Environmental sex reversal,Trojan sex genes,and sex ratio adjustment: conditions and population consequences

The great diversity of sex determination mechanisms in animals and plants ranges from genetic sex determination (GSD, e.g. mammals, birds, and most dioecious plants) to environmental sex determination (ESD, e.g. many reptiles) and includes a mixture of both, for example when an individual’s genetically determined sex is environmentally reversed during ontogeny (ESR, environmental sex reversal, e.g. many fish and amphibia). ESD and ESR can lead to widely varying and unstable population sex ratios. Populations exposed to conditions such as endocrine‐active substances or temperature shifts may decline over time due to skewed sex ratios, a scenario that may become increasingly relevant with greater anthropogenic interference on watercourses. Continuous exposure of populations to factors causing ESR could lead to the extinction of genetic sex factors and may render a population dependent on the environmental factors that induce the sex change. However, ESR also presents opportunities for population management, especially if the Y or W chromosome is not, or not severely, degenerated. This seems to be the case in many amphibians and fish. Population growth or decline in such species can potentially be controlled through the introduction of so‐called Trojan sex genes carriers, individuals that possess sex chromosomes or genes opposite from what their phenotype predicts. Here, we review the conditions for ESR, its prevalence in natural populations, the resulting physiological and reproductive consequences, and how these may become instrumental for population management.     

Mechanisms of rapid sympatric speciation by sex reversal and sexual selection in cichlid fish

Mechanisms of speciation in cichlid fish were investigated by analyzing population genetic models of sexual selection on sex-determining genes associated with color polymorphisms. The models are based on a combination of laboratory experiments and field observations on the ecology, male and female mating behavior, and inheritance of sex-determination and color polymorphisms. The models explain why sex-reversal genes that change males into females tend to be X-linked and associated with novel colors, using the hypothesis of restricted recombination on the sex chromosomes, as suggested by previous theory on the evolution of recombination. The models reveal multiple pathways for rapid sympatric speciation through the origin of novel color morphs with strong assortative mating that incorporate both sex-reversal and suppressor genes. Despite the lack of geographic isolation or ecological differentiation, the new species coexists with the ancestral species either temporarily or indefinitely. These results may help to explain different patterns and rates of speciation among groups of cichlids, in particular the explosive diversification of rock-dwelling haplochromine cichlids.     

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.     

The operational sex ratio influences choosiness in a pipefish

If more females than males are available for mating in the breedingpopulation (i.e., the operational sex ratio, OSR, is femalebiased), males can afford to be choosy. In the pipefish (Syngnathustyphle) females compete for males, who are choosy. In natureOSRs are typically female biased, but may occasionally be malebiased. In a series of experiments, males were allowed to choosebetween a large and a small female under a perceived excessof either males or females. Under female bias, males preferredthe large female: they spent more time close to her than tothe small female; they courted the large female sooner thanthe small; and they tended to copulate sooner and more oftenwith the large female. Under male bias all these differencesvanished and males mated at random with respect to female size.Males reproduced at a faster rate under male than under femalebias because they received more eggs in their brood pouches.Thus, males switched from maximizing mate quality (i.e., beingchoosy) to minimizing the risk of not reproducing (i.e., beingquick) as the OSR became male biased.     

The influence of age and sex on phagocyte chemiluminescence

The process of ageing is associated with increased susceptibility to infection. Phagocytes form the primary defence mechanism against infecting microorganisms, but the influence of ageing on phagocyte function remains controversial. In this study we have applied a microtitre plate phagocyte chemiluminescence (CL) assay suitable for clinical use to compare phagocyte oxidative metabolism in younger healthy subjects (age 20–60 years) and healthy older (60–70 years) subjects. Polymorphonuclear leukocytes (PMNL) and monocytes were stimulated using phorbol myristate acetate (PMA), serum opsonized zymosan (SOZ), and non-opsonized zymosan (ZYM) in the presence of both lucigenin and luminol. Monocytes showed a higher luminolenhanced CL response to PMA in males compared with females in the younger age group. No PMNL differences were observed between the sexes. Although no difference were found in relation to age when cells were stimulated with PMA and SOZ, significantly lower background (unstimulated) CL was obtained from PMNL with luminol. PMNL luminol-enhanced CL responses were also lower in response to ZYM. The findings suggest a reduced response of PMNL from older subjects to minimal stimulation. This could be related to abnormalities in the triggering of the respiratory burst or myeloperoxidase release due to ageing. The influence of age and sex should be taken into account in clinical studies of phagocyte CL.     

