Sperm performance in conspecific and heterospecific female fluid |
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Authors: | Emily R A Cramer Even Stensrud Gunnhild Marthinsen Silje Hogner Lars Erik Johannessen Terje Laskemoen Marie‐Christine Eybert Tore Slagsvold Jan T Lifjeld Arild Johnsen |
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Institution: | 1.Natural History Museum, University of Oslo, PO Box 1172, Blindern, 0318, Oslo, Norway;2.EA7316, Université de Rennes I, 263 Av. du Général Leclerc, 35042, Rennes Cedex, France;3.Department of Biosciences, Center for Ecological and Evolutionary Synthesis (CEES), University of Oslo, PO Box 1066, Blindern, 0316, Oslo, Norway |
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Abstract: | Divergent sexual selection within allopatric populations may result in divergent sexual phenotypes, which can act as reproductive barriers between populations upon secondary contact. This hypothesis has been most tested on traits involved in precopulatory sexual selection, with less work focusing on traits that act after copulation and before fertilization (i.e., postcopulatory prezygotic traits), particularly in internally fertilizing vertebrates. However, postcopulatory sexual selection within species can also drive trait divergence, resulting in reduced performance of heterospecific sperm within the female reproductive tract. Such incompatibilities, arising as a by‐product of divergent postcopulatory sexual selection in allopatry, can represent reproductive barriers, analogous to species‐assortative mating preferences. Here, we tested for postcopulatory prezygotic reproductive barriers between three pairs of taxa with diverged sperm phenotypes and moderate‐to‐high opportunity for postcopulatory sexual selection (barn swallows Hirundo rustica versus sand martins Riparia riparia, two subspecies of bluethroats, Luscinia svecica svecica versus L. s. namnetum, and great tits Parus major versus blue tits Cyanistes caeruleus). We tested sperm swimming performance in fluid from the outer reproductive tract of females, because the greatest reduction in sperm number in birds occurs as sperm swim across the vagina. Contrary to our expectations, sperm swam equally well in fluid from conspecific and heterospecific females, suggesting that postcopulatory prezygotic barriers do not act between these taxon pairs, at this stage between copulation and fertilization. We therefore suggest that divergence in sperm phenotypes in allopatry is insufficient to cause widespread postcopulatory prezygotic barriers in the form of impaired sperm swimming performance in passerine birds. |
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Keywords: | Cryptic female choice postcopulatory prezygotic barriers sexual selection speciation sperm competition sperm motility sperm velocity |
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