Sex Ratio In Natural Populations Research Articles

The hot sex: is there evidence for climate‐driven sex allocation in mammals?

Abstract Climate change predictions indicate that future conditions will be hotter and drier, and rainfall will be more variable. Climatic conditions can affect individual condition and shape offspring sex ratios. The implications that climate change have for mammalian sex allocation are vast but are currently poorly understood. There are several scenarios under which climate‐driven sex allocation responses might emerge. Here, I make predictions on the direction of offspring sex ratio biases under hypothetical situations of restricted resource availability and altered social/spatial structures that may result from hotter, drier conditions. I searched the literature for existing evidence of climate‐driven offspring sex ratio biases among individuals studied in wild populations. There was no strong, directional support for the production of sons or daughters under ‘good’ or ‘poor’ climatic conditions among the 30 studies had reported on climatic variables and offspring sex ratio. I also searched the literature for studies that assessed maternal condition and offspring sex ratio in natural populations (i.e. there is a link between climate, resource availability and maternal condition). Of the 27 investigations, half reported a bias in favour of sons born to mothers in ‘good’ condition (56%). I discuss how this outcome could be due to a publication bias, or that it may indicate that low resource availability has the potential to lead to mothers in poor condition and the production of fewer sons. I consider the conservation implications of climate‐driven offspring sex ratios skews being carried through to the population level. I discuss the challenge of being able to account for the multiple factors that influence offspring sex ratios in natural populations and emphasise the growing need for more research in this important area of mammalian evolutionary ecology.

First notes on plant diversity, finding sites and sex ratio in natural populations of Thrips tabaci Lindeman (Thysanoptera: Thripidae) in Algeria (Biskra province)

"Thrips tabaci Lindeman (Thysanoptera: Thripidae), commonly known as onion thrips, is a serious global pest of commercial onion, causing direct and indirect important damages. This survey carried out in natural areas of Biskra province (Algeria) during two periods, 2008/2009 and 2011/2012, aims to review the plant species harbouring T. tabaci in this region. Algerian and Spanish researchers confirmed twenty-three thrips species. T. tabaci is the most abundant and polyphagous. Studies have indicated that it settled in fifty one plant species belonging to nineteen botanical families. The most important are Asteraceae, Brassicaceae, Fabaceae, and Amaranthaceae. In Biskra, T. tabaci was found in sites between -32 m and 1000 m of sea level. The results also indicate the presence of sexual and asexual populations. This study shows that T. tabaci is ubiquitous in the natural habitat of Biskra province. Further research is needed to confirm its host plants and the most common mode of reproduction in this region by studying the largest number of plants in various environments and demonstrating the sex ratio over a broad survey spectrum."

Sex ratio effects on reproductive success of male and female Daphnia.

The intensity of mating competition and the opportunity for sexual selection are thought to depend on the operational sex ratio, the ratio of sexually active males to fertilizable females. Cyclic parthenogens, organisms that alternate between sexual reproduction and female-only parthenogenesis, show particularly high variation in sex ratios in natural populations but the effects of this variation on mating competition and reproductive success of each sex are poorly understood. In a series of experiments with Daphnia magna, we experimentally imposed five sex ratio categories, varying from one male per 81 females to an even sex ratio. We found that, in males, reproductive success strongly and monotonically decreased with decreasing number of females per male. In females, in contrast, mating success and reproductive success were reduced only at the most female-biased sex ratio (1:81), when many females remained unmated and unfertilized, and then again at equal sex ratios, probably due to negative effects of high density or stress induced by numerous males. Our results suggest that females experienced male limitation at heavily female-biased sex ratios below one male to about 50 females. As this is well within the sex ratio variation observed in natural Daphnia populations, we conclude that mating competition and the opportunity for sexual selection may exist not only in males but, at least periodically, also in females.

