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（sex reversal）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。有假说指出，遗传雄性的偶发性性反转可阻止因有害突变（deleterious mutations）积累所引发的非重组Y染色体演化退化。与此同时，性反转也会对种群增长率产生负面影响。
本研究针对亚北极环境下的普通林蛙（Rana temporaria）展开表型与基因型性别鉴定——该区域的成体性比呈现强烈的雌性偏倚，由此推测该区域可能频繁发生性反转。我们为该物种开发了2个全新的性连锁微卫星标记（microsatellite markers），并结合1个已有的标记与贝叶斯建模（Bayesian modelling）方法，探究性反转的发生情况并测定卵团（egg clutches）的初级性比（primary sex ratios）。
研究结果显示，显著比例（0.09，95%可信区间（credible interval）：0.04–0.18）的遗传雌性成体表现出雄性表型，但未发现可对抗Y染色体退化所需的遗传雄性性反转证据。卵团的初级性比整体趋近均等，但其中3个卵团仅含遗传雌性个体，另有3个卵团呈现显著的雌性偏倚。经性反转的遗传雌性个体的繁殖似乎可产生全雌性卵团，这可能会使种群成体性比（adult sex-ratio）向雌性偏倚，与野外观测结果相符。不过基于模拟模型的结果，此类偏倚预计规模较小且具有短暂性，因此无法完全解释野外观测到的雌性偏倚现象。