A1 Refereed original research article in a scientific journal
Sperm as a speciation phenotype in promiscuous songbirds
Authors: Lifjeld, Jan T; Cramer, Emily R A; Leder, Erica H; Voje, Kjetil Lysne
Editors: Kronforst Marcus, Connallon Tim
Publisher: Oxford University Press (OUP)
Publication year: 2025
Journal: Evolution
Journal name in source: Evolution
Journal acronym: Evolution
Article number: qpae154
Volume: 79
Issue: 1
ISSN: 0014-3820
eISSN: 1558-5646
DOI: https://doi.org/10.1093/evolut/qpae154
Web address : https://doi.org/10.1093/evolut/qpae154
Self-archived copy’s web address: https://research.utu.fi/converis/portal/detail/Publication/470940832
Sperm morphology varies considerably among species. Sperm traits may contribute to speciation if they diverge fast in allopatry and cause conspecific sperm precedence upon secondary contact. However, their role in driving prezygotic isolation has been poorly investigated. Here we test the hypothesis that, early in the speciation process, female promiscuity promotes a reduction in overlap in sperm length distributions among songbird populations. We assembled a data set of 20 pairs of populations with known sperm length distributions, a published estimate of divergence time, and an index of female promiscuity derived from extrapair paternity rates or relative testis size. We found that sperm length distributions diverged more rapidly in more promiscuous species. Faster divergence between sperm length distributions was caused by the lower variance in the trait in more promiscuous species, and not by faster divergence of the mean sperm lengths. The reduced variance is presumably due to stronger stabilizing selection on sperm length mediated by sperm competition. If divergent sperm length optima in allopatry causes conspecific sperm precedence in sympatry, which remains to be shown empirically, female promiscuity may promote prezygotic isolation and rapid speciation in songbirds.
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Funding information in the publication:
The study was funded by The Research Council of Norway (grant numbers 170853, 196554, and 301592 to J.T.L.) and the European Research Council (grant number 948465 to K.L.V.).
