Data from: Male offspring production by asexual Potamopyrgus antipodarum, a New Zealand snail

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.

由于仅雌性可直接推动种群增长，与仅产生雌性后代的无性繁殖个体相比，对雌雄后代投入均等资源的有性繁殖个体，需承受两倍于前者的“雄性成本”。既往研究多聚焦于可抵消这一“雄性成本”的有性繁殖演化优势，以此解释有性繁殖在自然界的主导地位。本文则转而探究无性繁殖类群是否同样需要承担雄性成本，具体通过量化45个由专性无性雌性新西兰扁卷螺（Potamopyrgus antipodarum）建立的实验种群（下称谱系 (lineages)）的雄性产生速率开展。这种新西兰本土螺类是研究有性繁殖演化的极具价值的模型：其表型相似的有性与无性繁殖型种群常共生共存，可实现有性与无性繁殖类群的直接对比。经过两年实验室培养，45个谱系中有23个成功产生雄性个体，证实无性繁殖的P. antipodarum能够产生雄性后代。我们采用最大似然分析法，基于“仅部分谱系具备产生雄性能力”的模型估算，约50%的无性谱系拥有产生雄性的能力，且在可产生雄性的谱系中，雄性后代占比约为5%。实验第一年即可产生雄性的谱系，在第二年更易继续产生雄性，这提示部分无性谱系相较其他无性谱系，或许需承担雄性成本。最后，我们借助简单的确定性种群动力学模型，评估雄性产生对无性谱系入侵有性种群的速率的影响，结果显示，经估算的无性P. antipodarum雄性产生速率过低，不足以对入侵动态产生显著影响。