Facultative parthenogenesis and sex-ratio evolution

Summary Phenotypic models of selection are used to determine the effect of facultative parthenogenesis on the production of males in a spatially variable environment when (i) sex determination is under strict genetic control, and (ii) when sex may be environmentally determined. The results show that when sex is under strict genetic control and there is some chance of maturing in isolation, selection favors a female-biased sex ratio. When sex can be environmentally induced by cues which indicate high density, selection favors a mixture of genetic and environmental control, such that half the individuals always become female and the other half become females when isolated and become males when not isolated.     

Viviparity as a constraint on sex-ratio evolution

Abstract 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.     

Male-biased reproduction and sex-ratio adjustment in muskrats

Summary Sex ratios of a population and of litters were sampled in muskrats in Ontario, Canada. Sex ratios of litters sampled from nests were male biased (54% male). Until weaning, no differential costs of producing and rearing male and female young were identified that could account for this greater production of males. Following weaning, however, male-biased dispersal of juveniles from their natal site and more frequent acquisition by females of these sites as breeding sites the following year suggested a greater investment by adult females in female young. Therefore, competition between female siblings for the acquisition of their natal site may be sufficient to result in the greater production of males. In addition, the simultaneous occupation of, and competition between, siblings and parents for the resources of the natal home range may not be necessary for local resource competition to result in a greater production of the dispersing sex. Greater-than-expected binomial variance in sex ratios of litters suggested that adjustment of sex-ratios occurred. However, we were unable to associate the adjustment of litter sex ratios with changes in maternal condition. The greater production of males and the predominance of monogamous associations between adults in this population may have lead to slightly greater variation in male fitness than female fitness. Therefore, a female in better-than-average condition may have benefited by producing more males. Similarly, a lower cost of producing dispersing males may allow nutritionally-stressed females to reduce their total expenditure on offspring by producing more males. Because these experiments were non-manipulative, maternal condition may not have varied sufficiently during this study to detect adjustments of litter sex ratios resulting from either of the above mechanisms acting separately, but the combined effects of small differences in matermal condition and selective pressures operating in the same direction may have resulted in the observed deviation from the binomial.     

Age and sex-ratio expression in Drosophila mediopunctata

The sex-ratio trait - production of progenies with excess of females due to X-linked meiotic drive in parental males - has a variable expression in Drosophila mediopunctata. We tested the effect of male age and found that aging increases the expression of sex-ratio, a fact relevant for the interpretation of field data and for experimental design.     

Strategic sperm allocation under parasitic sex-ratio distortion

Parasitic sex-ratio distorters are a major selective force in the evolution of host mating behaviour and mate choice. Here, we investigate sperm limitation in the amphipod Gammarus duebeni and the impact of the microsporidian sex-ratio distorter Nosema granulosis on sperm allocation strategies. We show that males become sperm limited after three consecutive matings and provide uninfected, high fecundity, females with more sperm than infected females. We show that sperm limitation leads to a decrease in female productivity. The outcome of sex-ratio distortion has been shown theoretically to be sensitive to the mating limits of males. Our results indicate that strategic sperm allocation under male rarity will have a greater impact on infected females and has the potential to regulate spread of parasitic feminisers in host populations.     

A two-generation study of human sex-ratio variation.

We report here the first vertical population study of human sex-ratio variation. Sex-ratio data for 2 generations from Akita, Japan, have been analyzed. Parental age, birth order, sequences of the sexes at birth, and generations have no statistically significant effect on sex ratio. There is a slight excess of males at birth, as is typical for human sex-ratio studies. There is evidence of sex-ratio-dependent family planning. An analysis of vertical transmission of sex-ratio modifying factors that excludes effects of birth order in both the parental and offspring generations has detected a marginally significant paternal effect. Genetic variability of the sex ratio, if present at all, is of a very minor magnitude.     

