Genetic variation in cricket calling song across a hybrid zone between two sibling species.
In the present study we examine the extent to which differences in calling song observed between two sister taxa of ground crickets, Allonemobius fasciatus and A. socius, are due to genetic and/or environmental causes. We also examine the heritability of calling song within populations. These crickets have been the object of considerable study, and they differ in body size (Mousseau and Roff 1989), diapause (Mousseau and Roff 1989; Mousseau 1991), development time (Mousseau 1988), ovipositor length (Mousseau and Roff 1995), and egg size (Mousseau 1988). Much of this variation is likely the result of adaptation to the different climatic conditions experienced by these two taxa. Allonemobius fasciatus is found north of 40 [degrees] N latitude in North America, and has a short development time, a univoltine life cycle, an intense diapause, a long ovipositor, and large eggs. Allonemobius socius occurs south of 40 [degrees] N latitude and relative to A. fasciatus has a long development time, a less intense and facultative diapause, a short ovipositor, and small eggs. Thus these two species display very different life-history syndromes that probably reflect an evolutionary history of very different climatic regimes.
Allonemobius socius and A. fasciatus meet along a mosaic hybrid zone of varying width that extends from the coastal plain of New Jersey to at least the Great Plains of Illinois (Howard 1986; Howard and Waring 1991). Within mixed populations, hybrid individuals are often found, but at a frequency far less than expected under random mating expectations (Howard 1986; Howard and Waring 1991). When heterospecific matings are "forced" in the laboratory, viable and fertile hybrid offspring are produced (Gregory and Howard 1993; TAM, unpubl. data). Moreover, hybrids survive well in the field (Howard et al. 1993). These observations indicate that the strong yet incomplete reproductive isolation that exists between the two taxa must reflect prezygotic barriers.
The results of early studies eliminated the possibility that the two species were isolated by habitat or phenological differences (Howard 1986; Howard and Furth 1986; Howard et al. 1993). In areas of sympatry, both species occur in wet, short-grass habitats such as cow pastures or the grassy edges of parking lots, and both exhibit a univoltine, egg-overwintering life cycle with adults appearing in mid to late summer and persisting until killed off by hard frosts in October or November. Thus, the potential for close interaction between the two species appears high and suggests that some sort of behavioral difference must be responsible for reproductive isolation.
The discovery of male calling-song differences between the two taxa (Howard and Furth 1986), as well as evidence of character displacement in song in areas of sympatry (Benedix and Howard 1991) focused our attention on male calling songs and led to more detailed studies of geographic and temporal variation in song (Veech et al. 1996). Recent studies of female phonotaxis in these crickets (e.g., Doherty and Howard 1996) suggest that variation in male calling song is not the predominant mechanism for the reproductive isolation currently observed between the two cricket species. Rather, present-day isolation appears to be mediated primarily by a postinsemination barrier to fertilization (Howard and Gregory 1993; Gregory and Howard 1994). It is not known if species-level differences in song (should they exist) ever played a role in the evolution of reproductive isolation in Allonemobius, as it has many other closely related crickets, and many acoustic organisms in general (e.g., Gwynne and Morris 1983; Lewis 1985; Bailey 1991). However, the presence of heritable genetic variation for calling song could provide the raw material for future selection.
In this paper, we examine the mechanisms underlying phenotypic variation in calling song across the mosaic hybrid zone that separates A. fasciatus and A. socius. We wish to determine the extent to which patterns of variation in the wild reflect genetic differences among taxa and the degree to which environmental variation influences song differences. To this end, we have designed a study that compares and contrasts patterns of song variation in the wild (where individuals experience the full range of natural environmental variation) with song variation observed in a common-garden environment (i.e., minimal environmental variation).
The objectives of this study were to test the following three hypotheses: (1) The patterns of phenotypic variation in calling song observed in the wild reflect genetic differentiation among populations. This hypothesis was tested by examining variation among populations when reared in a common garden. (2) The different developmental environments experienced by wild and laboratory-reared crickets will result in different calling-song characteristics, reflecting the influence of different developmental environments. (3) Calling song is heritable. An absence of heritable variation in song would pose a fundamental constraint to the evolution of this character.
