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Experimental Evolution of Sperm Competitiveness in a Mammal Renée C Firman*, Leigh W Simmons

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Experimental Evolution of Sperm Competitiveness in a Mammal Renée C Firman*, Leigh W Simmons Firman and Simmons BMC Evolutionary Biology 2011, 11:19 http://www.biomedcentral.com/1471-2148/11/19 RESEARCHARTICLE Open Access Experimental evolution of sperm competitiveness in a mammal Renée C Firman*, Leigh W Simmons Abstract Background: When females mate with multiple partners, sperm from rival males compete to fertilise the ova. Studies of experimental evolution have proven the selective action of sperm competition on male reproductive traits. However, while reproductive traits may evolve in response to sperm competition, this does not necessarily provide evidence that sperm competitive ability responds to selection. Indeed, a study of Drosophila failed to observe divergence in sperm competitive ability of males in lines selected for enhanced sperm offence and defence. Results: Adopting the naturally polygamous house mouse (Mus domesticus) as our vertebrate model, we performed an experimental evolution study and observed genetic divergence in sperm quality; males from the polygamous selection lines produced ejaculates with increased sperm numbers and greater sperm motility compared to males from the monogamous lines. Here, after 12 generations of experimental evolution, we conducted competitive matings between males from lineages evolving under sperm competition and males from lineages subject to relaxed selection. We reduced variation in paternity arising from embryo mortality by genotyping embryos in utero at 14 days gestation. Our microsatellite data revealed a significant paternity bias toward males that evolved under the selective regime of sperm competition. Conclusion: We provide evidence that the sperm competitiveness phenotype can respond to selection, and show that improved sperm quality translates to greater competitive fertilisation success in house mice. Background or rival males. In some species, the female genotype When females mate with multiple partners, sperm from plays a major role in determining the outcome of sperm rival males compete to fertilise the ova [1]. Sperm com- competition [18-20]. Additionally, it seems that sperm petition has been shown to influence the evolution of competitive ability may also be contingent on the geno- testes size and efficiency [2-5], and sperm form and types of the competing males, and ejaculate-by-ejaculate function [6-11]. Studies of experimental evolution have interactions. Indeed, males have been shown to display shown that male reproductive traits can evolve in non-transitivity in their sperm competitiveness [21,22]. response to sperm competition [12-17]. However, this In Drosophila, the repeatability of sperm competitive- does not prove that sperm competitive ability responds ness declines when ejaculate × ejaculate × female to selection or how different genotypes contribute to combinations become inconsistent [23], and in vivo future generations. The assumption that divergence in observations of sperm within the female tract have sperm number and/or quality translates to competitive demonstrated the complexity of these interactions [24]. ability has been demonstrated in only two cases [15,17], On top of this, fertilisation and/or paternity success is and never before in a vertebrate. only a relative predictor of sperm competitive ability It is now recognised that the sperm competitiveness dependent upon the stochastic ‘background’ against phenotype is a multifarious, transitional trait that can be which it is measured [25]. Collectively, these factors influenced by genetic interactions between females and/ might constrain sperm competitiveness from responding to selection. * Correspondence: [email protected] We have generated replicate selection lines of house Centre for Evolutionary Biology (M092), University of Western Australia, 35 mice evolving with and without sperm competition. Stirling Hwy, Nedlands, 6009, Australia © 2011 Firman and Simmons; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Firman and Simmons BMC Evolutionary Biology 2011, 11:19 Page 2 of 6 http://www.biomedcentral.com/1471-2148/11/19 In the eighth generation, we observed genetic divergence paternity bias toward males from the polygamous selec- in ejaculate quality; males evolving under sperm compe- tion lines. Thus, there was a significant effect of male tition had higher sperm numbers and better sperm selection history on the proportion of embryos sired by motility compared to males with a selection history of the second male to mate (P2) (Figure 2), but no effect of monogamy [12]. Here, in the 12th generation we con- female