Variation In Population Sex Ratio Research Articles

Do Genetic Drift and Gene Flow Affect the Geographic Distribution of Female Plants in Gynodioecious Lobelia siphilitica?

Variation in population sex ratio is particularly pronounced in gynodioecious angiosperms. Extremely high female frequencies in gynodioecious populations cannot be readily explained by selective forces alone. To assess the contributions of drift and gene flow to extreme sex-ratio variation, we documented sex ratio and population size in 92 populations of Lobelia siphilitica across its range and genotyped plants using plastid and nuclear genetic markers. Similarity in spatial patterns of genetic and demographic variables may suggest that drift and/or gene flow have contributed to population sex-ratio variation in L. siphilitica. We found strong spatial structuring of extremely high female frequencies: populations with >50% female plants are restricted to the south–central portion of the range. However, we did not detect any spatial structuring in population size nor metrics of genetic diversity, suggesting that extreme variation in female frequency is not strongly affected by drift or gene flow. Extreme sex-ratio variation is frequently observed in gynodioecious plants, but its causes are difficult to identify. Further investigation into mechanisms that create or maintain the spatial structure of sex ratios in gynodioecious species will provide much needed insight.

Beyond balancing selection: frequent mitochondrial recombination contributes to high-female frequencies in gynodioecious Lobelia siphilitica (Campanulaceae).

Gynodioecy is a sexual system in which females and hermaphrodites co-occur. In most gynodioecious angiosperms, sex is determined by an interaction between mitochondrial male-sterility genes (CMS) that arise via recombination and nuclear restorer alleles that evolve to suppress them. In theory, gynodioecy occurs when multiple CMS types are maintained at equilibrium frequencies by balancing selection. However, some gynodioecious populations contain very high frequencies of females. Highfemale frequencies are not expected under balancing selection, but could be explained by the repeated introduction of novel CMS types. To test for balancing selection and/or the repeated introduction of novel CMS, we characterised cytoplasmic haplotypes from 61 populations of Lobelia siphilitica that vary widely in female frequency. We confirmed that mitotype diversity and female frequency were positively correlated across populations, consistent with balancing selection. However, while low-female populations hosted mostly common mitotypes, high-female populations and female plants hosted mostly rare, recombinant mitotypes likely to carry novel CMS types. Our results suggest that balancing selection maintains established CMS types across this species, but extreme female frequencies result from frequent invasion by novel CMS types. We conclude that balancing selection alone cannot account for extreme population sex-ratio variation within a gynodioecious species.

Size‐based sexual dimorphism of nitrogen excretion in livebearing fishes

AbstractNitrogen (N) and phosphorus (P) are important limiting or co‐limiting nutrients in many aquatic ecosystems. Consumers such as fishes can significantly impact the balance and redistribution of these nutrients through consumer‐driven nutrient recycling. Intraspecific variation in nutrient excretion rates can therefore have significant ecosystem impacts. Among individuals of sexually dimorphic consumers, variation in population size structure and sex ratio could potentially have impacts of similar magnitude. We tested for the effects of body size and sex on consumer‐driven nutrient recycling by measuring N and P excretion rates from eight species of poeciliid fishes. We found a strong positive effect of size on N excretion rates, as has been previously described among species. However, we found no effect of size on P excretion rates, nor did we find any difference in N or P excretion rates between sexes. Our work provides a preliminary analysis of how sexual dimorphism can lead to disparate nutrient excretion rates within consumer populations. These results indicate that variation in population sex ratios of sexually dimorphic consumers could have impacts at the ecosystem scale.

Rapid population sex-ratio changes in the moss Ceratodon purpureus.

Sex-ratio variation occurs widely in dioecious plants, but the mechanisms of population sex-ratio bias are poorly understood. In bryophytes, sex ratios are often female biased, and little information is available about how and when bias forms. To test whether population sex-ratio variation can emerge during the gametophytic phase and is not purely a product of spore sex ratios, we created artificial populations of the moss Ceratodon purpureus, with male- and female-biased sex ratios, and placed half under a stress treatment. We hypothesized that male-majority populations would become female-biased and that stress would increase this transition. After 18 mo, when sporophytes were initially forming, we used sex-specific molecular markers to determine population sex ratios. Female-majority populations did not differ significantly from their original bias, whereas male-majority populations became significantly more female biased. The plants had only just produced their first spores, so these sex-ratio changes occurred during the gametophytic generation, as a result of sex-specific growth or survival. Sporophytes occurred only in populations with female-biased final sex ratios, which suggests that females in male-majority populations may have invested energy in ramets rather than in sporophyte production. The stress treatment was mild and had no effect on sex ratio. Our results suggest that female bias can be generated during the gametophytic generation, before plants reach sexual maturity. These results, combined with those of previous work, suggest that both the gametophytic and the sporophytic stages drive population sex ratios in C. purpureus, thus indicating that multiple mechanisms operate to create biased population sex ratios.

