Sex ratios deviate across killifish species without clear links to life history

Sex ratios can differ from an expected equal proportion of males and females, carrying substantial implications for our understanding of how mating systems evolve. Typically, macro-evolutionary studies have been conducted without assessing how deviations from an equal sex ratio could be explained by sex-biased mortality or dispersal. Our understanding of sex ratio evolution independent of these confounds, in addition to any putative links between skewed sex ratios and other factors (e.g. life history), therefore remains largely unexplored. Here, we conducted an exploratory study investigating differences in sex ratios across closely related species while controlling for extrinsic mortality. We also tested two factors, non-overlapping/overlapping generations and the social environment, which have both been hypothesised to affect sex ratios. Specifically, we raised 15 species of killifish, which have either overlapping or discrete generations, under both solitary and social treatments. We found substantial divergences in sex ratios across closely related species, which exhibited both male and female biases. In conjunction with a low phylogenetic signal, our results suggest that sex ratios can evolve rapidly in this group. However, we found no evidence that overlapping generations or the social environment affected sex biases, suggesting that other factors drive the rapid evolution of sex ratios in killifishes.

Introduction to the special issue-Developmental plasticity in reptiles: Physiological mechanisms and ecological consequences

Scientific interest in developmental plasticity spans many disciplines, and research on reptiles has provided many insights into this field. We highlight these contributions, review the field's history, and introduce the special issue on this topic .

Daphnia females adjust sex allocation in response to current sex ratio and density

Cyclical parthenogenesis presents an interesting challenge for the study of sex allocation, as individuals' allocation decisions involve both the choice between sexual and asexual reproduction, and the choice between sons and daughters. Male production is therefore expected to depend on ecological and evolutionary drivers of overall investment in sex, and those influencing male reproductive value during sexual periods. We manipulated experimental populations, and made repeated observations of natural populations over their growing season, to disentangle effects of population density and the timing of sex from effects of adult sex ratio on sex allocation in cyclically parthenogenetic Daphnia magna. Male production increased with population density, the major ecological driver of sexual reproduction; however, this response was dampened when the population sex ratio was more male-biased. Thus, in line with sex ratio theory, we show that D. magna adjust offspring sex allocation in response to the current population sex ratio.

Variations upon a theme: Australian lizards provide insights into the endocrine control of vertebrate reproductive cycles

Australian lizards exhibit a broad array of different reproductive strategies and provide an extraordinary diversity and range of models with which to address fundamental problems in reproductive biology. Studies on lizards have frequently led to new insights into hormonal regulatory pathways or mechanisms of control, but we have detailed knowledge of the reproductive cycle in only a small percentage of known species. This review provides an overview and synthesis of current knowledge of the hormonal control of reproductive cycles in Australian lizards. Agamid lizards have provided useful models with which to test hypotheses about the hormonal regulation of the expression of reproductive behaviors, while research on viviparous skinks is providing insights into the evolution of the endocrine control of gestation. However, in order to better understand the potential risks that environmental factors such as climate change and endocrine disrupting chemicals pose to our fauna, better knowledge is required of the fundamental characteristics of the reproductive cycle in a broader range of lizard species.

Experimental evidence for homeostatic sex allocation after sex-biased reintroductions

First principles predict negative frequency-dependent sex allocation, but it is unproven in field studies and seldom considered, despite far-reaching consequences for theory and practice in population genetics and dynamics as well as animal ecology and behaviour. Twenty-four years of rhinoceros calving after 45 reintroductions across southern Africa provide the first in situ experimental evidence that unbalanced operational sex ratios predicted offspring sex and offspring sex ratios. Our understanding of population dynamics, especially reintroduction and invasion biology, will be significantly impacted by these findings.

Hormone-mediated adjustment of sex ratio in vertebrates

The ability to adjust sex ratios at the individual level exists among all vertebrate groups studied to date. In many cases, there is evidence for facultative adjustment of sex ratios in response to environmental and/or social cues. Because environmental and social information must be first transduced into a physiological signal to influence sex ratios, hormones likely play a role in the adjustment of sex ratio in vertebrates, because the endocrine system acts as a prime communicator that directs physiological activities in response to changing external conditions. This symposium was developed to bring together investigators whose work on adjustment of sex ratio represents a variety of vertebrate groups in an effort to draw comparisons between species in which the sex-determination process is well-established and those in which more work is needed to understand how adjustments in sex ratio are occurring. This review summarizes potential hormone targets that may underlie the mechanisms of adjustment of sex ratio in humans, non-human mammals, birds, reptiles, and fishes.