selection history (Table 1). The average propor- ducted competitive matings using males from our selec- tion of offspring sired by males from the polygamous tion lines, and investigated whether the sperm lines when they competed against males from the competitiveness phenotype responds to selection by monogamous lines (0.58 ± 0.06) was significantly higher determining whether improved sperm quality translates than the proportion of offspring sired by monogamous to superior sperm competitiveness in house mice. males when competing against males from polygamous lines (0.24 ± 0.06) (Figure 2). In both the defensive (i.e. Results first to mate) and offensive (i.e. second male to mate) We used sexually mature male and female house mice roles, males from the polygamous lines gained exclusive from our established selection lines that had been evol- paternity of 33% of the litters, while males from the ving with (polygamous) and without (monogamous) monogamous lines gained exclusive paternity of just sperm competition for 12 generations, and performed 14% of the litters. competitive matings between males (Figure 1). A first male to mate advantage (relative to the time of ovula- Discussion tion) is a general pattern of paternity in mammals [26]. We conducted competitive matings between male house For example, in competitive matings of mice the pro- mice from polygamous and monogamous selection lines, portion of offspring sired by the second male to mate and found that males evolving with sperm competition (P2) is close to 0.20 [27]. Here we assess P2 for males (polygamy) had a significant paternity bias over males with a polygamous selection history when competing evolving without sperm competition (monogamy). We against males with a monogamous selection history, and thus provide evidence that the sperm competitiveness vice versa. To quantify female effects on sperm use, the phenotype can respond to selection, and show that experimental females were taken from both the mono- improved sperm quality translates to greater competitive gamous and polygamous selection lines. We examined fertilisation success in a mammal. the relative sperm competitiveness of males from the Male mice have been shown to adjust their ejaculate monogamous and polygamous lines by including the expenditure according to the perceived risk of sperm proportion of offspring sired by a male when mating in competition [28]. Under the established mating regimes the offensive role of sperm competition (P2) in a nested of the selection experiment, it is possible that males ANOVA. Although 53% of the litters were multiply from the polygamous lines became more sensitive to sired, our microsatellite data revealed a significant cues that are indicative of female mating history, which Monogamous Polygamous 16 vs 2 n = 32 Polygamous Monogamous 16 vs Figure 1 The experimental design. Competitive male combinations were generated via a semi-factorial design, creating 32 male combinations. Using all replicate lines the different combinations of monogamous line and polygamous line males were assigned to females with either selection history. To eliminate potential confounding affects due to coevolution within the lines, males and females from the same replicate line did not mate. The entire design was replicated. Firman and Simmons BMC Evolutionary Biology 2011, 11:19 Page 3 of 6 http://www.biomedcentral.com/1471-2148/11/19 0.9 0.8 0.7 0.6 0.5 (± se) 2 P 0.4 0.3 0.2 0.1 0.0 12341234 Monogamous Polygamous Selection history Figure 2 Sperm competitiveness scores (mean P2 ± s.e.) of males from the polygamous and monogamous selection lines. thereby influenced their copulatory behaviour and com- lines [12]. Our analyses revealed genetic divergence petitive ability. In a previous experiment we conducted between the two selective regimes, and provided evi- remote observations of copulatory behaviour in a mon- dence that sperm traits had evolved in response to androus and polyandrous context [27]. Mating beha- sperm competition. We found that males from the poly- viour did not differ between the two treatments, gamous lines had ejaculates with greater numbers of suggesting that males do not adjust their copulatory sperm and better sperm motility compared to males behaviour according to the number of previous partners from the monogamous lines [12]. Given that the mean a female had mated with [27]. Following eight genera- sperm quality scores and the mean P2 values were tions of selection, we assessed the sperm quality of derived from different generations (eight and 12 respec- males from the monogamous and polygamous selection tively) we are unable to test directly for a correlation between sperm quality and sperm competitiveness among our selection lines. However,
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