Abundance and reproduction variables of two species of harpacticoid copepods along an increasing salinity gradient

Spatial and temporal complexities of reproductive behaviour and sex ratios of two harpacticoid species Canuella perplexa Scott T. & Scott A., 1893 and Bryocamptus sp. were evaluated monthly from September 2007 to August 2008. Collections were made from four ponds at different salinities in the solar saltern of Sfax, Tunisia, considering other environmental features as well. The salinities ranged from 38.0 to 86.1 psu. With rising salinity, the abundance of C. perplexa adults declined from 4550 ± 4160 ind m−3 (pond A1, salinity = 41.5 ± 2.3 psu) to 880 ± 780 ind m−3 (pond C31, salinity = 71.0 ± 4.3 psu). The abundance of Bryocamptus sp. adults increased from 3560 ± 2810 ind m−3 (A1) to 10,080 ± 6890 ind m−3 (C31). The abundance of total adults of C. perplexa was negatively correlated with both salinity (r = − 0.441, n = 18, p < 0.05) and abundance of dinoflagellates (r = − 0.291, n = 18, p < 0.05) and positively correlated with suspended matter (r = 0.389, n = 18, p < 0.05), temperature (r = 0.354, n = 18, p < 0.05), and abundance of diatoms (r = 0.410, n = 18, p < 0.05). The abundance of adults of Bryocamptus sp. displayed only a positive correlation with salinity (r = 0.452, n = 18, p < 0.05). In both species, the sex ratio (males/females) was driven by the variability of female abundance but never exceeded 1.0. For C. perplexa, the egg number per female ranged between 28 and 31; the ovisac length was between 0.39 ± 0.002 and 0.45 ± 0.018 mm. The egg diameter ranged from 0.062 ± 0.001 to 0.065 ± 0.002 mm, and the total length of ovigerous females ranged between 1.2 and 1.35 mm. For Bryocamptus sp., the egg number per female ranged between 19 and 22, the ovisac length between 0.28 ± 0.008 and 0.31 ± 0.003 mm, the egg diameter between 0.041 ± 0.001 and 0.044 ± 0.001 mm, and the total length of ovigerous females between 0.6 and 0.74 mm. The outcome of this study demonstrated that under extreme environmental conditions and increasing salinity the percentage of diatoms and dinoflagellates was found to be an essential factor in controlling the abundance and reproduction traits of both harpacticoids in this study. Furthermore, the sex ratios in these harpacticoid populations are complex as many factors can influence and alter the sex ratios in natural populations.

Population genetic structure and variability in Lindera glauca (Lauraceae) indicates low levels of genetic diversity and skewed sex ratios in natural populations in mainland China.

Lindera glauca (Lauraceae) is a tree of economic and ecological significance that reproduces sexually and asexually via apomictic seeds. It is widely distributed in the low-altitude montane forests of East Asia. Despite the potential implications of a mixed reproductive system in terms of genetic diversity, few studies have focused on this aspect. In this study, the genetic structure of wild populations of L. glauca was investigated via genetic analyses. Overall, 13 nuclear microsatellites (nSSRs) and five chloroplast microsatellites (cpSSRs) were used to genotype 300 individual plants, taken from 20 wild populations (a small sample size in some wild populations is due to the limitation of its specific reproduction, leading to certain limitations in the results of this study) and two cultivated populations ranging across nearly the entire natural distribution of mainland China. The populations exhibited low levels of genetic diversity (nSSR: AR = 1.75, Ho = 0.32, He = 0.36; cpSSR: Nb = 2.01, Hrs = 0.40), and no significant effect of isolation by distance between populations existed, regardless of marker type (nSSR: R2 = 0.0401, P = 0.068; cpSSR: R2 = 0.033, P = 0.091). Haplotype networks showed complex relationships among populations, and the H12 haplotype was predominant in most populations. Analyses of molecular variance obtained with nuclear markers (Fsc = 0.293, FST = 0.362) and chloroplast markers (Fsc = 0.299, FST = 0.312) were similar. The migration ratio of pollen flow versus seed flow in this study was negative (r = −1.149). Results suggest that weak barriers of dispersal between populations and/or the similarity of founders shared between neighbors and distant populations are indicative of the gene flow between populations more likely involving seeds. Wild L. glauca in mainland China was inferred to have highly skewed sex ratios with predominant females. In addition, some populations experienced a recent bottleneck effect, especially in Gujianshan, Chongqing, and southwest China (population GJS). It is suggested that few wild male individuals should be conserved in order to maintain overall genetic diversity in the wild populations of this species. These findings provide important information for the sustainable utilization and preservation of the overall genetic diversity of L. glauca.

