Sperm performance in conspecific and heterospecific female fluid

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.异域种群内的分化性选择（divergent sexual selection）可能导致分化的性表型，当种群发生二次接触时，此类表型可作为种群间的生殖屏障。该假说已在交配前性选择（precopulatory sexual selection）相关性状中得到最为广泛的验证，而针对交配后、受精前的性状（即交配后受精前性状，postcopulatory prezygotic traits）的研究则相对匮乏，在体内受精脊椎动物（internally fertilizing vertebrates）中这一研究缺口尤为显著。不过，物种内部的交配后性选择（postcopulatory sexual selection）同样可推动性状分化，进而导致异种精子（heterospecific sperm）在雌性生殖道（female reproductive tract）内的功能表现受损。这类由异域种群间分化的交配后性选择所产生的副产物式生殖不相容性，可构成生殖屏障，其作用机制类似物种选择性交配偏好。本研究针对三对具备分化精子表型、且拥有中等至高强度交配后性选择机会的类群，检验其是否存在交配后受精前生殖屏障：分别为家燕（Hirundo rustica）与崖沙燕（Riparia riparia）、蓝喉歌鸲的两个亚种Luscinia svecica svecica与L. s. namnetum、以及大山雀（Parus major）与蓝山雀（Cyanistes caeruleus）。我们通过雌性生殖道外层的流体环境检测精子游动性能，因为鸟类的精子数量锐减主要发生在精子穿越阴道的过程中。与我们的预期相悖，精子在同种与异种雌性的流体中游动性能并无显著差异，这表明在交配后至受精前的这一阶段，上述类群间并不存在交配后受精前生殖屏障。因此我们认为，异域种群间的精子表型分化，尚不足以在雀形目鸟类中引发以精子游动性能受损为表现的广泛交配后受精前生殖屏障。