Sperm competition promotes diversity of sperm bundles in <Emphasis Type="Italic">Ohomopterus</Emphasis> ground beetles |
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Authors: | Yasuoki Takami Teiji Sota |
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Institution: | (1) Department of Zoology, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwake, Sakyo, Kyoto 606-8502, Japan |
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Abstract: | Diversification of sperm morphology has been investigated in the context of sperm competition, but the adaptive significance
of sperm bundles is still unclear. In analyzing 10 taxa of the genus Carabus subgenus Ohomopterus and one related Carabus ground beetles, we found that dimorphic sperm bundles occurred in most species with varied degrees of bimodality, whereas
sperm were generally monomorphic. Comparative analyses with phylogenetically independent contrasts revealed that the sizes
of large and small sperm bundles evolved more rapidly than, and were not correlated with, the length of sperm, suggesting
more intense selection on sperm bundle sizes and their independent responses to different evolutionary forces. The size of
large sperm bundles was positively correlated with male genital morphology (pertinent to displacement of rival spermatophores)
and postcopulatory guarding duration as well as male body length, suggesting that larger sperm bundles have been favored when
the risk of spermatophore displacement is high. Larger sperm bundles may be advantageous because of their ability to migrate
more rapidly into the spermatheca. In contrast, no clear association was detected between the small sperm bundle size and
mating traits despite its rapid diversification. The present study provides the first record of heteromorphic sperm bundles,
the diversity of which may be promoted by sperm competition. |
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Keywords: | Genitalia Independent contrast Sexual selection Spermatodesm Sperm heteromorphism |
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