Is there a relationship between sex of cystic fibrosis carriers and sex ratio of their offspring?

Summary Data on the offspring of 198 aunts and 179 uncles of 100 cystic fibrosis index cases were analyzed. Aunts showed higher average number of liveborn sons than uncles. No significant difference was observed in the number of liveborn female offspring. When the sample was subdivided with respect to family size, the proportion of liveborn sons of aunts appeared higher than that of uncles in all classes.The present observations suggest that a female carrier may have a higher probability of male offspring than a male carrier and that she may be mainly responsible for the sex ratio deviations previously reported in sibships of cystic fibrosis patients.     

Litter sex ratios in Richardson’s ground squirrels: long-term data support random sex allocation and homeostasis

When costs of producing male versus female offspring differ, parents may vary allocation of resources between sons and daughters. We tested leading sex-allocation theories using an information-theoretic approach and Bayesian hierarchical models to analyse litter sex ratios (proportion males) at weaning for 1,049 litters over 24 years from a population of Richardson’s ground squirrels (Urocitellus richardsonii), a polygynandrous, annually reproducing mammal in which litter size averages from six to seven offspring and sons are significantly heavier than daughters at birth and weaning. The model representing random Mendelian sex-chromosome assortment fit the data best; a homeostatic model received similar support but other models performed poorly. Embryo resorption was rare, and 5 years of litter data in a second population revealed no differences in litter size or litter sex ratio between birth and weaning, suggesting that litter size and sex ratio are determined in early pregnancy. Sex ratio did not vary with litter size at weaning in any of 29 years, and the observed distribution of sex ratios did not differ significantly from the binomial distribution for any litter size. For 1,580 weaned litters in the two populations, average sex ratio deviated from parity in only 3 of 29 years. Heavier females made a greater reproductive investment than lighter females, weaning larger and heavier litters composed of smaller sons and daughters, but litter sex ratio was positively related to maternal mass in only 2 of 29 years. Such occasional significant patterns emphasize the importance of multi-season studies in distinguishing infrequent events from normal patterns.     

Climate change and temperature-dependent sex determination: can individual plasticity in nesting phenology prevent extreme sex ratios?

Under temperature-dependent sex determination (TSD), temperatures experienced by embryos during development determine the sex of the offspring. Consequently, populations of organisms with TSD have the potential to be strongly impacted by climatic warming that could bias offspring sex ratio, a fundamental demographic parameter involved in population dynamics. Moreover, many taxa with TSD are imperiled, so research on this phenomenon, particularly long-term field study, has assumed great urgency. Recently, turtles with TSD have joined the diverse list of taxa that have demonstrated population-level changes in breeding phenology in response to recent climate change. This raises the possibility that any adverse impacts of climate change on populations may be alleviated by individual plasticity in nesting phenology. Here, we examine data from a long-term study on a population of painted turtles (Chrysemys picta) to determine whether changes in phenology are due to individual plasticity and whether individual plasticity in the timing of nesting has the capacity to offset the sex ratio effects of a rise in climatic temperature. We find that individual females show plasticity in the date of first nesting each year, and that this plasticity depends on the climate from the previous winter. First nesting date is not repeatable within individuals, suggesting that it would not respond to selection. Sex ratios of hatchlings within a nest declined nonsignificantly over the nesting season. However, small increases in summer temperature had a much stronger effect on nest sex ratios than did laying nests earlier in the season. For this and other reasons, it seems unlikely that individual plasticity in the timing of nesting will offset the effects of climate change on sex ratios in this population, and we hypothesize that this conclusion applies to other populations with TSD.     

Avian sex determination: what, when and where?

Sex is determined genetically in all birds, but the underlying mechanism remains unknown. All species have a ZZ/ZW sex chromosome system characterised by female (ZW) heterogamety, but the chromosomes themselves can be heteromorphic (in most birds) or homomorphic (in the flightless ratites). Sex in birds might be determined by the dosage of a Z-linked gene (two in males, one in females) or by a dominant ovary-determining gene carried on the W sex chromosome, or both. Sex chromosome aneuploidy has not been conclusively documented in birds to differentiate between these possibilities. By definition, the sex chromosomes of birds must carry one or more sex-determining genes. In this review of avian sex determination, we ask what, when and where? What is the nature of the avian sex determinant? When should it be expressed in the developing embryo, and where is it expressed? The last two questions arise due to evidence suggesting that sex-determining genes in birds might be operating prior to overt sexual differentiation of the gonads into testes or ovaries, and in tissues other than the urogenital system.     

