985). All loci assayed were polymorphic within each population of P. compressa, with the exception of PGDH, which was fixed in the KBO and HAW populations.
TABLE 3. Comparisons of estimates of genotypic diversity between samples of Porites compressa assayed at different loci. |G.sub.o~, observed genotypic diversity, and D*, number of unique genotypes/N. No. of alleles No. of found/ No. loci population genotypes assayed |G.sub.o~ D* NKB 1986 5 10 16 11.29 0.356 (w/GPI) 6 13 28 22.27 0.622 1988 7 16 29 17.92 0.644 SKB 1986 5 10 23 14.89 0.511 1988 7 19 43 41.32 0.956 HAW 1986 5 10 26 19.36 0.578 1988 7 15 39 34.60 0.886
The seven enzyme systems (total of 21 alleles) used in the 1988 analyses resolved all but one of the 45 genotypes assumed to be unique in the CI population. The probability of assigning genetically different individuals of P. compressa to the same multilocus genotype using these assays (Type II error) was thus estimated at 2.2%. A plot of the cumulative number of resolved genotypes against sequential addition of all permutations of loci approaches an asymptote between the sixth and seventh loci. However. variations in local gene frequencies dictate that more loci may be required to identify and distinguish between all clones in all populations.
The five loci (ten alleles) assayed in the 1986 samples were not sufficient to resolve true clonal structure in the NKB, SKB, and HAW populations (Hunter 1985). Only 16 (NKB), 23 (SKB), and 26 (HAW) genotypes were detected among the 45 samples from NKB and 44 samples each from SKB and HAW populations, yielding correspondingly low indices of diversity among the three populations. The addition of one more locus (GPI: 3 alleles) increased the total number of genotypes scored from 16 to 28 for the NKB population and essentially doubled the diversity indices. GPI produced clearly resolvable bands for fresh tissue in P. compressa, but only frozen samples were available for GPI analyses for SKB and HAW in 1986. Estimated relative levels of diversity as measured by |G.sub.o~ and D* changed from HAW |is greater than~ SKB |is greater than~ NKB to SKB |is greater than~ HAW |is greater than~ NKB in the 1986 and 1988 analyses, respectively.
Variation among Populations
Genotypic diversity and structure varied among Porites compressa populations. Proportions of asexually derived colonies (1 - D*) ranged from 4% in the SKB population to 36% at the NKB site. Although most recruitment at the SKB site was apparently from sexual propagules, two genotypes were represented by two colonies each suggesting that asexual propagation can begin relatively soon after establishment of a population. The two sets of clonal replicates were within 0.6 to 3.0 m of their clone mates, the shortest distances of clone mates among any of the study sites. In contrast, the NKB population had the lowest clonal diversity (29 genotypes in 45 colonies sampled), the highest number of colonies per clone (6), and the highest average clone size (1.55 colonies) of all populations in this study. Sampled clone mates were up to 15.9 m apart. The KBO, LAN, and HAW populations were characterized by intermediate diversities. with proportions of asexually derived colonies ranging from 11 to 24%, and clone mates were scattered from 1.0 to 16.5 m apart within the 256 |m.sup.2~ sampling areas.
The relative genotypic diversities observed in TABULAR DATA OMITTED six populations of P. compressa are portrayed in figure 3. For those populations sampled with the nested quadrat design (NKB, SKB, KBO, HAW), the various levels of diversity observed with increasing numbers of colonies and total area sampled provide descriptions of the effective dispersal distributions of clones. In the NKB population, clone mates were found within all quadrat sizes greater than 1 |m.sup.2~. Clone mates in the KBO population were distributed over a wider area than detected by the 1 |m.sup.2~, 4 |m.sup.2~, or 16 |m.sup.2~ quadrats but were most apparent at 32 |m.sup.2~ as evidenced by an abrupt drop in relative clonal diversity within this quadrat. The HAW population showed a high degree of clonal replication in the smallest quadrats (1 |m.sup.2~ and 4 |m.sup.2~), less in the intermediate sizes, and widely dispersed clones were again identified in the largest quad-rat. Clone mates were found only in quadrats less than 32 |m.sup.2~ in the SKB population. It should be noted that the sampling regime was not designed to detect all clones or the distributions of each genotype in a population but rather to provide a reasonable estimate of levels of clonal replication at each site.
