Rapid sperm evolution in the bluethroat (Luscinia svecica) subspecies complex
Spermatozoa are among the most variable animal cell types, and much research is currently directed towards explaining inter- and intraspecific variation in sperm form and function. Recent comparative studies in passerine birds have found associations between the level of sperm competition and both sperm length and sperm velocity. In species with sperm competition, postcopulatory sexual selection may shape the morphology of sperm as adaptations to the female environment. The speed of evolutionary change in sperm morphology at the species level is largely unknown. In this study, we analysed variation in sperm morphology among morphologically distinct and geographically isolated bluethroat subspecies in Europe. Consistent with previous studies, our analyses of mtDNA and nuclear introns suggest recent divergence and lack of lineage sorting among the subspecies. We found significant divergence in total sperm length and in the length of some sperm components (i.e. head and midpiece). There was a significantly positive relationship between pairwise divergences in sperm morphology and mitochondrial DNA, suggesting a role for genetic drift in sperm divergence. The magnitude of sperm length divergence was considerably higher than that in other geographically structured passerines, and even higher than that observed between several pairs of sister species. We hypothesize that the rapid sperm evolution in bluethroats is driven by sperm competition, and that strong postcopulatory sexual selection on sperm traits can lead to rapid speciation through reproductive incompatibilities.
KeywordsSperm competition Sperm morphology Sperm size variation Reproductive isolation
We thank Marc Naguib for providing us with blood samples of L. megarhynchos, Patrick Bonnet, Sophie Questiau and Matthieu Marquet for help in the field, Eirik Rindal for statistical support, and Melissah Rowe, Becky Cramer and Gunnhild Marthinsen for helpful comments on the manuscript. Funding was provided by the Natural History Museum, University of Oslo (PhD fellowship to SH) and by the Ministry of Education of the Czech Republic (MSM6198959212) to VP.
All authors declare that the present study complies with the current laws and ethical standards of animal research in Czech Republic, France, Germany, Norway, Poland, Russia and Spain.
Conflict of interest
The authors declare that they have no conflict of interest.
Genbank accession numbers
BRM15: KC595725–K C595778
Control region: KC595779–KC595862
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