Complex selection associated with Hox genes in a natural population of lizards

Hox genes are recognized for their explanatory power of bilateral development. However, relatively little is known about natural variation in, and the evolutionary dynamics of, Hox genes within wild populations. Utilizing a natural population of sand lizards (Lacerta agilis), we screened HoxA13 for genetic variation and an association with incidence of offspring malformations. We found significant effects of parental genetic similarity and offspring sex, and their interaction, on risk of hatching malformed as an offspring. We also found within population genetic variation in HoxA13, and identified a significant effect of a three-way interaction among Hox genotype, parental genetic similarity, and offspring sex on the risk of hatching malformation. Since malformed offspring in this population do not survive to maturity, this study reveals complex and ongoing selection associated with Hox genes in a wild reptile population. Importantly, this demonstrates the utility of natural populations in unveiling microevolutionary processes shaping variation in highly conserved genes.

Paternal condition drives progeny sex-ratio bias in a lizard that lacks parental care

Sex-allocation theory predicts that females in good condition should preferentially produce offspring of the sex that benefits the most from an increase in maternal investment. However, it is generally assumed that the condition of the sire has little effect on progeny sex ratio, particularly in species that lack parental care. We used a controlled breeding experiment and molecular paternity analyses to examine the effects of both maternal and paternal condition on progeny sex ratio and progeny fitness in the brown anole (Anolis sagrei), a polygynous lizard that lacks parental care. Contrary to the predictions of sex-allocation theory, we found no relationship between maternal condition and progeny sex ratio. By contrast, progeny sex ratio shifted dramatically from female-biased to male-biased as paternal condition increased. This pattern was driven entirely by an increase in the production of sons as paternal condition improved. Despite strong natural selection favoring large size and high condition in both sons and daughters, we found no evidence that progeny survival was related to paternal condition. Our results emphasize the importance of considering the paternal phenotype in studies of sex allocation and highlight the need for further research into the pathways that link paternal condition to progeny fitness.

Variation in levels of reactive oxygen species is explained by maternal identity, sex and body-size-corrected clutch size in a lizard

Many organisms show differences between males and females in growth rate and crucial life history parameters, such as longevity. Considering this, we may expect levels of toxic metabolic by-products of the respiratory chain, such as reactive oxygen species (ROS), to vary with age and sex. Here, we analyse ROS levels in female Australian painted dragon lizards (Ctenophorus pictus) and their offspring using fluorescent probes and flow cytometry. Basal level of four ROS species (singlet oxygen, peroxynitrite, superoxide and H(2)O(2)) measured with a combined marker, and superoxide measured specifically, varied significantly among families but not between the sexes. When blood cells from offspring were chemically encouraged to accelerate the electron transport chain by mitochondrial uncoupling, net superoxide levels were three times higher in daughters than sons (resulting in levels outside of the normal ROS range) and varied among mothers depending on offspring sex (significant interaction between maternal identity and offspring sex). In offspring, there were depressive effects on ROS of size-controlled relative clutch size, which relies directly on circulating levels of vitellogenin, a confirmed antioxidant in some species. Thus, levels of reactive oxygen species varies among females, offspring and in relation to reproductive investment in a manner that makes its regulatory processes likely targets of selection.

Reproducing lizards modify sex allocation in response to operational sex ratios

Sex-allocation theory suggests that selection may favour maternal skewing of offspring sex ratios if the fitness return from producing a son differs from that for producing a daughter. The operational sex ratio (OSR) may provide information about this potential fitness differential. Previous studies have reached conflicting conclusions about whether or not OSR influences sex allocation in viviparous lizards. Our experimental trials with oviparous lizards (Amphibolurus muricatus) showed that OSR influenced offspring sex ratios, but in a direction opposite to that predicted by theory: females kept in male-biased enclosures overproduced sons rather than daughters (i.e. overproduced the more abundant sex). This response may enhance fitness if local OSRs predict survival probabilities of offspring of each sex, rather than the intensity of sexual competition.

