Female Control of Offspring Sex Ratios Based on Male Attractiveness in the Guppy

The sex allocation hypothesis predicts that females manipulate the offspring sex ratios according to mate attractiveness. Although there is increasing evidence to support this prediction, it is possible that paternal effects may often obscure the relationship between female control of offspring sex ratios and male attractiveness. In the present study, we examined whether females played a primary role in the manipulation their offspring sex ratios based on male attractiveness, in the guppy Poecilia reticulata, a live‐bearing fish. We excluded the paternal effects by controlling the relative sexual attractiveness of the male by presenting them to the females along with a more attractive or less attractive stimulus male. The test male was perceived to be relatively more attractive by females when it was presented along with a less attractive stimulus male, or vice versa. Subsequently, test male was mated in two different roles (relatively more and less attractive) with two females. If females were responsible for offspring sex ratio manipulation, the sex ratio of the brood would be altered on the basis of the relative attractiveness of the test male. On the other hand, if males play a primary role in offspring sex ratio manipulation, the sex ratios would not differ with the relative attractiveness of the test male. We found that females gave birth to more male‐biased broods when they mated with test males in the attractive role than when they mated with males in the less attractive role. This finding suggests that females are responsible for the manipulation of offspring sex ratios based on the attractiveness of their mates.     

Adaptive Offspring Sex Ratio Depends on Male Tail Length in the Guppy

A biased sex ratio in a brood is considered to be an adaptive strategy under certain circumstances. For example, if the expected reproductive success of one sex is greater than that of the other, parents should produce more offspring of the former sex than the latter. A previous study has documented that in the guppy, Poecilia reticulata, the female offspring of males possessing proportionally longer tails exhibit smaller body sizes and show decreased reproductive outputs than those of males having shorter tails. On the other hand, the total lengths of the male offspring of the long‐tailed males are larger because of their longer tails; consequently, they exhibit greater sexual attractiveness to females. Therefore, it has been hypothesized that this asymmetry in the expected reproductive success between the male and female offspring of long‐tailed males may result in a biased sex ratio that is dependent on the tail lengths of their fathers. This hypothesis was tested in the present study. The results showed that the females that mated with long‐tailed males produced more male offspring than those that mated with short‐tailed males. Logistic regression analysis showed that the ratio of tail length to the standard length of the fathers is a determinant factor of the sex of their offspring. These results suggest that the manipulation of the offspring sex ratios by parents enhances the overall fitness of the offspring.     

A Trivers-Willard Effect in Contemporary Humans: Male-Biased Sex Ratios among Billionaires

Background

Natural selection should favour the ability of mothers to adjust the sex ratio of offspring in relation to the offspring''s potential reproductive success. In polygynous species, mothers in good condition would be advantaged by giving birth to more sons. While studies on mammals in general provide support for the hypothesis, studies on humans provide particularly inconsistent results, possibly because the assumptions of the model do not apply.

Methodology/Principal Findings

Here, we take a subset of humans in very good condition: the Forbe''s billionaire list. First, we test if the assumptions of the model apply, and show that mothers leave more grandchildren through their sons than through their daughters. We then show that billionaires have 60% sons, which is significantly different from the general population, consistent with our hypothesis. However, women who themselves are billionaires have fewer sons than women having children with billionaires, suggesting that maternal testosterone does not explain the observed variation. Furthermore, paternal masculinity as indexed by achievement, could not explain the variation, since there was no variation in sex ratio between self-made or inherited billionaires.

Conclusions/Significance

Humans in the highest economic bracket leave more grandchildren through sons than through daughters. Therefore, adaptive variation in sex ratios is expected, and human mothers in the highest economic bracket do give birth to more sons, suggesting similar sex ratio manipulation as seen in other mammals.     

