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Depending on the genetic architecture of male and female fitness, sex‐specific selection can have negative, positive, or neutral consequences for the opposite sex. Theory predicts that conflict between male and female function may drive the breakdown of intrasexual genetic correlations, allowing sexual dimorphism in sexually antagonistic traits. Reproductive traits are the epitome of this, showing highly differentiated proximate functions between the sexes. Here we use divergent artificial selection lines for female reproductive investment to test how female‐specific selection on a sex‐limited trait affects male reproductive success in a precocial bird, the Japanese quail (Coturnix japonica). We demonstrate that selection for increased egg investment in females positively affects male reproductive success both in competitive and non‐competitive mating situations. This increased reproductive success was linked to a relatively larger left testis in males originating from lines selected for high female reproductive investment. Given that female quail have functional gonads only on their left side, this correlated response indicates that selection has acted on the shared developmental basis of male and female gonads. Our study thereby provides evidence for a positive genetic correlation between key reproductive traits in males and females despite a high degree of sexual dimorphism, and suggests that, in this system, selection on reproductive function is sexually concordant.
Depending on the genetic architecture of male and female fitness, sex‐specific selection can have negative, positive, or neutral consequences for the opposite sex. Theory predicts that conflict between male and female function may drive the breakdown of intrasexual genetic correlations, allowing sexual dimorphism in sexually antagonistic traits. Reproductive traits are the epitome of this, showing highly differentiated proximate functions between the sexes. Here we use divergent artificial selection lines for female reproductive investment to test how female‐specific selection on a sex‐limited trait affects male reproductive success in a precocial bird, the Japanese quail (Coturnix japonica). We demonstrate that selection for increased egg investment in females positively affects male reproductive success both in competitive and non‐competitive mating situations. This increased reproductive success was linked to a relatively larger left testis in males originating from lines selected for high female reproductive investment. Given that female quail have functional gonads only on their left side, this correlated response indicates that selection has acted on the shared developmental basis of male and female gonads. Our study thereby provides evidence for a positive genetic correlation between key reproductive traits in males and females despite a high degree of sexual dimorphism, and suggests that, in this system, selection on reproductive function is sexually concordant.
Despite sharing an autosomal genome, the often divergent reproductive strategies of males and females cause selection to act in a sex-specific manner. Selection acting on one sex can have negative, positive, or neutral fitness consequences on the opposite sex. Here we test how female-limited selection on reproductive investment in Japanese quail (Coturnix japonica) affects male fertility-related traits. Despite there being no difference in the size of males’ testes from lines selected for high female reproductive investment (H-line) or low female reproductive investment (L-line), in both lines, the left testis had a greater volume of sperm-producing tissue. Since H-line females have a larger left-side restricted oviduct, this suggests a positive genetic correlation between male and female gonad function, and that internal testis structure is a target of sexual selection. However, despite H-line males having previously been found to have greater fertilisation success in a competitive scenario, we found little evidence of a difference between the lines in sperm number, motility, velocity, length, or the number of sperm that reached the ova. Pre-copulatory cues and/or the role of seminal fluid in sperm motility may thus be more likely to contribute to the H-line male fertilisation advantage in this species.
The extent to which parasites reduce host survival should depend upon how hosts balance trade-offs between reproduction and survival. For example, parasites are predicted to impose greater survival costs under polygynous or promiscuous mating systems in which competition for mates favours increased reproductive investment, particularly in males. We provide, to our knowledge, the first comparative test of the hypothesis that the mating system of the host is an important determinant of (i) the extent to which parasites reduce survival, and (ii) the extent to which males and females differ in the survival cost of parasitism. Using meta-analysis of 85 published estimates of the survival cost of parasitism from 72 studies of 64 species representing diverse animal lineages, we show that parasites impose a mean 3.5-fold increase in the odds of mortality on their hosts. Although this survival cost does not differ significantly across monogamous, polygynous and promiscuous mating systems, females incur a greater survival cost than males in monogamous species, whereas males incur a greater survival cost than females in polygynous and promiscuous species. Our results support the idea that mating systems shape the relative extent to which males and females invest in reproduction at the expense of defence against parasites.
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