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

Abstract

27Fish can be threatened by distorted sex ratios that arise during sex differentiation. It is 28 therefore important to understand sex determination and differentiation, especially in river-29 dwelling fish that are often exposed to environmental factors that may interfere with sex 30 differentiation. However, sex differentiation is not sufficiently understood in keystone taxa 31 such as the Thymallinae, one of the three salmonid subfamilies. Here we study a wild 32 grayling (Thymallus thymallus) population that suffers from distorted sex ratios. We found 33 sex determination in the wild and in captivity to be genetic and linked to the sdY locus. We 34 therefore studied sex-specific gene expression in embryos and early larvae that were bred and 35 raised under different experimental conditions, and we studied gonadal morphology in five 36 monthly samples taken after hatching. Significant sex-specific changes in gene expression 37 (affecting about 25,000 genes) started around hatching. Gonads were still undifferentiated 38 three weeks after hatching, but about half of the fish showed immature testes around seven 39 weeks after hatching. Over the next few months, this phenotype was mostly replaced by the 40 "testis-to-ovary" or "ovaries" phenotypes. The gonads of the remaining fish, i.e. 41 approximately half of the fish in each sampling period, remained undifferentiated until six 42 months after fertilization. Genetic sexing of the last two samples revealed that fish with 43 undifferentiated gonads were all males, who, by that time, were on average larger than the 44 genetic females (verified in 8-months old juveniles raised in another experiment). Only 12% 45 of the genetic males showed testicular tissue six months after fertilization. We conclude that 46 sex differentiation starts around hatching, goes through an all-male stage for both sexes 47 (which represents a rare case of "undifferentiated" gonochoristic species that usually go 48 through an all-female stage), and is delayed in males who, instead of developing their gonads, 49 grow faster than females during these juvenile stages. 50 51 peer-reviewed)

Effects of global warming on sex ratios in fishes

“…In fishes with environmental influence on sex determination, i.e ., with either environmental sex determination or environmental sex reversal (environmental factors overruling genetic factors), the primary sex ratio can only be determined at a time when the decisive processes that occur early in development have set the path to the development of testes or ovaries. It is not surprising that at this time, typically during embryogenesis and/or early larval stages (Devlin & Nagahama, 2002), the sexes show sex‐specific gene expression (Geffroy et al ., 2016; Maitre et al ., 2017). At that time, they also show sex‐specific reactions to environmental stress such as, for example, exposure to exogenous sex hormones at ecologically relevant concentrations (Selmoni et al ., 2019).…”

“…Maitre et al . (2017) then used sex‐linked molecular markers to study the gonad development of genetic males and females, finding marked differences in the timing of gonad development: females start during their first summer ( i.e ., at a time when the natural population is exposed to increasingly higher water temperatures) whereas males grow faster during these months and only start developing testes in autumn, after the warm period. Such different life histories could potentially cause female‐biased juvenile mortality linked to climate change.…”

Low adaptive potential for tolerance to ethynylestradiol, but also low toxicity, in a grayling population (Thymallus thymallus)

“…A further sample of in total 1,555 embryos was assigned to the present study ( Figure 1). After embryo and larval performance had been recorded (see below), a mixed sample of these differently treated larvae and of the remaining ones were pooled and further raised in aquaria to study sex differentiation [25,26]. All remaining samples were euthanized with an overdose (1 mL/L) of Koimed Sleep (Ethylenglycolmonophenylether; Koimed, Ulmiz, Switzerland).…”

“…Regarding our second main question: Because grayling males do not provide any parental care, significant sire effects on offspring traits reveal additive genetic variance in full-factorial breeding experiments [26]. The dam effect then represents a combination of additive genetic variance and maternal environmental effects [26]. In salmonids, maternal environmental effects comprises characteristics such as egg size [50] and compounds that females allocate to their eggs [e.g.…”

“…The presence of this stressor could have induced rapid evolution in some exposed salmonid populations that happened to have additive genetic variation for the tolerance to this new type of pollution [14,15]. Continuous selection over several generations would then be expected to reduce the toxicity of EE2 but also deplete the genetic variance for its tolerance and thereby slow down adaptation [26]. If so, we would predict on average lower toxicity of, and lower genetic variation for the tolerance to, EE2 in river-spawning than in lake-spawning salmonids.…”