Should Attractive Males Sneak: The Trade-Off between Current and Future Offspring

Alternative reproductive tactics are predicted to be adopted by less competitive males when competition for fertilization is intense. Yet, in some species, competitively superior males use an alternative tactic alongside the conventional tactic. This can jeopardize their success through the conventional tactic, but surprisingly little attention has been paid to this cost. We investigated 1) the degree to which competitive males sneak fertilize eggs in the polygamous threespine stickleback, Gasterosteus aculeatus, and 2) if males balance the cost of sneaking against its benefit. We found competitive males that succeeded in establishing a territory and in attracting spawning females to perform most sneak fertilizations. However, when we reduced the benefit of sneak attempts, by reducing visibility and the success rate of sneak attempts, males sneaked less. When we increased the cost of sneak attempts, by increasing the perceived value of current offspring (by mating males to preferred females rather than unpreferred females or no females), the interest of males in sneak opportunities decreased. Intriguingly, larger males, who presumably had a higher probability of future reproduction, were more willing to risk their current offspring for sneak opportunities. These findings suggest that competitive males that are attractive to females carefully balance costs against benefits in their sneaking decisions. More broadly, our results imply that changes in the environment can influence the cost-benefit ratio of sneaking and alter the distribution of fertilizations in a population. We end with discussing the implications that alterations in sneaking behavior could have for the operation of sexual selection in changing environments.     

Resource Elasticity of Offspring Survival and the Optimal Evolution of Sex Ratios

The fitness of any organisms includes the survival and reproductive rate of adults and the survival of their offspring. Environmental selection pressures might not affect these two aspects of an organism equally. Assuming that an organism first allocates its limited resources to maintain its survival under environmental selection pressure, our model, based on the evolutionarily stable strategy theory, surprisingly shows that the sex ratio is greatly affected by the environmental pressure intensity and by the reproductive resource elasticity of offspring survival. Moreover, the concept of the resource elasticity of offspring survival intrinsically integrates the ecological concepts of K selection and r selection. The model shows that in a species with reproductive strategy K, increased environmental selection pressure will reduce resource allocation to the male function. By contrast, in a species with reproductive strategy r, harsher environmental selection pressure will increase allocation to the male function. The elasticity of offspring survival might vary not only across species, but also across many other factors affecting the same species (e.g., age structure, spatial heterogeneity), which explains sex ratio differences across species or age structures and spatial heterogeneity in the same species.     

Sex Determination and Sex Ratios in Crocodylus palustris

Incubation temperature determines sex in the mugger crocodile,Crocodylus palustris. Exclusively females are produced at constanttemperatures of 28.0°C through 31°C. At 32.5°C,only males are produced. Both sexes are produced in varyingproportions at 31.5, 32.0, and 33.0°C. Embryo survival isnot affected within this range, but developmental rate and totalincubation time are strongly temperature dependent. In naturalnests laid in breeding enclosures, cool incubation temperaturesproduced only females whereas males were produced only in warmnests. Clutch sex ratios were female or male biased. Yearlysex ratios (=percent male) varied from 0.05 to 0.58; overallsex ratio during six nesting seasons was 0.24 (1 male: 3 females).Sex ratio and incubation time vary with nest location and temperaturein a manner consistent with the constant temperature results.Incubation time decreases with increasing incubation temperature,and is an accurate predictor of sex ratio in the field and laboratory. To date, temperature-dependent sex determination (TSD) has beenreported in five species of Crocodylus and in three speciesof Alligatorinae; but the TSD patterns in these groups differ.The TSD pattern of C. palustris is similar to that of C. porosus.Nesting in C. palustris is synchronized with the seasonal availabilityof thermal regimes suitable for incubation. Resultant sex ratiosare a consequence of when and where eggs are laid. Early nestsare located in warm, sunny sites; in contrast, late season nestsare located in the shade. An egg transplant experiment demonstratedthat sex ratios could be altered by simple manipulations ofnest temperatures in the field. The adaptive significance ofTSD in crocodilians may relate to the influence of incubationtemperature on various hatchling attributes, particularly growth.     

Changing Sex Ratios in Diabetes

A survey of sexes and ages in patients attending the Hartlepool diabetic clinic was carried out for the 20-year period 1948-67. The proportion of men aged 50 and over rose from 25·9% in the first decade to 38·2% in the second. An attempt has been made to explain this pronounced and rapid change.     