Cricket Collection, Maintenance, and Pedigree Generation
Crickets were collected from six populations within a 50-km radius of Camden, New Jersey. In this region, A. fasciatus and A. socius meet and form a mosaic hybrid zone. With one exception, population designations follow Howard and Waring (1991), who described the structure of the zone in this area and characterized five of the six populations as A. fasciatus, A. socius, or mixed on the basis of species-specific allozyme markers. Two populations (M23 and M26) were composed of A. socius genotypes; two were composed of A. fasciatus genotypes (NS and RS); and a fifth population (HF) contained a mix of A. socius, A. fasciatus, and hybrid genotypes. The sixth population (Lippincott Farm [LF]) was characterized using the same methods (DJH, unpubl. data) and is composed primarily of A. fasciatus genotypes.
Laboratory Rearing and Pedigree Generation
Between 100 and 400 late-instar nymphs were collected from each site during August 1990 and returned to the laboratory for the completion of development in a common-garden environment of 29 [degrees] C, 11:13 h L:D. Sexes were reared separately to assure virginity for subsequent matings. Following the attainment of sexual maturity (1 to 14 days post-collection), males and females were randomly paired (within populations) to form [P.sub.1] parentals. Eggs collected from these pairs exhibited embryonic diapause (because of the 11:13 h L:D) and were placed at 4 [degrees] C for four to six months to break diapause. Following diapause, eggs were returned to the common garden environment of 29 [degrees] C, 11:13 h L:D. Hatchlings were caged in plastic boxes (10 X 15 x 20 cm) containing a water vial, a slice of carrot, a small amount of crushed Purina Cat Chow (original formula), and strips of unbleached paper towels at a density of [less than] 30 nymphs per cage. Between 50 and 200 [F.sub.1] full-sib families per population were successfully reared. These species are very amenable laboratory organisms with egg-to-egg survival often exceeding 90%, which dramatically reduces the opportunity for selection in the lab. This design was employed to allow contrasts between field and laboratory rearing environments.
A 30-sec segment of the calling song of all male parents ([P.sub.1]) and four or five randomly chosen [F.sub.1] male offspring were recorded at between 25.5 [degrees] C and 27 [degrees] C on a Teac X-2000 open-reel tape recorder or a Marantz PMD 201 portable cassette recorder using a Realistic Super Cardioid Dynamic Microphone. Males were recorded while sequestered individually in a plastic cage (17.5 x 12 X 6 cm) that was covered with fine mesh netting. Most males were recorded one to two weeks after adult eclosion.
A 10- to 20-sec portion of each song was digitized on a Macintosh SE/30 computer by using the Farallon Mac-Recorder Sound System. After ascertaining dominant frequency and the mean number of pulses per chirp in the Sound-edit portion of this program, the digitized song was shipped to a program developed by John Doherty for the rapid analysis of temporal patterns in sounds. Using both methods, the following seven calling-song parameters were estimated: mean number of pulses per chirp (MP), mean chirp period (CP), mean interchirp interval (ICI), mean carrier frequency (dominant frequency, FREQ), mean pulse period (PP), mean pulse rate (PR), and mean pulse duration (PD). Detailed descriptions of the calling-song characters are provided by Benedix and Howard (1991), Olvido and Mousseau (1995) and Veech et al. (1996). Statistical analyses of calling variation were conducted using SAS (1990) on either a 486DX PC or a SUN SPARC workstation. The calling songs from a total of 2733 (754 wild caught, 1979 [F.sub.1] laboratory-reared) male crickets were recorded and analyzed in the manner described above.
Broad-sense heritabilities were estimated for laboratory-reared full-sib families. A total of 1979 individual males, divided into 504 families, were analyzed. SAS's (1990) VARCOMP procedure was used to generate variance components attributable to family effects. The restricted maximum-likelihood method (REML) was used to estimate variance components. Heritability was estimated as twice the family variance divided by total phenotypic variance (Falconer 1989). Family effects were tested for statistical significance using SAS's (1990) GLM procedure. Heritability estimates generated using full-sib data include the influence of maternal effects as well as common environment effects, and they represent an upper limit to the true heritability of a trait (Falconer 1989).
We found species-specific differences in all aspects of these crickets' calling song. Figure 1 summarizes the variation observed among the study populations for wild and laboratory-reared crickets. In general, the two A. socius populations (M23 and M26) exhibited a significantly higher pulse rate (PR), a shorter chirp period (CP), a shorter mean interchirp interval (ICI), fewer number of pulses per chirp (MP), a shorter pulse period (PP), a shorter pulse duration (PD), and a higher dominant carrier frequency (FREQ) than the three A. fasciatus populations. Because many of the calling-song components were not statistically independent, a multivariate analysis of variance (MANOVA) was performed to test for variation in calling song (Table 1). The largest contributor to variation in calling song came from species-level effects (F = 160, P [less than] 0.0001), followed by differences between field-collected and laboratory-reared crickets (F = 39, P [less than] 0.0001). [TABULAR DATA FOR TABLE 1 OMITTED] Although significant, differences among populations (within species) were relatively small (F = 6.1, P [less than] 0.0001). This analysis indicates that species-level effects were the largest contributor to overall variation in calling song. Results from univariate analyses of variance for individual song components were similar (SPECIES, POPULATION(SPECIES), and ENVIRONMENT effects were all significant; statistics not reported), except that environmental differences between wild and laboratory-reared crickets were more important than species-level effects for PR.