Sex-specific sibling interactions and offspring fitness in vertebrates: patterns and implications for maternal sex ratios

Vertebrate sex ratios are notorious for their lack of fit to theoretical models, both with respect to the direction and the magnitude of the sex ratio adjustment. The reasons for this are likely to be linked to simplifying assumptions regarding vertebrate life histories. More specifically, if the sex ratio adjustment itself influences offspring fitness, due to sex-specific interactions among offspring, this could affect optimal sex ratios. A review of the literature suggests that sex-specific sibling interactions in vertebrates result from three major causes: (i) sex asymmetries in competitive ability, for example due to sexual dimorphism, (ii) sex-specific cooperation or helping, and (iii) sex asymmetries in non-competitive interactions, for example steroid leakage between fetuses. Incorporating sex-specific sibling interactions into a sex ratio model shows that they will affect maternal sex ratio strategies and, under some conditions, can repress other selection pressures for sex ratio adjustment. Furthermore, sex-specific interactions could also explain patterns of within-brood sex ratio (e.g. in relation to laying order). Failure to take sex-specific sibling interactions into account could partly explain the lack of sex ratio adjustment in accordance with theoretical expectations in vertebrates, and differences among taxa in sex-specific sibling interactions generate predictions for comparative and experimental studies.

Do operational sex ratios influence sex allocation in viviparous lizards with temperature-dependent sex determination?

Under certain environmental situations, selection may favour the ability of females to adjust the sex ratio of their offspring. Two recent studies have suggested that viviparous scincid lizards can modify the sex ratio of the offspring they produce in response to the operational sex ratio (OSR). Both of the species in question belong to genera that have also recently been shown to exhibit temperature-dependent sex determination (TSD). Here we test whether pregnant montane water skinks (Eulamprus tympanum) utilise TSD to select offspring sex in response to population wide imbalances in the OSR, by means of active thermoregulation. We use a combination of laboratory and field-based experiments, and conduct the first field-based test of this hypothesis by maintaining females in outdoor enclosures of varying OSR treatments throughout pregnancy. Although maternal body temperature during pregnancy was influenced by OSR, the variation in temperature was not great enough to affect litter sex ratios or any other phenotypic traits of the offspring.

Sex-ratio bias across populations of a freshwater turtle (Testudines : Chelidae) with genotypic sex determination

Adult sex ratios vary considerably among populations of single species and across years, but the best evidence is drawn from species with temperature-dependent sex determination. It is difficult to disentangle the effects of bias in the production of the sexes and the effects of a range of other factors contributing to biased adult sex ratios. In this paper, we survey sex ratios across populations of a species constrained to produce 1 : 1 offspring sex ratios by genotypic sex determination and show considerable variation in adult sex ratios. Raw adult sex ratios of Emydura macquarii emmottii were significantly biased in nine of the 11 populations examined. In all but one case, the bias was strongly in favour of males. Part of the bias in sex ratio was attributed to the differing ages of maturity of males and females – males mature younger than females – which leads to more male cohorts being included in the calculations of sex ratio than female cohorts. However, correcting for this effect brought the sex ratios of the populations closer to parity, as expected, and accounted for an overall 62% of the male surplus evident in the adult sex ratio. Even so, it was insufficient to explain the strong male bias (1.2–2.9) in five of the nine populations initially showing such bias. This provides support to those who advise caution in interpreting adult sex ratio data for freshwater turtles in the context of demography, sex allocation or evaluating the impact of climate change.

Female common lizards (Lacerta vivipara) do not adjust their sex-biased investment in relation to the adult sex ratio

Sex allocation theory predicts that facultative maternal investment in the rare sex should be favoured by natural selection when breeders experience predictable variation in adult sex ratios (ASRs). We found significant spatial and predictable interannual changes in local ASRs within a natural population of the common lizard where the mean ASR is female-biased, thus validating the key assumptions of adaptive sex ratio models. We tested for facultative maternal investment in the rare sex during and after an experimental perturbation of the ASR by creating populations with female-biased or male-biased ASR. Mothers did not adjust their clutch sex ratio during or after the ASR perturbation, but produced sons with a higher body condition in male-biased populations. However, this differential sex allocation did not result in growth or survival differences in offspring. Our results thus contradict the predictions of adaptive models and challenge the idea that facultative investment in the rare sex might be a mechanism regulating the population sex ratio.