Sex Differences in Discrimination of Shoal Size in the Guppy (Poecilia reticulata)

For a diversity of species, differences in sexual and parental roles, along with differences in body morphology, often result in males and females having different diets, distinct predators and even different patterns of habitat use. As a consequence, the two sexes often face different environmental challenges and selection may favour the evolution of sex differences in cognition. We tested this prediction in the guppy (Poecilia reticulata). Under perceived hazard, individual guppies join the larger available social group, a behaviour that is thought to minimise predation risk. In this species, females are more frequently exposed to predation and more averse to predation risk; we therefore expected greater accuracy in shoal size discrimination in females. We compared the accuracy of male and female guppies in discriminating shoals of 4 and 6 conspecifics, which represents the upper limit of discrimination for this species. Overall, we found no sex differences in the accuracy of discriminating the two shoals. However, while females showed this ability at the beginning of the test, males began to select the larger group only after several minutes. In three control experiments, we found indications that this sex difference cannot be accounted for by differences in motivation or antipredator strategies between the two sexes, suggesting female guppies are more efficient at rapidly estimating shoal size.     

ART中影响新生儿性别比的相关因素分析

辅助生殖技术(assisted reproductive technology,ART)在给广大不孕不育患者带来福音的同时,也带来了男女出生性别比的失衡,相关影响因素有待研究分析。通过收集并分析2013年1月至2014年12月于广州医科大学附属第三医院接受ART治疗并成功分娩4 635个新生儿的3 462个周期患者的临床资料发现,ICSI组与IVF组比较,活产男婴比例没有统计学差异(P=0.07),但PESA/TESA-ICSI组出生男婴比例较IVF组显著降低(P=0.036);同时,移植囊胚期胚胎出生的男婴比例要显著高于移植卵裂期胚胎的男婴比例(P=0.005)。进一步的分层卡方检验结果显示,除去授精方式的影响,移植胚胎期别仍然可以影响出生婴儿的性别比(P=0.021)。但多因素logistic回归分析提示,以上因素均不能构成独立预测因素。上述结果表明在体外受精-胚胎移植周期中,移植囊胚期胚胎可显著增加男婴出生比例,而PESA/TESA-ICSI授精方式则会显著降低男婴出生比例。     

Fetal Sex Ratios in Southwestern Montana Elk

ABSTRACT The Trivers-Willard (1973) model suggests maternal control of offspring sex, in utero or by the end of parental investment, may be an adaptive advantage in some species. We tested for differential sex allocation using 11,408 known-sex fetal elk (Cervus elaphus) from biological collections and hunter harvest returns from 2 southwestern Montana, USA, elk populations (1961–2007). We included maternal and environmental condition covariates measured pre- and postconception and throughout pregnancy. Results suggested that adult female elk in southwest Montana did not differentially invest in male offspring when conditions were beneficial. We found evidence that, when the Northern Yellowstone elk herd was at low density, beneficial spring (May-Jun) growing conditions, as indexed by a local precipitation measure and a regional drought indicator, correlated with production of more female fetuses (1 SD increase in precipitation and 1 SD decrease in drought resulted in 6% and 5% more F fetuses, respectively). In the same herd, we found evidence that improved maternal condition, as indexed by kidney fat mass and heart fat mass, also correlated with production of more female fetuses (1 SD increase in kidney fat mass and heart fat mass resulted in 8% more F fetuses). When the same elk herd reached higher densities under different ecological conditions, no covariate was associated with a deviation in the 50:50 female-to-male sex ratio. Similarly, there was no association between covariates and fetal sex ratios in a nearby elk herd at high population density. In modeling, wildlife managers should consider factors that could alter sex ratios at birth, and also how biased sex ratios postpartum could affect population models.     

Sex Chromosome Meiotic Drive in DROSOPHILA MELANOGASTER Males

In Drosophila melanogaster males, deficiency for X heterochromatin causes high X-Y nondisjunction and skewed sex chromosome segregation ratios (meiotic drive). Y and XY classes are recovered poorly because of sperm dysfunction. In this study it was found that X heterochromatic deficiencies disrupt recovery not only of the Y chromosome but also of the X and autosomes, that both heterochromatic and euchromatic regions of chromosomes are affected and that the "sensitivity" of a chromosome to meiotic drive is a function of its length. Two models to explain these results are considered. One is a competitive model that proposes that all chromosomes must compete for a scarce chromosome-binding material in Xh(-) males. The failure to observe competitive interactions among chromosome recovery probabilities rules out this model. The second is a pairing model which holds that normal spermiogenesis requires X-Y pairing at special heterochromatic pairing sites. Unsaturated pairing sites become gametic lethals. This model fails to account for autosomal sensitivity to meiotic drive. It is also contradicted by evidence that saturation of Y-pairing sites fails to suppress meiotic drive in Xh(- ) males and that extra X-pairing sites in an otherwise normal male do not induce drive. It is argued that meiotic drive results from separation of X euchromatin from X heterochromatin.     

