sex determination; fish; salmonids; global warming; demographics; evolution; ecology; genetics; experimental breeding; minnow (Phoxinus phoxinus)
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Background - Sex determination in fish is often genetic, with many species relying on the segregation of sex chromosomes for assignment of gender. However, the phenotypic sex can be modified by environmental influences after fertilization. Sex hormones or hormone mimics, endocrine-disrupting chemicals, and even water temperatures or pH above or below normal during a discrete period after conception can induce environmental sex reversal (ESR) and thereby create biased sex ratios. Information on the physiological and reproductive consequences of such environmental changes is accumulating, and there seems to be an increasing recognition that environmental factors may induce sex reversal in many natural fish populations. However, the conditions for ESR in fish are not well understood yet. Moreover, the prevalence of ESR in natural populations, its fitness effects relative to genetic and maternal environment effects, and hence its consequences on population demography, population genetics, and the potential for an evolutionary response to ESR are largely unknown. First theoretical treatments of the issue concluded that ESR can have dramatic effects on the demographics of a population and on the evolution of sex-linked genes or chromosomes. On the one hand, such effects can directly threaten natural populations. Indeed, first field observations suggest that ESR happens in several salmonid populations, including a Swiss population of grayling (Thymallus thymallus) in which we found dramatically distorted sex ratios that are linked to changed temperatures at the time when sex determination happens, and that could potentially explain an otherwise unexplained population decline. On the other hand, ESR can potentially be used to control invasive species like the goldfish (Carassius auratus) or the pumpkinseed sunfish (Lepomis gibbosus) in the pre-Alpine region, or the European minnow (Phoxinus phoxinus) and the brown trout (Salmo trutta) in other parts of the world: repeated introduction of, for example, YY-females could produce extreme male-biased sex ratios and thereby lead to population decline or extinction (the “Trojan Y-chromosome hypothesis”). Working Hypotheses - Sex determination in various fish species of the pre-Alpine region is influenced by changed temperature regimes and by other environmental factors. This affects population demographics, genetics, and evolution. ESR not only threatens some natural populations but can also be used to control invasive species by introduction of sex-reversed individuals.Specific Aims - We will experimentally study the conditions for ESR and the associated fitness costs in various populations of graylings, brown trout, whitefish (Coregonus sp.), and minnows of the pre-Alpine region. Our first three study species are members of the charismatic and economically important family Salmonidae, i.e. of a family that has been well studied with regard to physiology, genetics, ecology, behavior, and evolution. We therefore consider them as excellent model species - like the minnow that is one of the best-studied members of the large family Cyprinidae. Quantitative estimates of key factors will be used to build data based models that explore the population consequences and the possible evolution of ESR in natural populations. We will also explore the potential of ESR for controlling fish species that are considered invasive in the pre-Alpine region or in other parts of the world.Experimental Design and Methods - We will draw representative samples of breeders from various natural populations. Full-factorial in vitro breeding experiments and the raising of offspring under various experimental conditions in the laboratory, under various field conditions, and in experimental ponds will allow us to study the within- and between- population variation in the conditions for, and the consequences of, ESR. We will also record the prevalence in ESR in the wild by inference from our breeding experiments and by analyzing the findings of ongoing monitoring programs.Expected Value of the Proposed Project - ESR may become increasingly relevant with greater anthropogenic interferences on watercourses and with changing global and local temperatures. Our research will close important gaps in our understanding of sex determination in various fish that are not only model species for various fields of basic research but also ecological keystone species in their respective habitats. Our research will provide a scientific basis for possible countermeasures in the case of populations that are declining because of ESR or that are threatened by ESR. It will also provide a scientific basis for possible exploitation of ESR to control invasive species that are threatening existing species communities in various parts of the world, including Switzerland.