Embryonic gonadal and sexual organ development in a small viviparous skink,Niveoscincus ocellatus

The majority of research into the timing of gonad differentiation (and sex determination) in reptiles has focused on oviparous species. This is largely because: (1) most reptiles are oviparous; (2) it is easier to manipulate embryonic developmental conditions (e.g., temperature) of eggs than oviductal embryos and (3) modes of sex determination in oviparous taxa were thought to be more diverse since viviparity and environmental sex determination (ESD)/temperature-dependent sex determination (TSD) were considered incompatible. However, recent evidence suggests the two may well be compatible biological attributes, opening potential new lines of enquiry into the evolution and maintenance of sex determination. Unfortunately, the baseline information on embryonic development in viviparous species is lacking and information on gonad differentiation and sexual organ development is almost non-existent. Here we present an embryonic morphological development table (10 stages), the sequence of gonad differentiation and sexual organ development for the viviparous spotted snow skink (Niveoscincus ocellatus). Gonad differentiation in this species is similar to other reptilian species. Initially, the gonads are indifferent and both male and female accessory ducts are present. During stage 2, in the middle third of development, differentiation begins as the inner medulla regresses and the cortex thickens signaling ovary development, while the opposite occurs in testis formation. At this point, the Müllerian (female reproductive) duct regresses in males until it is lost (stage 6), while females retain both ducts until after birth. In the later stages of testis development, interstitial tissue forms in the medulla corresponding to maximum development of the hemipenes in males and the corresponding regression in the females.

Pivotal range and thermosensitive period of the pig-nosed turtle, Carettochelys insculpta (Testudines: Carettochelydidae), from northern Australia

Understanding temperature-dependent sex determination in nature often depends on knowledge of species-specific attributes that are integrated into the relationship between temperature and sex. We determined two such attributes for the pig-nosed turtle, Carettochelys insculpta Ramsay, 1886, in tropical Australia: the pivotal range in temperature that separates the male-producing domain from the female-producing domain, and the thermosensitive period during which the embryonic sex is influenced by temperature. The pivotal range for C. insculpta was very narrow, spanning only about 1 °C, and was centered on 32 °C, which is high but consistent with temperatures reported for other tropical species. The thermosensitive period spanned developmental stages 17–21 for temperature influence in the direction of maleness and 18–21 for temperature influence in the direction of femaleness. This period is slightly narrower than that for other reptile species but broadly consistent with the middle third of incubation.

Sex choice in plants: facultative adjustment of the sex ratio in the perennial herb Begonia gracilis

Sex allocation theory predicts that reproducing individuals will increase their fitness by facultatively adjusting their relative investment towards the rarer sex in response to population shifts in operational sex ratio (OSR). The evolution of facultative manipulation of sex ratio depends on the ability of the parents to track the conditions favouring skewed sex allocation and on the mechanism controlling sex allocation. In animals, which have well-developed sensorial mechanisms, facultative adjustment of sex ratios has been demonstrated on many occasions. In this paper, we show that plants have mechanisms that allow them to evaluate the population OSR. We simulated three different conditions of population OSR by manipulating the amount of pollen received by the female flowers of a monoecious herb, and examined the effect of this treatment on the allocation to male vs. female flowers. A shortage of pollen on the stigmas resulted in a more male-skewed sex allocation, whereas plants that experienced a relatively pollen rich environment tended to produce a more female-skewed sex allocation pattern. Our results for Begonia gracilis demonstrate that the individuals of this species are able to respond to the levels of pollination intensity experienced by their female flowers and adjust their patterns of sex allocation in accordance to the expectations of sex allocation theory.