Toxoplasma gondii and sex: essential or optional extra?

The evolutionary and biological significance of a female-biased sex ratio within apicomplexan parasites has been the subject of much discussion. It is proposed that the sex allocation theory, as applied to inbreeding populations, can explain the sex ratios observed for this diverse group of parasites. This is based on a mathematical model, which assumes that the majority of microgametes will succeed in fertilizing macrogametes. Is this a realistic assumption? It is possible, for different reasons, that the theory may not to be applicable to either malaria parasites or Toxoplasma gondii.     

Avoidance and prohibition of brother–sister sex in humans

Sex between full sibs is unusual in birds, mammals, and humans. These species likely possess an innate avoidance mechanism based on early proximity (i.e., the Westermarck hypothesis), and the rare occurrences may be attributable to error. Alternatively, an inclusive fitness argument shows that a low rate of sib mating may be an adaptation. The widespread occurrence of a prohibition against brother–sister sex in human societies is often invoked as evidence against an innate avoidance mechanism, since if the latter were to exist the former would be superfluous. However, given that punishing violators is costly, a prohibition is more likely to spread through an egalitarian society when the prohibited behavior is already avoided. I describe a model of the cultural dynamics of the sibling incest taboo which we have used to investigate this possibility. The predictions derived from this model are consistent with, and add rigor to, Westermarcks theory of the origin of the incest taboo.     

Advancing sex and gender competency in medicine: sex & gender women’s health collaborative

Research conducted to date has deepened our understanding of sex and gender differences in the etiology, diagnosis, treatment, and outcomes for many conditions that affect both women and men. The Sex and Gender Women’s Health Collaborative (SGWHC) is supported by the coordinated efforts of our founding partners: the American Medical Women’s Association, the American College of Women’s Health Physicians and Society for Women’s Health Research to address the gaps in medical education with regard to sex and gender competency in the care of women. The SGWHC initiated and continues to build a novel digital resource library of sex and gender specific materials to be adopted and adapted into medical education and clinical practice, residing @ http://www.sgwhc.org. This article presents a case for the inclusion of sex and gender focused content into medical curricula and describes a means for students, faculty, and practitioners to access a centralized, interactive repository for these resources.     

Pipefishes and seahorses: Are they all sex role reversed?

The male pregnancy of pipefishes and seahorses has led to the inference that females compete most intensely for access to mates, because males limit female reproduction. However, recent work has shown that in different species either sex may be the predominant competitor for mates. In this family, there is an apparent association between the mating pattern and the sex roles: polygamous species show reversed sex roles whereas monogamous species exhibit 'conventional' sex roles. These studies emphasize that sex role reversal is not synonymous with male parental care.     

Why is mutual mate choice not the norm? Operational sex ratios,sex roles and the evolution of sexually dimorphic and monomorphic signalling.

Biases in the operational sex ratio (OSR) are seen as the fundamental reason behind differential competition for mates in the two sexes, and as a strong determinant behind differences in choosiness. This view has been challenged by Kokko and Monaghan, who argue that sex-specific parental investment, mortalities, mate-encounter rates and quality variation determine the mating system in a way that is not reducible to the OSR. We develop a game-theoretic model of choosiness, signalling and parental care, to examine (i) whether the results of Kokko and Monaghan remain robust when its simplifying assumptions are relaxed, (ii) how parental care coevolves with mating strategies and the OSR and (iii) why mutual mate choice is observed relatively rarely even when both sexes vary in quality. We find qualitative agreement with the simpler approach: parental investment is the primary determinant of sex roles instead of the OSR, and factors promoting choosiness are high species-specific mate-encounter rate, high sex-specific mate-encounter rate, high cost of breeding (parental investment), low cost of mate searching and highly variable quality of the opposite sex. The coevolution of parental care and mating strategies hinders mutual mate choice if one parent can compensate for reduced care by the other, but promotes it if offspring survival depends greatly on biparental care. We argue that the relative rarity of mutual mate choice is not due to biases in the OSR. Instead, we describe processes by which sexual strategies tend to diverge. This divergence is prevented, and mutual mate choice maintained, if synergistic benefits of biparental care render parental investment both high and not too different in the two sexes.     

