Introduction of Trojan sex chromosomes to boost population growth

On Large and Small Data Blow-Up Solutions in the Trojan Y Chromosome Model

The Trojan Y Chromosome Strategy (TYC) is the only genetic biological control method in practice in North America for controlling invasive populations with an XX–XY sex determinism. Herein a modified organism, that is a supermale or feminised supermale, is introduced into an invasive population to skew the sex ratio over time, causing local extinction. We consider the three species TYC reaction diffusion model, and show that introduction of supermales above certain thresholds, and for certain initial data, solutions can blow-up in finite time. Thus, in order to have biologically meaningful solutions, one needs to restrict parameter and initial data regimes, in TYC type models.

Can an introduced predator select for adaptive sex allocation?

Most species produce equal numbers of sons and daughters, and sex differences in survival after parental care do not usually affect this pattern. Temporary overproduction of the scarcer sex can be adaptive when generations overlap, the sexes differ in life-history expectations, and parents can anticipate future mating opportunities. However, an alternative strategy of maximizing the competitiveness of the more abundant sex in these circumstances remains unexplored. We develop theory showing how mothers can maximize reproductive value when future mate competition will be high by producing more sons in the advantageous early hatching positions within their broods. Our model for optimal birth order was supported by long-term data of offspring sex in a parrot facing catastrophic female mortality caused by introduced predators. Swift parrots (Lathamus discolor) suffer high female mortality due to introduced sugar gliders (Petaurus breviceps) creating fluctuating male-biased adult sex ratios. Offspring hatched early within broods fledged in better condition, and in support of our model were more likely to be male in years with higher adult female mortality. We found a highly significant rank-order correlation between observed and predicted birth sex ratios. Our study shows the potential for mothers to maximize reproductive value via strategic biases in offspring sex depending on the advantages conferred by birth order and the predictability of future mate competition. Our long-term data support the predictions and appear to suggest that sex allocation strategies may evolve surprisingly quickly when anthropogenic pressures on populations are severe.

The Genetics and Epigenetics of Sex Change in Fish

Fish show extraordinary sexual plasticity, changing sex naturally as part of their life cycle or reversing sex because of environmental stressors. This plasticity shows that sexual fate is not an irreversible process but the result of an ongoing tug-of-war for supremacy between male and female signaling networks. The behavioral, gonadal, and morphological changes involved in this process are well described, yet the molecular events that underpin those changes remain poorly understood. Epigenetic modifications emerge as a critical link between environmental stimuli, the onset of sex change, and subsequent maintenance of sexual phenotype. Here we synthesize current knowledge of sex change, focusing on the genetic and epigenetic processes that are likely involved in the initiation and regulation of sex change. We anticipate that better understanding of sex change in fish will shed new light on sex determination and development in vertebrates and on how environmental perturbations affect sexual fate.

Climate-driven shifts in adult sex ratios via sex reversals: the type of sex determination matters

Sex reversals whereby individuals of one genetic sex develop the phenotype of the opposite sex occur in ectothermic vertebrates with genetic sex-determination systems that are sensitive to extreme temperatures during sexual differentiation. Recent rises in global temperatures have led researchers to predict that sex reversals will become more common, resulting in the distortion of many populations' sex ratios. However, it is unclear whether susceptibility to climate-driven sex-ratio shifts depends on the type of sex determination that varies across species. First, we show here using individual-based theoretical models that XX/XY (male-heterogametic) and ZZ/ZW (female-heterogametic) sex-determination systems can respond differentially to temperature-induced sex reversals. Interestingly, the impacts of climate warming on adult sex ratio (ASR) depend on the effects of both genotypic and phenotypic sex on survival and reproduction. Second, we analyse the temporal changes of ASR in natural amphibian populations using data from the literature, and find that ASR shifted towards males in ZZ/ZW species over the past 60 years, but did not change significantly in XX/XY species. Our results highlight the fact that we need a better understanding of the interactions between genetic and environmental sex-determining mechanisms to predict the responses of ectotherms to climate change and the associated extinction risks.This article is part of the themed issue 'Adult sex ratios and reproductive decisions: a critical re-examination of sex differences in human and animal societies'.

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.