Effects of the interaction between temperature and steroid hormones on gametogenesis and sex ratio in the European flat oyster (Ostrea edulis)

Throughout Europe, populations of Ostrea edulis have been in decline since the 1970s. Temperature has an important influence on physiological, biochemical and reproductive attributes of oysters. It is also the most easily modulated environmental factor in hatcheries, so it is useful to understand the implications of temperature variation in driving gametogenesis and sex development in a protandrous sequential hermaphrodites such as O. edulis. To understand the effect of temperature on gametogenesis and sex ratio, as well as the potential mechanism of sex determination through the role of steroid hormone homologues, oysters were exposed to three temperatures (10, 14, and 18 °C) for four months. Gametogenic stage and sex ratio were assessed histologically for each treatment. In parallel, concentrations of estradiol (E2)- and testosterone (T)- were determined in developing gonads. Our data show that by some biometric parameters, gametogenesis and sex ratio were significantly influenced by temperature during the experiment. There was a weak but significant correlation between E2 and T concentration during the treatments. However, and importantly, a direct relation between gonadal maturation, sex determination and hormones concentration was not found. These results suggest that gametogenesis and sex determination are predominantly affected by temperature in this species, and that steroids may not be actively involved as endogenous modulators in sex determination. Rising sea water temperatures and warmer condition through the year could cause an accelerated gametogenesis and skewed sex ratios in natural populations of O. edulis.

The role of predation risk in metamorphosis versus behavioural avoidance: a sex-specific study in a facultative paedomorphic amphibian

Evolutionary theory predicts the evolution of metamorphosis over paedomorphosis (the retention of larval traits at the adult stage) in response to life in unfavourable habitats and to the benefits of dispersal. Although many organisms are canalised into obligatory complex or simple life cycles, some species of newts and salamanders can express both processes (facultative paedomorphosis). Previous research highlighted the detrimental effect of fish on both metamorphic and paedomorphic phenotypes, but it remains unknown whether predation risk could induce shifts from paedomorphosis to metamorphosis, whether behavioural avoidance could be an alternative strategy to metamorphosis and whether these responses could be sex-biased. Testing these hypotheses is important because metamorphosed paedomorphs are dispersal individuals which could favour the long-term persistence of the process by breeding subsequently in more favourable waters. Therefore, we quantified the spatial behaviour and timing of the metamorphosis of facultative paedomorphic palmate newts Lissotriton helveticus in response to predation risk. We found that fish induced both male and female paedomorphs to hide more often, but behavioural avoidance was not predictive of metamorphosis. Paedomorphs did not metamorphose more in the presence of fish, yet there was an interaction between sex and predation risk in metamorphosis timing. These results improve our understanding of the lower prevalence of paedomorphs in fish environments and of the female-biased sex ratios in natural populations of paedomorphic newts. Integrating sex-dependent payoffs of polyphenisms and dispersal across habitats is therefore essential to understand the evolution of these processes in response to environmental change.

Female-biased sex ratios unrelated to Wolbachia infection in European species of the Jaera albifrons complex (marine isopods)