Sex-specific morphological and physiological differences in the moss Ceratodon purpureus (Dicranales).

Dioecy and sexual dimorphism occur in many terrestrial plant species but are especially widespread among the bryophytes. Despite the prevalence of dioecy in non-vascular plants, surprisingly little is known about how fine-scale sex-specific cell and leaf morphological traits are correlated with sex-specific physiology and population sex ratios. Such data are critical to understanding the inter-relationship between sex-specific morphological and physiological characters and how their relationship influences population structure. In this study, these data types were assessed to determine how they vary across three populations within one moss species and whether fine-scale morphological traits scale up to physiological and sex ratio characteristics. Twenty cell-, leaf- and canopy-level traits and two photochemical measurements were compared between sexes and populations of the dioecious moss Ceratodon purpureus . Field population-expressed sex ratios were obtained for the same populations. Male and female plants differed in cell, leaf and photochemical measures. These sexual dimorphisms were female biased, with females having larger and thicker leaves and greater values for chlorophyll fluorescence-based, leaf photochemistry measurements than males. Female traits were also more variable than male traits. Interestingly, field population sex ratios were significantly male biased in two study populations and female biased in the third study population. The results demonstrate that the larger morphology and the greater physiological output of female C. purpureus gametophytes compared with males occurs across populations and is likely to have significant effects on resource allocation and biotic interactions. However, this high level of dimorphism does not explain population sex ratio variation in the three study populations tested. This research lays the groundwork for future studies on how differential sex-specific variation in cell and leaf traits influences bryophyte plant fitness.

Meiotic sex ratio variation in natural populations of Ceratodon purpureus (Ditrichaceae).

• Sex ratio variation is a common but often unexplained phenomenon in species across the tree of life. Here we evaluate the hypothesis that meiotic sex ratio variation can contribute to the biased sex ratios found in natural populations of the moss Ceratodon purpureus.• We obtained sporophytes from several populations of C. purpureus from eastern North America. From each sporophyte, we estimated the mean spore viability by germinating replicate samples on agar plates. We estimated the meiotic sex ratio of each sporophyte by inferring the sex of a random sample of germinated spores (mean = 77) using a PCR-RFLP test. We tested for among-sporophyte variation in viability using an ANOVA and for deviations from 1:1 sex ratio using a χ(2)-test and evaluated the relationship between these quantities using a linear regression.• We found among-sporophyte variation in spore viability and meiotic sex ratio, suggesting that genetic variants that contribute to variation in both of these traits segregate within populations of this species. However, we found no relationship between these quantities, suggesting that factors other than sex ratio distorters contribute to variation in spore viability within populations.• These results demonstrate that sex ratio distortion may partially explain the population sex ratio variation seen in C. purpureus, but more generally that genetic conflict over meiotic segregation may contribute to fitness variation in this species. Overall, this study lays the groundwork for future studies on the genetic basis of meiotic sex ratio variation.

Between-Year Variation in Population Sex Ratio Increases with Complexity of the Breeding System in Hymenoptera

While adaptive adjustment of sex ratio in the function of colony kin structure and food availability commonly occurs in social Hymenoptera, long-term studies have revealed substantial unexplained between-year variation in sex ratio at the population level. In order to identify factors that contribute to increased between-year variation in population sex ratio, we conducted a comparative analysis across 47 Hymenoptera species differing in their breeding system. We found that between-year variation in population sex ratio steadily increased as one moved from solitary species, to primitively eusocial species, to single-queen eusocial species, to multiple-queen eusocial species. Specifically, between-year variation in population sex ratio was low (6.6% of total possible variation) in solitary species, which is consistent with the view that in solitary species, sex ratio can vary only in response to fluctuations in ecological factors such as food availability. In contrast, we found significantly higher (19.5%) between-year variation in population sex ratio in multiple-queen eusocial species, which supports the view that in these species, sex ratio can also fluctuate in response to temporal changes in social factors such as queen number and queen-worker control over sex ratio, as well as factors influencing caste determination. The simultaneous adjustment of sex ratio in response to temporal fluctuations in ecological and social factors seems to preclude the existence of a single sex ratio optimum. The absence of such an optimum may reflect an additional cost associated with the evolution of complex breeding systems in Hymenoptera societies.

