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.性别反转（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。有假说提出，遗传雄性的偶发性性别反转，可阻滞因有害突变积累所引发的不重组Y染色体的演化退化。与此同时，性别反转亦会对种群增长率产生负面影响。本研究于亚北极环境中针对普通蛙（林蛙，*Rana temporaria*）的表型性别与基因型性别展开探究——该区域存在强烈的雌性偏倚性比，由此引发了频繁发生性别反转的推测。我们为该物种开发了2个全新的性别关联微卫星标记（microsatellite marker），并结合已有的第3个标记与贝叶斯建模方法，对性别反转的发生情况进行研究，同时测定卵块的初级性比。研究结果表明，占比0.09（95%置信区间（credible interval）：0.04–0.18）的遗传雌性成体呈现雄性表型，但未发现可对抗Y染色体退化所需的遗传雄性性别反转证据。初级性比整体趋近均等，但有3个卵块仅含遗传雌性个体，另有3个卵块呈现显著的雌性偏倚。发生性别反转的遗传雌性个体的繁殖行为，似乎会产生全雌性卵块，这可能会如野外观测结果所示，使种群水平的成体性比偏向雌性。不过，基于模拟模型的结果，此类性比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。