A total of 212 unique seven-locus genotypes was recognized among the 269 colonies of Porites compressa scored in the 1988 samples giving a total diversity overall populations of |G.sub.o~ = 163.20 (D* = 0.7881). For the 180 samples collected from the Kaneohe Bay area (NKB, SKB, CI, and KBO), |G.sub.o~ was 109.89 (D* = 0.8280). Thirteen genotypes (4.8%) were found in more than one population (three shared between NKB and SKB, three between HAW and LAN, two between KBO and LAN, two between CI and SKB, and one each between SKB--LAN, KBO--HAW, and HAW--SKB). This overlap reflects the limits of resolution of the seven-locus system for detection of individuals over the entire species. No genotype was found in more than two populations.
Genet Size and Distributions
Sampled genet sizes within each population ranged from one to six colonies with means of 1.02 to 1.55 colonies/clone. Clonal distributions within each of the populations sampled by the nested quadrat design are shown in figure 4. Distances between clone mates varied within and among populations, with HAW having both the closest (1.0 m) and most distant (16.5 m) colonies with identical multilocus genotypes.
Mean colony size was significantly different (log-transformed, t-tests, P |is less than~ 0.01) among each of the sites (NKB = 2586 |+ or -~ 2079 |cm.sup.2~ SKB = 366 |+ or -~ 449 |cm.sup.2~, CI = 96 |+ or -~ 70 |cm.sup.2~) within Kaneohe Bay. Although colony size is not always an accurate predictor of absolute age of a coral genet (Hughes and Jackson 1980; Hughes and Connell 1987), it can provide a rough estimate of the minimum age of a colony if growth rates are known. Mean linear growth of Porites compressa colonies measured in NKB (3.1 |+ or -~ 0.39 cm/yr; Hunter 1988) and in south Kaneohe Bay (3.5 |+ or -~ 0.40 cm/yr; Jokiel 1985) suggests that colonies at both sites grow at about the same rate. With the assumption that a new sexual recruit grows to a diameter of 1 to 2 cm in its first year (Fitzhardinge 1985) and that colonies have a mean radial growth rate of 2.46 cm/yr (Polacheck 1978), approximate colony age structures were generated for each site. Ages of sampled colonies ranged from 4 to 23 yr at the NKB site, from 2 to 12 yr at the SKB site, and 2 to 5 yr for colonies on the wall at the CI site. Clonally derived colonies were restricted to the smaller age classes at SKB but were distributed across all age classes at the NKB site.
Mean colony size at the SKB site extrapolated to an average age of approximately 3.9 yr. Mean colony size in the NKB population suggested an average age of 10.3 yr, although genet age could be substantially higher if recruitment was from an asexually derived fragment of an established clone. Several colonies greater than 4 m in diameter occur at this site (although none fell under the randomly assigned sampling points of the survey), suggesting ages of well over a century.
Total area occupied per genet averaged 4156 |+ or -~ 6128 |cm.sup.2~ for NKB with a range of one to six colonies/clone as compared to 383 |+ or -~ 454 |cm.sup.2~ for SKB with one to two colonies/clone. There was no significant difference in mean colony size between one-colony clones |Mathematical Expression Omitted~ and clones with more than one colony |Mathematical Expression Omitted~ in the NKB population.
Disturbance Indices and Genotypic Diversity
Criteria describing each of the six sites were ranked and statistically compared with ranks for the respective genotypic diversities for each population. Depth, wave exposure, percentage of unoccupied substratum, the period of time (|D.sub.T~) since disturbance events of low, moderate, or high intensity, or the combined mean for all disturbance indicators at each site did not reflect the observed levels of clonal diversity in this range of habitats. However, genotypic diversity was significantly correlated (P |is less than~ 0.05) with colony size, maximum disturbance intensity (|D.sub.I~), and the time since disturbance events of maximum (very high) intensity. In addition, relative genotypic diversity was correlated with the disturbance index that incorporated both time and intensity of disturbance (|D.sub.I~/|D.sub.T~).