Sex ratio variation in gynodioecious Lobelia siphilitica: effects of population size and geographic location

Variation in population sex ratio can be influenced by natural selection on alternate sex phenotypes as well as nonselective mechanisms, such as genetic drift and founder effects. If natural selection contributes to variation in population sex ratio, then sex ratio should covary with resource availability or herbivory. With nonselective mechanisms, sex ratio should covary with population size. We estimated sex ratio, resource availability, herbivory and size of 53 populations of gynodioecious Lobelia siphilitica. Females were more common in populations with higher annual temperatures, lower soil moisture and lower predation on female fruits, consistent with sex-specific selection. Females were also more common in small populations, consistent with drift, inbreeding or founder effects. However, small populations occurred in areas with higher temperatures than large populations, suggesting that female frequencies in small populations could be caused by sex-specific selection. Both selective and nonselective mechanisms likely affect sex ratio variation in this gynodioecious species.

Pollination success across an elevation and sex ratio gradient in gynodioecious Daphne laureola

Gynodioecy is a dimorphic breeding system in which hermaphrodite and female individuals coexist in populations. Sex ratio and gender-relative lifetime seed production determine the stability of gynodioecy, and both genetic and ecological factors may influence these parameters. I analyzed the consequences of variation in population sex ratio and site elevation for the relative pollination success of female and hermaphrodite individuals of Daphne laureola in southern Spain, where previous studies failed to detect female fecundity advantages at two mid-elevation sites. Pollination success, estimated as stigmatic pollen loads, number of pollen tubes per style, and percentage of fertilized flowers, was higher for hermaphrodites than females in populations with 20-56% females. Furthermore, female quantitative disadvantage in pollination success increased with elevation, suggesting that the higher availability of pollen due to the increased proportion of hermaphrodites could not mitigate the negative effect that other factors associated with elevation apparently had on pollination. Supplemental hand pollinations showed that female seed production was pollen limited in populations with a proportion of females >50%, although both pollination success and natural fruit set of females in these sites were the highest recorded.

Population sex ratio variation in a monogamous long‐lived bird, the wandering albatross

Summary Factors affecting population birth sex ratio and variations of sex ratios within population in very long‐lived animals are poorly known, apart for humans that have attracted much attention. We measured the age and sex structure of a population of a monogamous long‐lived seabird, the wandering albatross Diomedea exulans, where reproductive performance of individuals is known, to examine factors affecting sex ratio variation and survival consequences of producing a particular sex. The overall sex ratio at hatching is significantly biased toward the production of males. Very young and old parents tend to produce an excess of female offsprings, whereas intermediate‐aged birds, which represent the bulk of the population, produce an excess of males. Low‐quality parents (quality is measured as average breeding success over the cumulated past breeding life, and is not related to condition or age) produce more female chicks compared with high‐quality parents that represent the bulk of the population. The combined effect of age and quality of parents produce an extensive sex ratio variation at the level of the population. The survival of male and female parents is influenced differently by their quality and by the sex of offspring. Male parents die in larger numbers when rearing a female chick, whereas low‐quality female parents have a lower survival, especially when they rear a male offspring, the sex needing higher parental investment. The differences between males and females parents’ survival are probably due their contrasted investment in parental care. After fledging, during the 5–8‐year period of immaturity, there is a much higher mortality of males that balances the surplus of males at hatching and fledging and results in a similar number of males and females at the time of recruitment. However, the overall operational sex ratio is male biased due to the slightly higher mortality of adult females that results in an excess of old widowed males: excluding these post‐reproductive males leads to an unbiased operational sex ratio. These results show that sex ratio at hatching varies extremely with the age and phenotypic quality of parents, and lead to a complex age and sex structure of the population in this strictly monogamous long‐lived species.

