Department of Biology, University of Iowa, Iowa City, IA 52242, USA.
Heredity (Edinb). 2012 Jul;109(1):57-62. doi: 10.1038/hdy.2012.13. Epub 2012 Apr 11.
As only females contribute directly to population growth, sexual females investing equally in sons and daughters experience a two-fold cost relative to asexuals producing only daughters. Typically, researchers have focused on benefits of sex that can counter this 'cost of males' and thus explain its predominance. Here, we instead ask whether asexuals might also pay a cost of males by quantifying the rate of son production in 45 experimental populations ('lineages') founded by obligately asexual female Potamopyrgus antipodarum. This New Zealand snail is a powerful model for studying sex because phenotypically similar sexual and asexual forms often coexist, allowing direct comparisons between sexuals and asexuals. After 2 years of culture, 23 of the 45 lineages had produced males, demonstrating that asexual P. antipodarum can make sons. We used maximum-likelihood analysis of a model of male production in which only some lineages can produce males to estimate that ~50% of lineages have the ability to produce males and that ~5% of the offspring of male-producing lineages are male. Lineages producing males in the first year of the experiment were more likely to make males in the second, suggesting that some asexual lineages might pay a cost of males relative to other asexual lineages. Finally, we used a simple deterministic model of population dynamics to evaluate how male production affects the rate of invasion of an asexual lineage into a sexual population, and found that the estimated rate of male production by asexual P. antipodarum is too low to influence invasion dynamics.
由于只有雌性直接影响人口增长，因此在同等程度上投资于儿子和女儿的有性雌性相对于只产雌性的无性个体而言，其成本会增加一倍。通常，研究人员关注的是性的益处，这些益处可以抵消这种“雄性成本”，从而解释其优势。在这里，我们转而通过量化 45 个由强制性无性雌性 Potamopyrgus antipodarum 建立的实验种群（“谱系”）中雄性的产生率，来询问无性个体是否也可能会付出雄性成本。这种新西兰蜗牛是研究性别的强大模型，因为表型相似的有性和无性形式通常共存，从而可以在有性个体和无性个体之间进行直接比较。经过 2 年的培养，45 个谱系中有 23 个产生了雄性，这表明无性 P. antipodarum 可以产生雄性。我们使用最大似然分析了雄性产生模型，该模型中只有某些谱系可以产生雄性，从而估计有能力产生雄性的谱系约占 50%，并且产生雄性的谱系的后代中约有 5%是雄性。在实验的第一年产生雄性的谱系在第二年更有可能产生雄性，这表明某些无性谱系相对于其他无性谱系可能会付出雄性成本。最后，我们使用种群动态的简单确定性模型来评估雄性产生如何影响无性谱系入侵有性群体的速度，并发现无性 P. antipodarum 的雄性产生率估计太低，不会影响入侵动态。
