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Variation in complex mating signals in an “ island ” hybrid zone between Stenobothrus grasshopper species

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ABSTRACT

Two grasshopper species Stenobothrus rubicundus and S. clavatus were previously shown to meet in a narrow hybrid zone on Mount Tomaros in northern Greece. The species are remarkable for their complex courtship songs accompanied by conspicuous movements of antennae and wings. We analyzed variations in forewing morphology, antenna shape, and courtship song across the hybrid zone using a geographic information system, and we documented three contact zones on Mount Tomaros. All male traits and female wings show abrupt transitions across the contact zones, suggesting that these traits are driven by selection rather than by drift. Male clines in antennae are displaced toward S. clavatus, whereas all clines in wings are displaced toward S. rubicundus. We explain cline discordance as depending on sexual selection via female choice. The high covariance between wings and antennae found in the centers of all contact zones results from high levels of linkage disequilibria among the underlying loci, which in turn more likely results from assortative mating than from selection against hybrids. The covariance is found to be higher in clavatus‐like than rubicundus‐like populations, which implies asymmetric assortative mating in parental‐like sites of the hybrid zone and a movement of the hybrid zone in favor of S. clavatus.

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Maps of Mount Tomaros with sampling locations of antenna phenotypes (A) and with interpolated antenna trait values at unsampled sites (B). Hybrid indices from 1 (S. clavatus) to 12 (S. rubicundus) indicated by different colors are shown at the right. Centers of the three contact zones are shown by the black lines.
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ece32265-fig-0008: Maps of Mount Tomaros with sampling locations of antenna phenotypes (A) and with interpolated antenna trait values at unsampled sites (B). Hybrid indices from 1 (S. clavatus) to 12 (S. rubicundus) indicated by different colors are shown at the right. Centers of the three contact zones are shown by the black lines.

Mentions: Analysis of the spatial distribution of the antenna phenotypes in the hybrid zone allowed us to distinguish three contact zones (Fig. 8), similar to our observations for the wing phenotypes. However, in contrast to the wing phenotype distribution, the extreme clavatus antenna phenotypes seem to occur equally often in the northern and southern parts of Mount Tomaros. The shapes of the antenna clines along the three transects are similar to the shapes of the wing clines in males, but this is not the case in females (Fig. 9). The male cline across CZI has a sigmoid shape; by contrast, the female cline has a much shallower slope (Fig. 9A). Moreover, the distribution of means across the contact zone was bimodal for males and unimodal for females, which corresponded to the normal distribution (Kolmogorov–Smirnov test, d = 0.09, P > 0.2). Meanwhile, the correlation between antenna means in males and females was significant (Spearman's rank‐order correlation, r = 0.6, P < 0.05). In CZII, the male cline corresponds to the values of allopatric S. rubicundus at the northern ends; it then falls around the center, almost reaching the values of allopatric S. clavatus and then rises again at the southern end (Fig. 9B). In CZIII, the northern tail of the male cline reaches the values of allopatric S. clavatus; the cline rises around the center, although not reaching the values of allopatric S. rubicundus, and it then falls again at the southeastern end (Fig. 9C). Thus, the distribution of the male phenotypes across CZII and CZIII was unimodal and similar to that found for the wing phenotypes, with dominating S. rubicundus phenotypes in CZII and of S. clavatus phenotypes in CZIII. By contrast, the female clines across CZII and CZIII have more complex shapes than the male clines, without any clear pattern. In both zones, the distribution of the female phenotypes corresponded to the normal distribution (d = 0.14–0.23, P > 0.2). We did not find any significant correlation between male and female antennae along transects II and III.


Variation in complex mating signals in an “ island ” hybrid zone between Stenobothrus grasshopper species
Maps of Mount Tomaros with sampling locations of antenna phenotypes (A) and with interpolated antenna trait values at unsampled sites (B). Hybrid indices from 1 (S. clavatus) to 12 (S. rubicundus) indicated by different colors are shown at the right. Centers of the three contact zones are shown by the black lines.
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Show All Figures
getmorefigures.php?uid=PMC4979727&req=5

ece32265-fig-0008: Maps of Mount Tomaros with sampling locations of antenna phenotypes (A) and with interpolated antenna trait values at unsampled sites (B). Hybrid indices from 1 (S. clavatus) to 12 (S. rubicundus) indicated by different colors are shown at the right. Centers of the three contact zones are shown by the black lines.
Mentions: Analysis of the spatial distribution of the antenna phenotypes in the hybrid zone allowed us to distinguish three contact zones (Fig. 8), similar to our observations for the wing phenotypes. However, in contrast to the wing phenotype distribution, the extreme clavatus antenna phenotypes seem to occur equally often in the northern and southern parts of Mount Tomaros. The shapes of the antenna clines along the three transects are similar to the shapes of the wing clines in males, but this is not the case in females (Fig. 9). The male cline across CZI has a sigmoid shape; by contrast, the female cline has a much shallower slope (Fig. 9A). Moreover, the distribution of means across the contact zone was bimodal for males and unimodal for females, which corresponded to the normal distribution (Kolmogorov–Smirnov test, d = 0.09, P > 0.2). Meanwhile, the correlation between antenna means in males and females was significant (Spearman's rank‐order correlation, r = 0.6, P < 0.05). In CZII, the male cline corresponds to the values of allopatric S. rubicundus at the northern ends; it then falls around the center, almost reaching the values of allopatric S. clavatus and then rises again at the southern end (Fig. 9B). In CZIII, the northern tail of the male cline reaches the values of allopatric S. clavatus; the cline rises around the center, although not reaching the values of allopatric S. rubicundus, and it then falls again at the southeastern end (Fig. 9C). Thus, the distribution of the male phenotypes across CZII and CZIII was unimodal and similar to that found for the wing phenotypes, with dominating S. rubicundus phenotypes in CZII and of S. clavatus phenotypes in CZIII. By contrast, the female clines across CZII and CZIII have more complex shapes than the male clines, without any clear pattern. In both zones, the distribution of the female phenotypes corresponded to the normal distribution (d = 0.14–0.23, P > 0.2). We did not find any significant correlation between male and female antennae along transects II and III.

View Article: PubMed Central - PubMed

ABSTRACT

Two grasshopper species Stenobothrus rubicundus and S.&nbsp;clavatus were previously shown to meet in a narrow hybrid zone on Mount Tomaros in northern Greece. The species are remarkable for their complex courtship songs accompanied by conspicuous movements of antennae and wings. We analyzed variations in forewing morphology, antenna shape, and courtship song across the hybrid zone using a geographic information system, and we documented three contact zones on Mount Tomaros. All male traits and female wings show abrupt transitions across the contact zones, suggesting that these traits are driven by selection rather than by drift. Male clines in antennae are displaced toward S.&nbsp;clavatus, whereas all clines in wings are displaced toward S.&nbsp;rubicundus. We explain cline discordance as depending on sexual selection via female choice. The high covariance between wings and antennae found in the centers of all contact zones results from high levels of linkage disequilibria among the underlying loci, which in turn more likely results from assortative mating than from selection against hybrids. The covariance is found to be higher in clavatus&#8208;like than rubicundus&#8208;like populations, which implies asymmetric assortative mating in parental&#8208;like sites of the hybrid zone and a movement of the hybrid zone in favor of S.&nbsp;clavatus.

No MeSH data available.


Related in: MedlinePlus