Data from: Sex reversal and primary sex ratios in the common frog (Rana temporaria)

Sex reversal has been suggested to have profound implications for the evolution of sex chromosomes and population dynamics in ectotherms. Occasional sex reversal of genetic males has been hypothesized to prevent the evolutionary decay of nonrecombining Y chromosomes caused by the accumulation of deleterious mutations. At the same time, sex reversals can have a negative effect on population growth rate. Here, we studied phenotypic and genotypic sex in the common frog (Rana temporaria) in a subarctic environment, where strongly female-biased sex ratios have raised the possibility of frequent sex reversals. We developed two novel sex-linked microsatellite markers for the species and used them with a third, existing marker and a Bayesian modelling approach to study the occurrence of sex reversal and to determine primary sex ratios in egg clutches. Our results show that a significant proportion (0.09, 95% credible interval: 0.04–0.18) of adults that were genetically female expressed the male phenotype, but there was no evidence of sex reversal of genetic males that is required for counteracting the degeneration of Y chromosome. The primary sex ratios were mostly equal, but three clutches consisted only of genetic females and three others had a significant female bias. Reproduction of the sex-reversed genetic females appears to create all-female clutches potentially skewing the population level adult sex-ratio consistent with field observations. However, based on a simulation model, such a bias is expected to be small and transient and thus does not fully explain the observed female-bias in the field.

性反转被认为对变温动物（ectotherms）的性染色体演化与种群动态具有深远影响。有假说提出，遗传雄性的偶发性性反转，可阻止由有害突变积累所引发的非重组Y染色体的进化退化。与此同时，性反转也可能对种群增长率产生负面影响。

本研究以亚北极环境中的普通林蛙（Rana temporaria）为研究对象，对其表型性别与基因型性别展开分析——该区域严重偏雌性的种群性别比例，提示性反转事件可能频繁发生。我们为该物种开发了2个全新的性连锁微卫星（microsatellite）标记，并结合1个已有的微卫星标记与贝叶斯建模（Bayesian modelling）方法，探究性反转事件的发生情况，并测定蛙卵块的初级性别比例。

研究结果显示，有显著比例（0.09，95%可信区间：0.04~0.18）的遗传雌性成体表现出雄性表型，但未发现可抵消Y染色体退化所需的遗传雄性性反转证据。多数卵块的初级性别比例均等，但有3个卵块仅含遗传雌性个体，另有3个卵块则表现出显著的雌性偏倚。经性反转的遗传雌性个体的繁殖，似乎可产生全雌性卵块，这可能会使种群成体性别比例偏向雌性，与野外观测结果相符。不过，基于模拟模型的结果显示，这类偏倚预计将较为微弱且短暂，因此无法完全解释野外观测到的雌性偏倚现象。