Mechanisms governing sex-ratio variation in dioecious Rumex nivalis

Sex ratios of flowering individuals in dioecious plant populations are often close to unity, or are male biased owing to gender-specific differences in flowering or mortality. Female-biased sex ratios, although infrequent, are often reported in species with heteromorphic sex chromosomes. Two main hypotheses have been proposed to account for female bias: (1) selective fertilization resulting from differential pollen-tube growth of female- versus male-determining microgametophytes (certation); (2) differences in the performance and viability of the sexes after parental investment. Here we investigate these hypotheses in Rumex nivalis (Polygonaceae), a European alpine herb with female-biased sex ratios in which females possess XX, and males XY1Y2, sex chromosomes. Using field surveys and a glasshouse experiment we investigated the relation between sex ratios and life-history stage in 18 populations from contrasting elevations and snowbed microsites and used a male-specific SCAR-marker to determine the sex of nonflowering individuals. Female bias among flowering individuals was one of the highest reported for populations of a dioecious species (mean female frequency = 0.87), but males increased in frequency at higher elevations and in the center of snowbeds. Female bias was also evident in nonflowering individuals (mean 0.78) and in seeds from open-pollinated flowers (mean 0.59). The female bias in seeds was weakly associated with the frequency of male flowering individuals in populations in the direction predicted when certation occurs. Under glasshouse conditions, females outperformed males at several life-history stages, although male seeds were heavier than female seeds. Poor performance of Y1Y2 gametophytes and male sporophytes in R. nivalis may be a consequence of the accumulation of deleterious mutations on Y-sex chromosomes.     

Maternal heterogeneity in primary sex-ratio of three tetranychid mites

The influence of maternal age on primary sex ratio was determined forTetranychus urticae, T. pacificus andT. turkestani (Acari:Tetranychidae). All females were allowed to mate once.Tetranychus urticae had the highest production of daughters and the lowest production of sons. The sex ratio ofT. urticae was nearly 21 (daughterssons), whereas for the other species it was 0.61. Net sex ratios were all shifted in favor of daughters, suggesting that most son-production occurs late in life. Demographic techniques are applied to tetranychid populations, including the concepts of reproductive concentration curves and expectation of future reproduction based on age and previous offspring.     

Life-history analysis of the Trivers and Willard sex-ratio problem

Phenotypic quality, such as condition or size, often variesbetween individuals. For species with extensive maternal care,the quality of offspring may partially be determined by thequality of their mother. Trivers and Willard (1973) predictedthat high quality females should prefer offspring of the sexwhose reproductive success is most strongly influenced by maternalcare, which in many cases will be sons. Correspondingly, lowquality females should prefer daughters. However, this predictionis not based on a proper analysis of variation in reproductivevalue. Using state-dependent life-history theory, I show herethat high quality females should prefer offspring of the sexwhose reproductive value is most strongly influenced by maternalcare. I also show that when offspring quality is strongly determinedby their mother's quality, but not influenced by their father'squality, high quality females can have higher reproductive valuethan high quality males, even though their reproductive successmay be much lower. In such cases, high quality females shouldprefer daughters and, correspondingly, low quality females shouldprefer sons.[Behav Ecol 7: 316–325 (1996)]     

Successful sons or superior daughters: sex-ratio variation in springbok

Mothers would often benefit from producing more offspring of one sex than the other. Although some species show an astonishing ability to skew their sex ratio adaptively, the trends found in many studies on vertebrates have proved inconsistent. Furthermore, evidence for a mechanism by which such a bias is achieved is equivocal at best. Here, we examine sex-ratio variation over 30 years, both at an individual and a population level, in the highly polygynous, size-dimorphic springbok (Antidorcas marsupialis). Many previous studies of similar species have shown that mothers in superior condition preferentially produce sons, whereas those in poorer condition produce more daughters. We found the opposite to be true in springbok, perhaps because daughters provide mothers in superior condition with a more rapid and secure fitness return. This theory was supported by the findings that earlier-conceived offspring tended to be female and that an increased proportion of daughters were produced with increasing rainfall (which was likely to reduce nutritional stress). We also show that selective reabsorption of embryos is unlikely to be the main mechanism by which deviations from an equal sex ratio are achieved. Hence, either differential implantation occurs or females are able to influence the sex of the sperm fertilizing an egg.     

Adult sex-ratio in ostracods and its implications for sexual selection

ABSTRACT

The Ostracoda – ubiquitous aquatic micro-crustaceans – show an exceptionally high incidence of female-biased adult sex ratio. Intraspecific sex ratio is known to vary in extant species and yet in the fossil record a species’ adult sex ratio can be highly stable across time. Sex ratio conditions the intensity of sexual selection and influences which sex undergoes stronger selective pressure. However, the impact of variation in spatial and temporal intraspecific sex ratio on the evolution of sexual selection remains an open question, calling for further investigations on the factors controlling adaptive sex ratio. This mini-review aims to introduce the system, and explores some of the key literature addressing factors influencing intraspecific variation in adult sex ratio (ASR) and its implication in the intensity of sexual selection and evolution of mating systems.