Data from: Sex differentiation in grayling (Salmonidae) goes through an all-male stage and is delayed in genetic males who instead grow faster

Fish populations can be threatened by distorted sex ratios that arise during sex differentiation. Here we describe sex differentiation in a wild grayling (Thymallus thymallus) population that suffers from distorted sex ratios. We verified that sex determination is linked to the sex determining locus (sdY) of salmonids. This allowed us to study sex-specific gene expression and gonadal development. Sex-specific gene expression could be observed during embryogenesis and was strong around hatching. About half of the fish showed immature testes around eleven weeks after fertilization. This phenotype was mostly replaced by the “testis-to-ovary” or “ovaries” phenotypes during development. The gonads of the remaining fish stayed undifferentiated until six months after fertilization. Genetic sexing revealed that fish with undifferentiated gonads were all males, who grew larger than the genetic females during the observational period. Only 12% of the genetic males showed testicular tissue six months after fertilization. We conclude that sex differentiation starts before hatching, goes through an all-male stage for both sexes (which represents a rare case of “undifferentiated” gonochoristic species that usually go through an all-female stage), and is delayed in males. During these juvenile stages males grow faster than females instead of developing their gonads.

鱼类种群可因性别分化过程中产生的性别比例失衡而受到威胁。本研究对一个存在性别比例失衡的野生欧洲茴鱼（Thymallus thymallus）种群的性别分化过程进行了描述。我们确认该种群的性别决定与鲑科鱼类的性别决定基因座（sdY）相关，这为我们开展性别特异性基因表达与性腺发育的研究提供了基础。研究人员在胚胎发生阶段即可观测到性别特异性基因表达，且该表达在孵化前后达到较高水平。受精后约11周，约半数个体出现未成熟睾丸表型；该表型在后续发育过程中大多被“睾丸向卵巢转化”或“卵巢”表型替代。剩余个体的性腺则在受精后6个月内始终保持未分化状态。遗传性别鉴定结果显示，性腺未分化的个体均为遗传雄性，且在观测期内其体型大于遗传雌性。受精后6个月时，仅12%的遗传雄性个体出现睾丸组织。本研究得出如下结论：性别分化始于孵化前，两类性别的个体均经历全雄阶段——这是一类罕见的“未分化型”雌雄异体物种的特征，这类物种通常会经历全雌阶段——且雄性的性别分化进程存在延迟。在该幼体阶段，雄性个体并未优先发育性腺，反而比雌性生长得更快。