The influence of female age and host size on the sex ratio of the parasitoid Opius concolor

The sex ratio of the progeny of single females parasitizing large hosts favoured the females (sex ratio=0.26); but on small hosts favoured the males (0.73). No differences in mortality of the sexes were detected. The sex ratio was independent of female age when large hosts were used. The percentage of males observed in the progeny of the first day of female oviposition was significantly greater than the mean, irrespective of the age at which female oviposition began. When females were exposed to small hosts, a greater percentage of females was observed in the progeny from the last days of oviposition.

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Résumé L'influence de la taille de l'hôteret de l'âge de la femelle sur le taux sexuel de la descendance a été étudiée sur le parasitoïde Opius concolor Szépl.Le taux sexuel de la descendance des femelles isolées est favorable aux femelles (t.s.=0,26) quand elles ont à leur disposition des hôtes de grande taille, tandis qu'avec des hôtes petits le taux sexuel est favorable aux mâles (t.s.=0.73). On n'a pas détecté de mortalité différentielle des sexes.Les pourcentage de mâles obtenu le premier jour de ponte des femelles sur les hôtes de grande taille est significativement différent de la moyenne, indépendamment de l'âge de la femelle à ce moment. Cependant, sur des hôtes petits, bien que restant favorable aux mâles dans l'ensemble, une plus grande proportion de femelles à partir des premiers jours de ponte a pu être observée.

     

Temperature‐induced sex reversal is not responsible for sex‐ratio distortions in grayling Thymallus thymallus or brown trout Salmo trutta

On the basis of the experiments carried out over various years, it was concluded that (1) grayling Thymallus thymallus and brown trout Salmo trutta are resistant to temperature‐induced sex reversal at ecologically relevant temperatures, (2) environmental sex reversal is unlikely to cause the persistent sex ratio distortion observed in at least one of the study populations and (3) sex‐specific tolerance of temperature‐related stress may be the cause of distorted sex ratios in populations of T. thymallus or S. trutta.     

Operational sex ratio,reproductive costs,and the potential for intrasexual competition

Even though numerous metrics exist, we still appear to be far from a consensual view on the best way of measuring or predicting the potential strength of sexual selection. One of the earliest and simplest metrics devised was the operational sex ratio (OSR) (i.e. the ratio between sexually active males and females in a population), and even, if heavily criticized, the OSR can still be viewed as a valuable measure of the potential levels of intrasexual competition. Because this ratio is influenced by the time that individuals spend in the mating pool, the OSR depends on our ability to determine who is indeed ready to mate. Moreover, because the proximate effects of OSR on mate monopolization might not be immediately apparent, we should be prepared to account for its association with the conditions making selection favour traits that reduce the time needed to acquire additional mating opportunities. Using the worm pipefish as a working model, we conducted a more stringent calculation of the OSR by eliminating individuals that, although present, do not appear to be able to reproduce, as determined by an analysis of female size classes with immature or spent ovaries and male classes without pregnancy events. Accordingly, the OSR was not only capable of correctly highlighting the potential for intrasexual competition, but also was able to translate the desertion of individuals from the mating pool as the breeding season progressed into meaningful correlations with variables associated with reproductive investment and costs (i.e. gonad investment and energy reserves). As demonstrated, the predictive power of the OSR can be broader than anticipated, depending primarily on our ability to effectively discriminate which individuals are indeed ready to mate. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 477–484.     

Socially controlled male-to-female sex reversal in the protogynous orange-spotted grouper,Epinephelus coioides

Socially controlled sex change in teleosts is a dramatic example of adaptive reproductive plasticity. In many cases, the occurrence of sex change is triggered by a change in the social context, such as the disappearance of the dominant individual. The orange-spotted grouper Epinephelus coioides is a typical protogynous hermaphrodite fish that changes sex from female to male and remains male throughout its life span. In this study, male-to-female sex reversal in male Epinephelus coioides was successfully induced by social isolation. The body length and mass, gonadal change, serum sex steroid hormone levels and sex-related gene expression patterns during the process of socially controlled male-to-female sex reversal in E. coioides were systematically examined. This report investigates the physiological mechanisms of the socially controlled male-to-female sex reversal process in a protogynous hermaphrodite grouper species. The results enable us to study the physiological control of sex change, not only from female to male, but also from male to female.     