Overall, the hybrid zone population (HF) was intermediate in phenotype between populations composed of pure A. socius or A. fasciatus [ILLUSTRATION FOR FIGURE 1 OMITTED]. A detailed examination of this mixed population using a hybrid index based on allozyme allele frequencies (Howard and Waring 1991) found a strong relationship between genotype and phenotype for all calling-song attributes [ILLUSTRATION FOR FIGURES 2, 3 OMITTED]. Allonemobius socius genotypes within the hybrid zone population had songs very much like A. socius in allopatry, A. fasciatus genotypes were very similar to A. fasciatus in allopatry, whereas individuals of mixed ancestry were for the most part intermediate between the two pure species types. Overall, the correlation between hybrid index scores and song phenotypes were high and significant [ILLUSTRATION FOR FIGURES 2, 3 OMITTED], except for PR, which previous analyses (above) showed to be strongly influenced by environmental effects. This pattern of association between genotype and phenotype was consistent in both the wild-caught and laboratory-reared crickets, although correlations were generally higher in the wild crickets.
A principal components analysis (PCA) was conducted to examine patterns of multivariate variation in calling song across the study populations. Figure 4 shows plots of the first two principal components of this analysis; the two species overlap very little in overall calling-song characteristics. Using a canonical discriminant function analysis (CDFA) it was found that individual crickets could be assigned to the proper taxa 90% of the time on the basis of song characteristics. The CDFA indicated that the song characters contributing most to species differentiation were ICI, CP, and MP, which is immediately obvious from inspection of Figure 1.
Family effects were significant for most calling-song components in the two A. socius populations (M23 and M26; Table 2), whereas many song components showed lower, nonsignificant family effects in the A. fasciatus populations (LF, RS, and NS); 13 of the 14 estimates generated for the A. socius populations were significant versus 11 of 21 for A. fasciatus. Family effects were uniformly significant, and heritability estimates were significantly higher for song components in the genotypically mixed, hybrid zone population (HF) than for the "pure" species populations (F = 6.4, df = 2,39, P [less than] 0.004), although there were no significant differences between the two species. Overall, these results suggest that interspecific gene flow in the hybrid zone population has contributed significantly to the high heritabilities observed in this genetically mixed population (HF).
In this study, we found evidence for significant, genetically based differences in calling song among populations of A. socius and A. fasciatus across a mosaic hybrid zone in southern New Jersey. A comparison of wild-caught crickets with their laboratory-reared descendants indicated that much of the variation in the wild is due to genetically programmed differences between the two sister species. Overall, cricket song following a generation of common-garden rearing in the laboratory was very much like that observed for field-collected individuals. Although environmental effects due to the change from wild to laboratory rearing conditions were significant, these effects were for the most part consistent among populations. However, it is necessary to emphasize that environmental effects (i.e., the difference between field-collected and laboratory-reared crickets) were significant for all traits, but especially for PR, PD, and PP [ILLUSTRATION FOR FIGURE 1 OMITTED].
The differences in song between the two species did not break down in the mixed population (HF) despite the fact that hybrids occurred and represented a possible conduit for gene flow between the two taxa. Males with A. socius genotypes had songs very similar to A. socius males from single-species populations. Similarly, males with A. fasciatus genotypes sang like allopatric A. fasciatus. Individuals with hybrid genotypes had songs that spanned the range of variation observed between the two species. Moreover, there was a correlation between hybrid index score and calling song for all song characteristics [ILLUSTRATION FOR FIGURES 2, 3 OMITTED]. This observed linkage between calling song and genotype, coupled with the continuous variation in calling song within the mixed population (HF), suggests that song has a multigenic basis and recombination has not acted to eliminate or dramatically reduce linkage disequilibrium between species-specific markers and genes associated with calling-song variation.