Analysis of genotypic diversity in natural populations of clonal species allows for a retrospective assessment of the relative success of sexual and asexual recruitment within and among populations. The potential for the production of asexual propagules by fragmentation in scleractinian corals has been well documented (Highsmith 1980, 1982; Tunnicliffe 1981; Bak and Criens 1982; Bothwell 1982; Kobayashi 1984; Heyward and Collins 1985; Wallace 1985). Highsmith (1982) suggested that recruitment of asexually produced clonal fragments would lead to considerably fewer genotypes than actual colonies on a reef. Model simulations of population structure for the reef coral, Acropora cervicornis, Neigel and Avise (1983) predicted that catastrophic disturbances combined with low rates of sexual recruitment could be important factors affecting levels of genotypic diversity. In the present study, the frequencies and distributions of replicated genotypes demonstrated that clonal diversity in natural populations of Porites compressa depends on both recency and intensity of disturbance events.
The introduction of new genotypes through sexual recruitment is apparently rare in most species of clonal corals (Fadlallah 1982; Neigel and Avise 1983; Stoddart 1984a). However, numbers of sexual recruits may equal or exceed those produced asexually by fission of adult colonies of Agaricia lamarcki and Leptoseris cucullata, respectively (Hughes 1988). Sexual recruitment in P. compressa averaged approximately 1.6 new recruits/|m.sup.2~/yr on artificial settling blocks in Kaneohe Bay (Fitzhardinge 1985). The long-term survival rates of such recruits are unknown, but may be assumed to be even lower in areas where available substratum is limited because of the presence of established adult coral colonies or other sessile species. Rates of asexual reproduction through fragmentation have been estimated at 96 propagules/|m.sup.2~/yr for P. compressa on the NKB reef, although the long-term fates of these fragments have not been directly observed (Hunter unpubl. data).
The relatively high levels of genotypic diversity of P. compressa compared to other clonal corals suggests that recruitment and persistence of sexual propagules remains important in this species even in populations with low levels of disturbance. Alternatively (or in addition) to the contributions of sexual recruitment to genotypic TABULAR DATA OMITTED TABULAR DATA OMITTED diversity, competitive networks among established clones may prevent complete competitive dominance by a single genotype (Rinkerich and Loya 1983). Similarly, microscale heterogeneity may generate differential selective pressures within a single population, so that no one genotype becomes dominant (Burdon 1980). Disturbance events may occur often enough across or within populations, relative to a species' generation time, to prevent domination of space by locally adapted clonal genotypes. Furthermore, the selective settlement responses of coral planulae (Wallace 1985) may enhance their survivorship relative to nonselective asexual recruitment of fragments. These factors may act alone or in conjunction to allow the coexistence of several clones and maintain relatively high genotypic diversity in this species.
Investigations of other invertebrate population structures, including anthozoans. have demonstrated a broad range of genotypic diversity, from populations that are almost completely dominated by single clones (Black and Johnson 1979; Mladenov and Emson 1990) to those with no evidence of asexual reproduction (Hunt and Ayre 1989). Histocompatibility assays suggested that fragmentation is important in vegetative propagation of the gorgonian, Plexaura A, but not in its congener, Plexaura homomalla (Lasker and Coffroth 1985). Fujii (1987), using acrorhagial responses and sex ratios as indicators of clonal identity, found that populations of the anemone, Anthopleura asiatica, are dominated by a few clones, some of which may be distributed over 5 km. Physical disturbance and unstable habitats have been suggested to contribute to higher clonal diversity in some anemones (Sebens 1982; Ayre 1984) and asteroids (Crump and Barker 1985). In contrast, zoanthid species may exhibit a decrease in clonal propagation with increasing levels of disturbance (Karlson 1991).