About the sex ratio in connection with early embryonic mortality in man

The problem of the functioning specificity of sex chromosomes during the early stages of embryogenesis in man and the associated problem of the sex ratio in spontaneous and induced abortions, as well as in newborns, remains open. We have conducted a cytogenetic examination of 342 spontaneous abortions divided into three clinical groups on the basis of the severity of the developmental disturbances of the embryo: spontaneous abortionssensu stricto with a developed embryo without any significant intrauterine delay of development (n=100), nondeveloping pregnancies (n=176), and anembryonic fetuses (n=66). The frequency of chromosomal mutations in these groups was 22.0, 48.3, and 48.5%, respectively. Statistical analysis has demonstrated significant differences between the studied groups in the frequencies of the normal and abnormal karyotypes: the major contributions to these differences were associated with autosomal trisomy, triploidy, and the 46.XY karyotype. The presence of 46.XY may reflect the specific genetic mechanisms of the prenatal mortality of embryos with the normal karyotype, associated with sex and/or with the imprinting of X-chromosomes. The sex ratio in spontaneous abortions with the normal karyotype was as follows: 0.77 for spontaneous abortions with well-developed embryos without any significant intrauterine delay of development; 0.60 for nondeveloping pregnancies; and 0.31 for anembryonic fetuses. An analysis of DNA from the embryos and their parents has demonstrated a low probability of contamination of cell cultures with mother cells as a possible source of the prevalence of embryos with the 46.XX karyotype among spontaneous abortions. Nondeveloping pregnancies and anembryonic fetuses showed statistically significant differences in the sex ratio from the control group consisting of medical abortions (1,11). Differences in the sex ratio were due to an increasingly lower proportion of embryos with karyotype 46.XY (relative to the expected one) among the fetuses with an increased severity of developmental disturbances. The statistical “chances ratio” index also provided evidence that embryos with the 46.XY karyotype had a higher propensity to produce a well-formed fetus as compared with the female embryos. We propose that the expression of genes of the maternal X-chromosome in XY embryos supports a more stable development during early embryogenesis as compared with XX embryos. In the latter case, normal development is coupled with the operation of an additional mechanism for compensation of the dose of X-linked genes. Operation of this mechanism increases the probability of disturbances in female embryos. A higher viability of XY embryos during the early stages of ontogenesis in man appears to explain their underrepresentation in samples of spontaneously aborted embryos and appears to be the major factor responsible for the deviation of the sex ratio from the theoretically expected value.     

Constant relative age and size at sex change for sequentially hermaphroditic fish

A general problem in evolutionary biology is that quantitative tests of theory usually require a detailed knowledge of the underlying trade-offs, which can be very hard to measure. Consequently, tests of theory are often constrained to be qualitative and not quantitative. A solution to this problem can arise when life histories are viewed in a dimensionless way. Recently, dimensionless theory has been developed to predict the size and age at which individuals should change sex. This theory predicts that the size at sex change/maximum size (L50/L(max)), and the age at sex change/age at first breeding (tau/alpha) should both be invariant. We found support for these two predictions across 52 species of fish. Fish change sex when they are 80% of their maximum body size, and 2.5 times their age at maturity. This invariant result holds despite a 60 and 25 fold difference across species in maximum size and age at sex change. These results suggest that, despite ignoring many biological complexities, relatively simple evolutionary theory is able to explain quantitatively at what point sex change occurs across fish species. Furthermore, our results suggest some very broad generalities in how male fitness varies with size and age across fish species with different mating systems.     

Novel genetic sex markers reveal high frequency of sex reversal in wild populations of the agile frog (Rana dalmatina) associated with anthropogenic land use

Populations of ectothermic vertebrates are vulnerable to environmental pollution and climate change because certain chemicals and extreme temperatures can cause sex reversal during early ontogeny (i.e. genetically female individuals develop male phenotype or vice versa), which may distort population sex ratios. However, we have troublingly little information on sex reversals in natural populations, due to unavailability of genetic sex markers. Here, we developed a genetic sexing method based on sex‐linked single nucleotide polymorphism loci to study the prevalence and fitness consequences of sex reversal in agile frogs (Rana dalmatina). Out of 125 juveniles raised in laboratory without exposure to sex‐reversing stimuli, 6 showed male phenotype but female genotype according to our markers. These individuals exhibited several signs of poor physiological condition, suggesting stress‐induced sex reversal and inferior fitness prospects. Among 162 adults from 11 wild populations in North‐Central Hungary, 20% of phenotypic males had female genotype according to our markers. These individuals occurred more frequently in areas of anthropogenic land use; this association was attributable to agriculture and less strongly to urban land use. Female‐to‐male sex‐reversed adults had similar body mass as normal males. We recorded no events of male‐to‐female sex reversal either in the laboratory or in the wild. These results support recent suspicions that sex reversal is widespread in nature, and suggest that human‐induced environmental changes may contribute to its pervasiveness. Furthermore, our findings indicate that sex reversal is associated with stress and poor health in early life, but sex‐reversed individuals surviving to adulthood may participate in breeding.     