Recruitment sex ratios in gray-tailed voles (Microtus canicaudus) in response to density, sex ratio, and season

We tested predictions associated with three widely used hypotheses for facultative sex-ratio adjustment of vertebrates using eight enclosed populations of gray-tailed voles, Microtus canicaudus. These were (i) the population sex ratio hypothesis, which predicts that recruitment sex ratios should oppose adult sex-ratio skews, (ii) the local resource competition hypothesis, which predicts female-biased recruitment at low adult population density and male-biased recruitment at high population density, and (iii) the first cohort advantage hypothesis, which predicts that recruitment sex ratios should be female biased in the spring and male biased in the autumn. We monitored naturally increasing population densities with approximately equal adult sex ratios through the spring and summer and manipulated adult sex ratios in the autumn and measured subsequent sex ratios of recruits. We did not observe any significant sex-ratio adjustment in response to adult sex ratio or high population density; we did detect an influence of time within the breeding season, with more female offspring observed in the spring and more male offspring observed in the autumn. Significant seasonal increases in recruitment sex ratios indicate the capacity of female gray-tailed voles to manipulate their offspring sex ratios and suggest seasonal variation in the relative reproductive value of male and female offspring to be a regular phenomenon.

Viviparity as a constraint on sex-ratio evolution

In polytocous mammals, the sex ratio during gestation can influence a variety of morphological, physiological, and life-history traits because of steroid leakage between fetuses. Similar phenomena have also recently been described for a viviparous lizard. Some of these effects have important fitness consequences by influencing reproductive success later in life. Thus, biasing the sex ratio toward one sex may lead to a decreased fitness for the other sex, and therefore constrain the evolution of skewed sex ratios. By incorporating effects of sex ratio on offspring fitness in a simple sex-allocation model, I show that, under some circumstances (1) skewed sex ratios are predicted to evolve, and (2) this cost can constrain the evolution of skewed sex ratios.

Pivotal temperature for loggerhead turtles (Caretta caretta) from Kyparissia Bay, Greece

Pivotal temperature (the constant temperature giving 50% of each sex) for two clutches of loggerhead sea turtles (Caretta caretta) from Kyparissia Bay, Greece, was 29.3°C. Pivotal incubation duration (the time from laying to hatching giving 50% of each sex) was 52.6 days. These values are close to those obtained for this species in Brazil and the United States, providing further evidence that these characteristics are relatively conservative in different populations. Methodological differences between different experiments and limitations on accuracy of equipment make the detection of small differences problematic. Comparison of incubation durations in the field with the pivotal durations obtained here suggest that hatchling sex ratio on some Mediterranean beaches is female biased but probably varies considerably within this region.

Constraints in the evolution of sex ratio adjustment

When the relative fitness of male and female offspring varies with environmental conditions, evolutionary theory predicts that parents should adjust the sex of their offspring accordingly. Qualitative and even quantitative support for this prediction is striking in some taxa but much less convincing in others. Explaining such variation across taxa in the fit of sex ratio theory remains a major challenge. We use meta-analysis to test the role of two constraints in the evolution of sex ratios. Based on analysis of sex ratio skews in birds and wasps, we show that (i) mechanisms of sex determination do not necessarily constrain the evolution of sex ratio adjustment, and (ii) parental ability to predict their offsprings' environment influences the evolution of sex ratio patterns across taxa. More generally, our results show that multiple constraints may determine the precision of adaptation.

Sex ratios

Sex ratio theory attempts to explain variation at all levels (species, population, individual, brood) in the proportion of offspring that are male (the sex ratio). In many cases this work has been extremely successful, providing qualitative and even quantitative explanations of sex ratio variation. However, this is not always the situation, and one of the greatest remaining problems is explaining broad taxonomic patterns. Specifically, why do different organisms show so much variation in the amount and precision with which they adjust their offspring sex ratios?

Sex determination. Viviparous lizard selects sex of embryos

No one suspected that temperature-dependent sex determination (TSD), whereby the sex of embryos depends on the temperature at which they develop, might occur in viviparous (live-bearing) reptiles, because thermoregulation in the mother results in relatively stable, raised gestation temperatures. But here we show that developing embryos of the actively thermoregulating viviparous skink Eulamprus tympanum are subject to TSD, offering the mother the chance to select the sex of her offspring and a mechanism to help to balance sex ratios in wild populations.