Female-biased sex ratios and reproductive isolation in arthropods can be caused by endosymbiotic Wolbachia bacteria that manipulate the reproductive system of their host. Wolbachia is particularly common in terrestrial host species, but its frequency in marine arthropods is less well known. Here we asked whether Wolbachia bacteria are accountable for the female-biased sex ratio and variation in reproductive isolation levels observed in the four European species of the Jaera albifrons complex (marine intertidal isopods). We analysed the sex ratio in young adults reared in the laboratory (indicative of the sex ratio at birth), compared it with the adult sex ratio in natural populations, and performed a molecular survey of Wolbachia infection based upon amplification of three gene targets using 11 different protocols tested in 817 individuals from all species of the Jaera albifrons complex. One species (J. ischiosetosa) had a female-biased sex ratio at birth but showed no sign of infection by Wolbachia bacteria. This species, together with two others (J. albifrons and J. forsmani) also displayed female-biased sex ratio in adults in nature, while the adult sex ratio in the fourth European species (J. praehirsuta) was unbiased. A new Wolbachia strain was identified in J. albifrons and J. praehirsuta, albeit at very low frequency in populations. We conclude that Wolbachia bacteria are present in at least two species of the Jaera albifrons complex, but their prevalence is too low to have any effect on sex ratio and reproductive isolation. A sex ratio distorter other than Wolbachia may be acting in some J. ischiosetosa populations, and we hypothesize that the adult female excess seen in most species results from habitat-dependent, male-biased mortality in natural conditions.

Avian predation mediates size-specific survival in a Neotropical annual fish: a field experiment

Predation and population density have fundamental size- and sex-specific effects on individual survival and demographic parameters. Given the overlap and interactions between different age cohorts in natural populations, separating the factors related to differential survival and growth based on longitudinal field-collected data is problematic. Using a Neotropical annual fish (Austrolebias minuano) with a single age cohort per generation, we used replicated field enclosures to experimentally test the roles of avian predation and fish population density on survival and growth over adult lifespan. We found that mortality risk was higher in larger males and smaller females when predation was experimentally excluded. Exposure to avian predation eliminated this sex-specific effect of body size on survival. No overall sex difference in survival was found in the experiment, despite a female-biased sex ratio in natural populations. Individually based growth rates were highest in enclosures at low population density with no predation risk. Overall, we demonstrate that annual fish suffer high sex-dependent size-specific mortality that is more strongly related to predation than to density-dependent processes. This has important implications for our understanding of the evolution of senescence and other life history traits in annual fishes.

Can the meiotic sex ratio explain the sex ratio bias in adult populations in the dioicous moss Drepanocladus lycopodioides?

Sex ratio variation is commonly observed in natural populations of many organisms with separate sexes and genetic sex determination, including bryophytes. Most bryophyte populations exhibit female-skewed expressed adult sex ratios, generally inferred from counts of sexually mature plants. For the rarely sexually reproducing perennial dioicous moss Drepanocladus lycopodioides, we showed that a female bias also exists in the genetic adult sex ratio, using a specifically designed molecular sex-associated marker. Here, we investigated whether the meiotic spore sex ratio contributes to the observed bias in genetic adult sex ratio in natural populations. Earlier attempts to study meiotic sex ratios have involved commonly cultivated ruderals that rapidly express sex in the laboratory. We established single-spore cultures from field-collected sporophytes from these populations and used the marker to assess the sex of individual sporelings. Spore germinability was (near) complete, and mortality among sporelings was virtually absent. The true meiotic sex ratio did not differ from equality, but strongly differed both from the observed genetic sex ratios in the natural adult populations, and from the European scale genetic sex ratio. We conclude that the biased population sex ratios in this species arise at life cycle stages after spore germination. Sexual dimorphism may selectively favour female proliferation during some phase of gametophyte development. Based on methodological progress, we successfully used a perennial study species with rare sexual reproduction, which significantly broadens the life history spectrum investigated in bryophyte sex ratio studies.

Male adaptations to minimize sexual cannibalism during reproduction in the funnel-web spider Hololena curta.

Males of many spider species risk being attacked and cannibalized while searching for, courting, and mating with conspecific females. However, there are exceptions. We show that the funnel-web spider, Hololena curta, has 3 adaptations that minimize risk to males during courtship and mating, and enhance reproductive success. First, males detected chemical or tactile signals associated with webs of virgin females, and differentiated them from webs of mated females, enabling males to increase encounter rates with virgin females and avoid aggressive mated females. Second, males produced stereotyped vibrational signals during courting which induced female quiescence and suppressed female aggression. Third, when touched by males, sexually receptive females entered a cataleptic state, allowing males to safely approach and copulate. Because males can mate multiple times and the sex ratio in natural populations of H. curta is female biased, overall reproductive output is likely increased by males of this species avoiding sexual cannibalism.