The influence of developmental mortality on optimal sex allocation under local mate competition

In panmictic populations, optimal sex allocation is, under theassumptions of Fisher's model, not influenced by the probabilityof offspring developmental mortality, or by differences in mortalitybetween the sexes. In contrast, when mating opportunities areconfined to siblings, developmental mortality can influenceoptimal sex allocation. Many animal species have both localmating and developmental mortality. We show that when developmentalmortality is random for individual offspring, optimal sex allocationis influenced by mortality among males but not among females.Male mortality increases the allocation to males, but this shouldnever be male biased, even under extreme male mortality. Thisresult applies both when mothers are able to control the sexof individual offspring precisely, and when sex is allocatedwith binomial probability. The influence of mortality becomesprogressively larger when the variance of the distribution ofmortality over clutches diminishes. The reduction in fitnessis greater than the proportion of mortality, especially at smallclutch sizes, and mortality reduces the advantage of producingprecise sex ratios, and of local mate competition in general.     

Effects of sexual differences of age at maturity and survival on population sex ratio

Summary Five demographical factors influencing the sex ratio of a population are classically considered. The influence of two of them is dependent on the longevity of individuals in the population. The effect of differential age at maturity between males and females is higher for animals with low annual survival, whereas the effect of differential annual survival between males and females is higher for animals with high annual survival. Such a conclusion applied to turtles, which are long life-span animals, allows us to retain differential survival between sexes as a major factor influencing the population sex ratio.     

Mechanisms that influence sex ratio variation in the invasive hymenopteran Sirex noctilio in South Africa

Sirex noctilio is an economically important invasive pest of commercial pine forestry in the Southern Hemisphere. Newly established invasive populations of this woodwasp are characterized by highly male‐biased sex ratios that subsequently revert to those seen in the native range. This trend was not observed in the population of S. noctilio from the summer rainfall regions in South Africa, which remained highly male‐biased for almost a decade. The aim of this study was to determine the cause of this persistent male bias. As an explanation for this pattern, we test hypotheses related to mating success, female investment in male versus female offspring, and genetic diversity affecting diploid male production due to complementary sex determination. We found that 61% of females in a newly established S. noctilio population were mated. Microsatellite data analysis showed that populations of S. noctilio from the summer rainfall regions in South Africa are far less genetically diverse than those from the winter rainfall region, with mean Nei's unbiased gene diversity indexes of 0.056 and 0.273, respectively. These data also identified diploid males at low frequencies in both the winter (5%) and summer (2%) rainfall regions. The results suggest the presence of a complementary sex determination mechanism in S. noctilio, but imply that reduced genetic diversity is not the main driver of the male bias observed in the summer rainfall region. Among all the factors considered, selective investment in sons appears to have the most significant influence on male bias in S. noctilio populations. Why this investment remains different in frontier or early invasive populations is not clear but could be influenced by females laying unfertilized eggs to avoid diploid male production in populations with a high genetic relatedness.     

Demonstration of viability and fertility and development of a molecular tool to identify YY supermales in a fish with both genotypic and environmental sex determination

The pejerrey possesses a genotypic sex determination system driven by the amhy gene and yet shows marked temperature‐dependent sex determination. Sex‐reversed XY females have been found in a naturally breeding population established in Lake Kasumigaura, Japan. These females could mate with normal XY males and generate YY “supermale” individuals that, if viable and fertile, would sire only genotypic male offspring. This study was conducted to verify the viability, gender, and fertility of YY pejerrey and to develop a molecular method for their identification. Production of YY fish was attempted by crossing a thermally sex‐reversed XY female and an XY male, and rearing the progeny until sexual maturation. To identify the presumable YY individuals, we first conducted a PCR analysis using amhy‐specific primers to screen only amhy‐positive (XY and YY) fish. This screening showed that 60.6% of the progeny was amhy‐positive, which suggested the presence of YY fish. We then conducted a second screening by qPCR in order to identify the individuals with two amhy copies in their genome. This screening revealed 13 individuals, all males, with values twice higher than the other 30 amhy‐positive fishes, suggesting they have a YY complement. This assumption as well as the viability, fertility, and “supermale” nature of these individuals was confirmed in progeny tests with XX females that yielded 100% amhy‐positive offspring. These results demonstrate that qPCR can obviate progeny test as a means to identify the genotypic sex and therefore may be useful for the survey of all three possible genotypes in wild populations.