title: Sperm performance in conspecific and heterospecific female fluid.
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.
authors: Natural History Museum University of Oslo PO Box 1172 Blindern 0318 Oslo Norway.
authors: Emily R A
authors: ER
authors: Cramer
authors: Natural History Museum University of Oslo PO Box 1172 Blindern 0318 Oslo Norway.
authors: Even
authors: E
authors: Stensrud
authors: Natural History Museum University of Oslo PO Box 1172 Blindern 0318 Oslo Norway.
authors: Gunnhild
authors: G
authors: Marthinsen
authors: Natural History Museum University of Oslo PO Box 1172 Blindern 0318 Oslo Norway.
authors: Silje
authors: S
authors: Hogner
authors: Natural History Museum University of Oslo PO Box 1172 Blindern 0318 Oslo Norway.
authors: Lars Erik
authors: LE
authors: Johannessen
authors: Natural History Museum University of Oslo PO Box 1172 Blindern 0318 Oslo Norway.
authors: Terje
authors: T
authors: Laskemoen
authors: EA7316 Université de Rennes I 263 Av. du Général Leclerc 35042 Rennes Cedex France.
authors: Marie-Christine
authors: MC
authors: Eybert
authors: Department of Biosciences Center for Ecological and Evolutionary Synthesis (CEES) University of Oslo PO Box 1066 Blindern 0316 Oslo Norway.
authors: Tore
authors: T
authors: Slagsvold
authors: Natural History Museum University of Oslo PO Box 1172 Blindern 0318 Oslo Norway.
authors: Jan T
authors: JT
authors: Lifjeld
authors: Natural History Museum University of Oslo PO Box 1172 Blindern 0318 Oslo Norway.
authors: Arild
authors: A
authors: Johnsen
keywords: Cryptic female choice; postcopulatory prezygotic barriers; sexual selection; speciation; sperm competition; sperm motility; sperm velocity