Interpretation of analyses of genetic structure in clonal populations is complicated by two major methodological difficulties. First, assays used for identifying and distinguishing individuals must be sensitive enough to clearly resolve all genotypes sampled. In addition, sampling of populations must be sufficient in intensity and scale to allow detection of the presence and distribution of clonal replicates as well as unique genotypes. Several types of assays have been utilized to identify clonal structure in coral populations. Tissue grafting techniques were employed to delineate clonal diversity in Montipora spp. (Jokiel et al. 1983) and Acropora cervicornis (Neigel and Avise 1983). Later work by Heyward and Stoddart (1985), Resing and Ayre (1985), and Stoddart et al. (1985) cautioned against the exclusive use of tissue grafting to determine clonal identity in corals, because some colonies that fuse in grafts have been demonstrated to have dissimilar electrophoretic banding patterns. Stoddart (1984a) identified clonal genotypes in populations of Pocillopora damicornis based on electrophoretic analysis of four allozyme loci (13 alleles) and found that genotypic diversity in this species, which is reported to asexually produce larvae as well as fragments (Stoddart 1983), was less than half of that expected for an exclusively sexually reproducing species. However, the power of this four-locus system to resolve all genotypes of Pocillopora damicornis could not be evaluated, and Stoddart (1984a, 1988) suggested that the total number of genets may have been underestimated.
Interpretations of electrophoretic assessments of genotypic diversity in clonal species have been severely limited by the lack of a priori methods for ascertaining the number of loci necessary to differentiate clones. Studies that have addressed the genetic structure of marine invertebrate populations have utilized relatively few loci to describe clonal diversity. A plot of mean genotypic diversity (D*) versus the number of alleles assayed shows a significant linear correlation for studies of coral and anemone populations, suggesting that assays utilizing more (or more polymorphic) loci may be better able to detect all or most unique genotypes in populations of cnidarians. In contrast, genotypic diversity and number of alleles assayed showed no apparent trend in studies of echinoderm populations. In the present study, the power of the electrophoretic assay to resolve all genotypes was tested by its application to an aclonal population consisting of individuals that were all derived from sexually produced larvae and therefore assumed to be genetically distinct. This test, along with the high |G.sub.o~:|G.sub.e~ values and low (4.8%) proportion of overlapping genotypes among populations, suggests that the assays used were sufficient to resolve most clones of Porites compressa.
Just as the scales of observation in space and time can result in different levels of predictability of ecological diversity (Jackson 1991), area and scale of sampling will affect estimated levels of genotypic diversity within populations (Willis and Ayre 1985; Ayre and Willis 1988). Sampling of clonal diversity in coral species has been done either by random linear transecting (Stoddart 1983, 1984a,b; Willis and Ayre 1985; Ayre and Willis 1988), by assay of colonies at fixed distances (Neigel and Avise 1983), or by arbitrary selection of colonies within patches (Jokiel et al. 1983). Linear transects can provide good estimates of overall genetic diversity, but they cannot elucidate two-dimensional clonal distributions; haphazard sampling may detect common clones but will overestimate clonality if rare or small colonies have less chance of being sampled. A nested sampling design applied to populations within a range of disturbance regimes was useful in the present study for elucidating both total genetic and genotypic diversity of the reef coral, Porites compressa, as well as the fine-scale and broad-scale patterns of clonality within this species.
In summary, physical factors may differentially alter the population structure of clonal species by affecting the relative production and long-term recruitment success of sexually and asexually produced propagules. Although studies of species diversity in coral reef communities have supported an "intermediate disturbance" hypothesis (Grigg and Maragos 1974; Connell 1978; Dollar 1982; Grigg 1983), genotypic diversity within the dominant coral species assessed in this study appears to be directly related to recency and severity of disturbance. The resolution power of both the genetic assay and the scale of sampling require careful consideration when applied to ecological and evolutionary studies of clonal species.
I thank the following people for their contributions to this study: R. Kinzie, C. Lau, W. Tyler, and numerous dive buddies for assistance in the field; D. Ayre, K. Carpenter, R. Fleisher, D. Ports, and C. Simon for help with electrophoretic methodologies; E. Grau and P. Helfrich for access to facilities at Coconut Island; P. Mladenov for data analysis of |G.sub.e~ using his modification of a program provided by J. Stoddart; W. Walsh for assistance with statistical analyses; J. Archie, H. Carson, M. Hadfield, A. Heyward, R. Kinzie, and S. Palumbi for comments on earlier drafts of this manuscript; J. Jackson and three anonymous reviewers for substantial contributions and improvements. This research was supported by the ARCS Foundation, Charles and Margaret Edmondson Fellowship, Alvin Yoshinaga Fund, and University of Hawaii Sea Grant. This is contribution number 917 from the Hawaii Institute of Marine Biology.
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|Author:||Hunter, Cynthia L.|
|Date:||Aug 1, 1993|
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