Demographic and genetic consequences of disturbed sex determination

During sex determination, genetic and/or environmental factors determine the cascade of processes of gonad development. Many organisms, therefore, have a developmental window in which their sex determination can be sensitive to, for example, unusual temperatures or chemical pollutants. Disturbed environments can distort population sex ratios and may even cause sex reversal in species with genetic sex determination. The resulting genotype-phenotype mismatches can have long-lasting effects on population demography and genetics. I review the theoretical and empirical work in this context and explore in a simple population model the role of the fitness vyy of chromosomally aberrant YY genotypes that are a consequence of environmentally induced feminization. Low vyy is mostly beneficial for population growth. During feminization, low vyy reduces the proportion of genetic males and hence accelerates population growth, especially at low rates of feminization and at high fitness costs of the feminization itself (i.e. when feminization would otherwise not affect population dynamics much). When sex reversal ceases, low vyy mitigates the negative effects of feminization and can even prevent population extinction. Little is known about vyy in natural populations. The available models now need to be parametrized in order to better predict the long-term consequences of disturbed sex determination.This article is part of the themed issue 'Adult sex ratios and reproductive decisions: a critical re-examination of sex differences in human and animal societies'.

ECOLOGICAL AND EVOLUTIONARY APPLICATIONS FOR ENVIRONMENTAL SEX REVERSAL OF FISH

Environmental sex reversal (ESR), which results in a mismatch between genotypic and phenotypic sex, is well documented in numerous fish species and may be induced by chemical exposure. Historically, research involving piscine ESR has been carried out with a view to improving profitability in aquaculture or to elucidate the processes governing sex determination and sexual differentiation. However, recent studies in evolution and ecology suggest research on ESR now has much wider applications and ramifications. We begin with an overview of ESR in fish and a brief review of the traditional applications thereof. We then discuss ESR and its potential demographic consequences in wild populations. Theory even suggests sex-reversed fish may be purposefully released to manipulate population dynamics. We suggest new research directions that may prove fruitful in understanding how ESR at the individual level translates to population-level processes. In the latter portion of the review we focus on evolutionary applications of ESR. Sex-reversal studies from the aquaculture literature provide insight in to the evolvability of determinants of sexual phenotype. Additionally, induced sex reversal can provide information about the evolution of sex chromosomes and sex-linked traits. Recently, naturally occurring ESR has been implicated as a mechanism contributing to the evolution of sex chromosomes.

The fitness consequences of environmental sex reversal in fish: a quantitative review

Environmental sex reversal (ESR) occurs when environmental factors overpower genetic sex-determining factors. The phenomenon of ESR is observed widely in teleost species, where it can be induced by exposing developing fish to endocrine disrupting chemicals (EDCs). EDC-induced ESR has been exploited by the aquaculture industry, while ecological and evolutionary models are also beginning to elucidate the potential roles that sex-reversed individuals play in influencing population dynamics. However, how EDC exposure affects individual fitness remains relatively unknown. To date, many experimental studies have induced sex reversal in fish and measured fitness-as indicated by related traits such as size, survival and gonadal somatic index (GSI), but the reported results vary. Here, we meta-analytically combine the results of 78 studies of induced ESR to gain insight into the fitness of sex-reversed individuals. Overall, our results suggest that the fitness of fish exposed to EDCs is reduced at the time of exposure, with exposed individuals having a smaller size and likely a smaller GSI. Given a period of non-exposure, fish treated with EDCs can regain a size equal to those not exposed, although GSI remains compromised. Interestingly, survival does not appear to be affected by EDC treatment. The published reports that comprise our dataset are, however, based on captive fish and the general small size resulting from exposure is likely to lead to reduced survival in the wild. Additionally, reduced fitness-related parameters are likely to be due to exposure to EDCs rather than ESR itself. We suggest that theoretical models of ESR should account for the fitness-related effects that we report. Whilst we are able to shed light on the physical fitness of EDC-exposed fish, the behaviour of such individuals remains largely untested and should be the focus of future experimental manipulation.

Population consequences of environmental sex reversal

When sex determination in a species is predominantly genetic but environmentally reversible, exposure to (anthropogenic) changes in the environment can lead to shifts in a population's sex ratio. Such scenarios may be common in many fishes and amphibians, yet their ramifications remain largely unexplored. We used a simple model to study the (short-term) population consequences of environmental sex reversal (ESR). We examined the effects on sex ratios, sex chromosome frequencies, and population growth and persistence after exposure to environmental forces with feminizing or masculinizing tendencies. When environmental feminization was strong, X chromosomes were driven to extinction. Analogously, extinction of normally male-linked genetic factors (e.g., Y chromosomes) was caused by continuous environmental masculinization. Although moderate feminization was beneficial for population growth in the absence of large viability effects, our results suggest that the consequences of ESR are generally negative in terms of population size and the persistence of sex chromosomes. Extreme sex ratios resulting from high rates of ESR also reduced effective population sizes considerably. This may limit any evolutionary response to the deleterious effects of ESR. Our findings suggest that ESR changes population growth and sex ratios in some counter-intuitive ways and can change the predominant factor in sex determination from genetic to fully environmental, often within only a few tens of generations. Populations that lose genetic sex determination may quickly go extinct if the environmental forces that cause sex reversal cease.