Are secondary sex traits, parasites and immunity related to variation in primary sex traits in the Arctic charr?

Primary and secondary sex traits are influenced by the same sex hormones, and the expression of secondary sex traits may consequently signal males' capacity for sperm production. Sperm quality may also be influenced by immune activity, as sperm are non-self to the male. Parasite infections alter immune activity and may thus reduce ejaculate quality. In the Arctic charr (Salvelinus alpinus) the red abdominal colour is considered an ornament that signals important information in mate choice. We captured and individually caged sexually mature male Arctic charr during the spawning period. Afterwards we estimated abdominal colour, parasite infections, gonad mass and several spermatological and immunological variables. Intensity of abdominal colour was positively correlated to testes mass, milt mass and sperm cell numbers produced. Additionally, males with low parasite intensities had high testes mass and produced milt with high sperm density, indicating a trade-off between parasite resistance and development of primary sex traits. Our measures of immunity were, however, not related to primary sex traits. We conclude that females evaluating male abdominal coloration may obtain information about differences between males in fertilization potential and parasite resistance.     

Parasitism and survival in a damselfly: does host sex matter?

We present experimental data on the survivorship of damselflies infested by parasitic water mites from a population in field cages. In addition, we show correlative laboratory data under simulated severe weather conditions. In the manipulative experiment, parasitized females' individual condition, which was measured as weight at emergence, was an important determinant of survival under field conditions. In contrast, such a relationship did not occur in males and unparasitized females. It was found in the laboratory experiment that water mites as well as weight at emergence both contributed significantly to the reduced survivorship of male and female damselflies. It was concluded that the impact of parasitism depends on environmental conditions and that host sexes differ in their responses to parasitism. This is discussed in the light of immunocompetence in invertebrates.     

Yolk androgens and embryo sex: maternal effects or confounding factors?

Maternal effects occur when offspring phenotype is affected by environmental factors experienced by the mother and, in egg-laying species, are often mediated via egg resources. There is currently great interest among behavioural ecologists in maternally allocated yolk androgens, especially their relationship with offspring sex and development. Such studies need embryonic tissue for sexing, however, requiring eggs to be incubated (usually for 3 days). Therefore, there are concerns about whether the androgen concentrations assayed reflect those allocated by the mother. In addition, studies showing sex biases in maternal allocation of androgens could be confounded if male and female embryos uptake or metabolise androgens at different rates. We ran a series of experiments using zebra finch (Taeniopygia guttata) eggs to address these potential confounding factors. First we showed, using eggs naturally incubated for up to 5 days, that eggs containing embryos had lower yolk androgen concentrations than eggs that had failed to form embryos. We then tested various hypotheses for this difference using controlled incubation treatments. Our results suggested that (a) embryo development causes the yolk to become progressively more diluted with albumin; and (b) between 3 and 5 days of incubation embryos start uptaking or metabolising androgens. Crucially, we found no decline in yolk androgen concentration at 3 days incubation, and no evidence for sex-specific rates of uptake or metabolism of androgens. This strongly suggests that yolk androgen levels up to 3 days incubation do reflect those allocated by the mother, and that studies of sex biased maternal allocation of yolk androgens are not confounded by sex differences in embryo development.     

Is sex necessary?

Fungal sexual reproductive modes have markedly high diversity and plasticity, and asexual species have been hypothesized to arise frequently from sexual fungal species. A recent study on the red yeasts provides further support for the notion that sexual ancestors may give rise to shorter-lived asexual species. However, presumed asexual species may also be cryptically sexual, as revealed by other recent studies.See research article: http://www.biomedcentral.com/1471-2148/11/249     

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.