Costs of Rearing the Wrong Sex: Cross-Fostering to Manipulate Offspring Sex in Tammar Wallabies

Sex allocation theory assumes that offspring sex (son vs. daughter) has consequences for maternal fitness. The most compelling experiment to test this theory would involve manipulating offspring sex and measuring the fitness consequences of having the “wrong” sex. Unfortunately, the logistical challenges of such an experiment limit its application. In tammar wallabies (Macropus eugenii), previous evidence suggests that mothers in good body condition are more likely to produce sons compared to mothers in poor condition, in support of the Trivers-Willard Hypothesis (TW) of condition-dependent sex allocation. More recently, we have found in our population of tammar wallabies that females with seemingly poor access to resources (based on condition loss over the dry summer) are more likely to produce sons, consistent with predictions from the Local Resource Competition (LRC) hypothesis, which proposes that production of sons or daughters is driven by the level of potential competition between mothers and philopatric daughters. We conducted a cross-fostering experiment in free-ranging tammar wallabies to disassociate the effects of rearing and birthing offspring of each sex. This allowed us to test the prediction of the LRC hypothesis that rearing daughters reduces the future direct fitness of mothers post-weaning and the prediction of the TW hypothesis that rearing sons requires more energy during lactation. Overall, we found limited costs to the mother of rearing the “wrong” sex, with switching of offspring sex only reducing the likelihood of a mother having a pouch young the following year. Thus, we found some support for both hypotheses in that rearing an unexpected son or an unexpected daughter both lead to reduced future maternal fitness. The study suggests that there may be context-specific costs associated with rearing the “wrong” sex.     

Linkage Analysis Reveals the Independent Origin of Poeciliid Sex Chromosomes and a Case of Atypical Sex Inheritance in the Guppy (Poecilia reticulata)

Among different teleost fish species, diverse sex-determining mechanisms exist, including environmental and genetic sex determination, yet chromosomal sex determination with male heterogamety (XY) prevails. Different pairs of autosomes have evolved as sex chromosomes among species in the same genus without evidence for a master sex-determining locus being identical. Models for evolution of Y chromosomes predict that male-advantageous genes become linked to a sex-determining locus and suppressed recombination ensures their co-inheritance. In the guppy, Poecilia reticulata, a set of genes responsible for adult male ornaments are linked to the sex-determining locus on the incipient Y chromosome. We have identified >60 sex-linked molecular markers to generate a detailed map for the sex linkage group of the guppy and compared it with the syntenic autosome 12 of medaka. We mapped the sex-determining locus to the distal end of the sex chromosome. We report a sex-biased distribution of recombination events in female and male meiosis on sex chromosomes. In one mapping cross, we observed sex ratio and male phenotype deviations and propose an atypical mode of genetic sex inheritance as its basis.     

Biased Sex Ratios in Plants: Theory and Trends

This paper examines the history of sex ratio theory and the effects of multiple variables on individual and population sex ratios. It also provides examples where plants have been used to test major predictions of sex ratio theory. Then, using over 200 studies from the literature, dioecious plant species are categorized based on their life form, pollination agent, fruit dispersal agent, and sex ratio. A loglinear analysis is used to look at possible correlations between the sex ratio of a population and other life history characteristics. These data are used to examine the predictions made by De Jong et al. (Journal of Evolutionary Biology 15:7, 2002), that relative pollen and seed dispersal distances can be used to predict sex ratio bias. Despite the limited sample size, strong relationships are still observed. 93% of insect pollinated dioecious vines that have biotically dispersed fruit have male-biased sex ratios. Conversely, 61% of shrubs that are wind pollinated and have abiotic fruit dispersal have female-biased sex ratios.     

Systemic Non-Reproductive Effects of Sex Steroids in Adult Males and Females

Sex steroids are well known for their reproductive actions, however, their roles are not confined to reproduction only and they have been shown to exert wide ranging effects on systemic physiology. Further, the effects of the so-called male and female sex steroids are not limited to their respective genders but they are present in both sexes where they have a significant impact upon systemic functions, reproductive as well as non-reproductive. This work reviews the existing knowledge base and recent reports on the effects of sex steroids on non-reproductive physiology.