Facultative sex ratio adjustment by western bluebird mothers with stay-at-home helpers-at-the-nest

Although population sex ratios rarely deviate from unity, the sex ratio of individual mothers should be labile, allowing them to bias their sex ratios in favour of the more successful sex when they can expect a difference in the mean reproductive value of their daughters and sons. Just how mothers should bias sex allocation is particularly complicated in cooperative breeders, because the adaptive sex ratio may be influenced by conflicting impacts of helpers on their parents' fitness via ‘local resource enhancement’ and ‘local resource competition’. In western bluebirds, Sialia mexicana, breeding-age helpers-at-the-nest are exclusively male and increase their parents' nesting success in the current year. As such, they are viewed as helpers, not hinderers, even though by failing to gain a breeding position they reduce their parents' annual inclusive fitness below that of parents with a breeding son. In this study, I tested the hypothesis that nonbreeding, stay-at-home helpers indicate poor breeding prospects for locally competing sons, and investigated one prediction of this hypothesis that such helpers also indicate a tendency for parents to produce substandard sons. A female removal experiment showed that female mates were in short supply. Pairs had daughter-biased broods when they had a stay-at-home helper son that failed to get a mate in the first place, but not when they had a helper that had been breeding on his own, but returned home to help sometime after his mother finished laying her eggs. Stay-at-home sons were not developmentally delayed relative to breeders of the same cohort, suggesting that helpers may not be inferior to breeders. Although the population sex ratio fluctuates about unity, a multiyear data set showed a negative relationship between the frequency of helping and the annual brood sex ratio for the population. These results suggest that local resource competition outweighs local resource enhancement in driving individual and population sex ratio variation in western bluebirds, a pattern that would not be predicted based on the simple question of whether helpers help or hinder at the nest.

Proximate and ultimate control of sex ratios in Myrmica brevispinosa colonies.

The literature on sex ratio evolution in ant colonies is dominated by inclusive fitness arguments. In general, genetic theory makes good predictions about sexual investment in ant populations, but understanding colony-level variance in sex investment ratios has proven more difficult. Recently, however, more studies have addressed ecological factors that influence colony-level sex investment ratios. Food availability, in particular, has been manipulated because larval nutrition influences female caste determination, thus implying that resource availability should be of critical importance for colony sex investment ratios. However, results from food supplementation experiments are equivocal, and it is clear that ant colony response to food supplementation is dependent on the ecological background of the population. We presented field colonies of the ant Myrmica brevispinosa with two food types (proteins and carbohydrates), and assessed their relative impact on colony-level sex investment ratios. We show that colonies receiving carbohydrate enhancement invested in more female sexuals and produced more female-biased sex allocation ratios than protein-fed or control colonies. Thus, our study is the first, to our knowledge, to demonstrate that sex ratios in social insect colonies might be sensitive to resource quality. Male investment was not influenced by food treatment, but was positively correlated with colony size. Therefore, the shift in sex ratios in our study must have been mediated through nutritional influences on female caste determination rather than male brood elimination. We also used our data to evaluate evidence for sex ratio compensation by queenright colonies in response to male production by workers from orphaned colonies.

Variation in Population Sex Ratio and Mating Success of Asexual Lineages of Alsophila pometaria (Lepidoptera: Geometridae)

Populations of Alsophila pometaria (Harris) are largely gynogenetic and composed of asexual females that must mate with a conspecific male to reproduce. Within this asexual contingent, there are a variety of electrophoretically defined multilocus genotypes. In this study we found that populations differ in sex ratio. We also found that there is an indication of differential mating success among asexual genotypes in a population of A. pometaria.

Sex Ratio Variation in Populations of a Dioecious Desert Perennial, Simmondsia Chinensis

The dioecious wind-pollinated shrub Simmondsia chinensis is common to much of the Sonoran Desert of Mexico, Arizona, and California. A survey of sexual expression over several years at two widely-separated sites indicates that males slightly but consistently outnumber females on the whole; that sex ratios vary along moisture gradients with male bias tending to be strongest in the driest sites; and that sexual expression is independent of plant size and not labile from year to year. These results are compared with those from other studies of Simmondsia sex ratios, and to the predictions of theories, including adaptive theories, of sexual expression. I conclude that surveys alone cannot distinguish among alternative causal hypotheses, and that experiments and demographic studies will be necessary in general to elucidate variation in sexual expression of plants.