Postcopulatory sexual selection may promote evolutionary diversification in sperm form, but the contribution of between-species divergence in sperm morphology to the origin of reproductive isolation and speciation remains little understood. To assess the possible role of sperm diversification in reproductive isolation, we studied sperm morphology in two closely related bird species, the common nightingale (Luscinia megarhynchos) and the thrush nightingale (Luscinia luscinia), that hybridize in a secondary contact zone spanning Central and Eastern Europe. We found: (1) striking divergence between the species in total sperm length, accompanied by a difference in the length of the mitochondrial sperm component; (2) greater divergence between species in sperm morphology in sympatry than in allopatry, with evidence for character displacement in sperm head length detected in L. megarhynchos; (3) interspecific hybrids showing sperm with a length intermediate between the parental species, but no evidence for decreased sperm quality (the proportion of abnormal spermatozoa in ejaculates). Our results demonstrate that divergence in sperm morphology between the two nightingale species does not result in intrinsic postzygotic isolation, but may contribute to postcopulatory prezygotic isolation. This isolation could be strengthened in sympatry by reinforcement.

Faculty of Science Department of Zoology Charles University Prague Praha 2 CZ 12844 Czech Republic

Faculty of Science Department of Zoology University of South Bohemia České Budějovice CZ 370 05 Czech Republic

Faculty of Science Institute for Environmental Studies Charles University Prague Praha 2 CZ 12100 Czech Republic

Institute of Vertebrate Biology v v i The Czech Academy of Sciences Brno CZ 60365 Czech Republic

Natural History Museum University of Oslo 0318 Oslo Norway

References provided by Crossref.org

Synthesis and Scope of the Role of Postmating Prezygotic Isolation in Speciation

Rapid gene content turnover on the germline-restricted chromosome in songbirds

Sperm morphology and performance in relation to postmating prezygotic isolation in two recently diverged passerine species

Germline-restricted chromosome shows remarkable variation in size among closely related passerine species

Comparison of Karyotypes in Two Hybridizing Passerine Species: Conserved Chromosomal Structure but Divergence in Centromeric Repeats

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Dryad
10.5061/dryad.fn07jg1