Many studies of geographic variation in the acoustic behavior of insects have been conducted. For example, Booij (1982) examined 11 populations of the plant hopper Muellerianella fairmairei and eight populations of a sibling species, M. brevipennis, and found evidence for significant variation among populations and species. However, because individuals within populations were not all reared in a common garden, it was not possible to conclude that phenotypic variation was due to genetic differences among populations or was the result of environmental effects. Butlin and Hewitt (1988) were probably the first to control for rearing environment through two generations of laboratory rearing. They examined genetic variation in the acoustic signals of three parapatric grasshopper populations (Chorthippus parallelus). One population was composed of the subspecies C. p. erythropus collected from the French Pyrenees and the other two populations were of the C. p. parallelus subspecies collected from the Pyrenees and the Massif Central regions of France. Butlin and Hewitt (1988) found evidence for significant genetic differences between the subspecies, but little difference between the two populations of C. p. parallelus. There was also good evidence for a polygenic basis to the observed genetic variation, and they suggest that this supports a hypothesis of divergence for song characters while in allopatry during the Pleistocene glaciations that probably separated the subspecies. In many ways, our study of A. fasciatus and A. socius parallel Butlin and Hewitt's (1988) observations of C. parallelus (i.e., genetic variation among sister taxa, but little interpopulation variation; i.e., song characters appear to be multigenic).
The results of the present study are significant for several reasons. First, they reinforce the taxonomic revisions of the A. fasciatus species complex proposed by Howard (1983) and [TABULAR DATA FOR TABLE 2 OMITTED] Howard and Furth (1986), which were based on electrophoretic differentiation and calling-song differences among crickets from the wild. Second, they confirm the species-level differences between the two taxa reported by Benedix and Howard (1991) and Veech et al. (1996). Third, they indicate that the genetic variation (i.e., heritability) necessary for calling-song divergence in areas of sympatry probably exists, at least for A. socius populations. Fourth, this study supports the notion that interspecific hybridation may play a significant role for the maintenance of quantitative genetic variation, as has been demonstrated for Darwin's finches (Grant and Grant 1994).
Despite the genetic variation for calling song found in this study, recent phonotactic work (Doherty and Howard 1996; Olvido and Mousseau, unpubl. data) indicates that while females of both species exhibit positive phonotaxis to male calling songs, they are relatively insensitive to the differences in song between the two species. Thus differences in male calling songs, which may account for the isolation of many closely related crickets (and many acoustic organisms in general; Gwynne and Morris 1983; Lewis 1985; Bailey 1991), probably play a minor role in the present-day reproductive isolation observed between A. fasciatus and A. socius. Current evidence indicates that reproductive isolation between the two taxa is controlled by a postinsemination barrier to fertilization (Howard and Gregory 1993; Gregory and Howard 1994). In other words, heterospecific sperm is less effective at fertilizing eggs than conspecific sperm. The consequent conspecific sperm priority, combined with a pattern of multiple mating in natural populations (Gregory and Howard 1994), ensures that most females in mixed populations will lay eggs fertilized by a male of their own species. It is possible that the observed differences in song arose as a consequence of past selection (prior to the evolution of the observed postinsemination barrier), as a correlated response to selection on other traits, or as the result of drift on these characters.
One is left with the question of why the calling-song differences persist in mixed populations. If calling-song differences are meaningless to females (i.e., are neutral), why do the genes controlling the differences not flow across the species boundary? At present, the best explanation appears to be that both gene pools are highly resistant to gene flow, despite the presence of hybrids in mixed populations. Working with a suite of old and new genetic markers, Chu et al. (1995) found little evidence of gene flow between A. fasciatus and A. socius outside the zone of overlap. Despite areas of hybridization, the abrupt discontinuity between the species, both in song and genetics, indicates that selection acts to maintain coadapted gene complexes in the two species.
The authors thank R. Heil, V. Groemminger, A. Olvido, K. Waddell, J. Dunn, C. Stockton, K. Serbesoff, E. Andazola, T. Martin, P. Pipiringos, S. Huddleston, G. Stamper, M. Mannino, and E. Powers for their expert assistance. Special thanks go to J. Doherty for all his help and advice, and to T. Price, D. Cowley, and S. Via for instructive discussion. This work was supported in part by National Science Foundation grants DEB 9409004 to TAM, IBN 9119330 to TAM and J. Doherty, DEB 9407229 to DJH, and BSR 9006484 to DJH, J. Doherty, and TAM.
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|Author:||Mousseau, Timothy A.; Howard, Daniel J.|
|Date:||Aug 1, 1998|
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