Abstract
Investigations into the evolution of reproductive barriers have traditionally focused on closely related species, and the prevalence of conspecific sperm precedence. The effectiveness of conspecific sperm precedence at limiting gene exchange between species suggests that gametic isolation is an important component of reproductive isolation. However, there is a paucity of tests for evidence of sperm precedence during the earlier stages of divergence, for example among isolated populations. Here, we sourced individuals from two allopatric populations of house mice (Mus domesticus) and performed competitive in vitro fertilisation assays to test for conpopulation sperm precedence specifically at the gametic level. We found that ova population origin did not influence the outcome of the sperm competitions, and thus provide no evidence of conpopulation or heteropopulation sperm precedence. Instead, we found that males from a population that had evolved under a high level of postcopulatory sexual selection consistently outcompeted males from a population that had evolved under a relatively lower level of postcopulatory sexual selection. We standardised the number of motile sperm of each competitor across the replicate assays. Our data therefore show that competitive fertilizing success was directly attributable to differences in sperm fertilizing competence.
Highlights
In many species females solicit copulations from multiple partners to generate competition among the ejaculates of rival males [1,2]
Competitive in vitro fertilization (IVF) assays We used a generalised linear mixed model (GLMM) fit by the Laplace approximation using the ‘lme4’ library in the R-statistical analysis package to test for conspecific sperm precedence [35]
It was shown that conspecificity was lower for species pairs where the male was of a species that had evolved under high levels of postcopulatory sexual selection, suggesting that sperm competition selects for ‘aggressive’ sperm and their ability to overcome the ovum barriers of closely related species [24]
Summary
In many species females solicit copulations from multiple partners to generate competition among the ejaculates of rival males [1,2]. Sexual conflict theory suggests that after periods of allopatry, heteropopulation males might be more successful during sperm competition because females will have evolved ‘resistance’ to conpopulation males [3,7]. This outcome will impede speciation as gene flow is enhanced. Males from the same population as females will have a competitive advantage over heteropopulation males when they are better adapted to females and their defenses Under this scenario sperm precedence will reduce gene flow, and create reproductive barriers that may eventually lead to the evolution of new species [8]. Little is known about the isolating mechanisms between insemination and fertilization
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