Sediment quality assessment using Gmelinoides fasciatus and Monoporeia affinis (Amphipoda, Gammaridea) in the northeastern Baltic Sea

Crustaceans in the order Amphipoda are sensitive organisms for the assessment of sediment quality. In this work we performed 10-day toxicity tests on muddy sediments collected from a total of 29 sites in the Gulf of Finland, the Gulf of Riga and the Gulf of Bothnia (northeastern Baltic Sea) using Baltic Sea species such as the native amphipod Monoporeia affinis (Bousfield, 1989) and the invasive amphipod Gmelinoides fasciatus (Stebbing, 1899), and also compared these results with those of bioassays carried out using the standard test species, laboratory-cultivated amphipod Hyalella azteca (Saussure, 1858). The sediment samples (three cm of the upper layer) were collected by a GEMAX Dual Corer during the R/V “Aranda” cruises in August and September of 2009 and 2010. Toxicity of sediments in bioassays with M. affinis and G. fasciatus gave varied results depending on the amphipod species used. The lowest quality of sediments determined using M. affinis was recorded at sites located in the offshore and deepwater areas (60-100 m depths) of the Gulf of Finland characterized by hypoxic/anoxic conditions. Toxicity testing applying G. fasciatus showed that sediments at &gt;50% of the study sites in the Gulf of Finland and in the Gulf of Riga can be assessed as highly contaminated. Males of G. fasciatus were significantly more sensitive to potential contamination in sediments than females. The lower survival of males under contaminant stress may result in a skewed sex ratio in natural populations and in a decline of reproduction success. The survival rate of G. fasciatus in the toxicity tests correlated positively with the Shannon diversity index (calculated for macrozoobenthos at the study sites), weight losses on ignition (%) in sediments, and it also showed a negative relation with the bottom water oxygen content (mg/l). The results suggest that G. fasciatus is the more sensitive species of the three amphipods tested and can be used as a indicator of sediment quality in the Baltic Sea and other water bodies.

When to estimate sex ratio in natural populations of insects? A study on sex ratio variations of gall midges within a generation

AbstractPrecise estimation of arthropods' sex ratio is an important issue in a wide range of ecological studies and biological control programs. Although, in many cases changes in arthropods' sex ratio may be under the control of parents or some symbiotic microorganisms, biased sex ratios in some other species are caused by some extrinsic factors, neglect of which may lead to under/overestimation of true sex ratio. In this paper, we pursued those factors that cause false estimation of sex ratio in insects' species. We studied the predatory gall midge, Aphidoletes aphidimyza Rondani (Diptera: Cecidomyiidae), an important biological control agent of aphids, that shows protandry (i.e. early male emergence), differential lifespan of sexes, and differential distribution of sexes across habitat. Ten populations of A. aphidimyza were released separately in transparent cages and their sex ratio variations were recorded every 12 hours. The primary sex ratio in this species seems to be slightly male‐biased (52.41% males), however early emergence of males biases the sex ratio up to 72% males in a few hours after emergence. Shortly after the emergence of females, the sex ratio reaches its primary situation, but as a result of male‐biased mortality after mating, the proportion of females increases gradually to 97% by the fourth and fifth days after emergence. These results explicitly suggest that direct estimation of sex ratio in natural populations may be affected by some secondary factors such as differential mortality of sexes, protandry, and differential distribution of males and females over time and/or across habitat.

Sex ratios in natural populations of Drosophila pseudoobscura from Mexico

Most species show an equal proportion of individuals of both sexes. In diploid species sex ratio is determined by a genic balance between sex chromosomes. In Drosophila sex is determined by the ratio of X- chromosomes versus autosomes and in some species of the genus it is related to the presence of an inversion in the sex chromosome. The present work analyses the sex ratio in 27 natural populations of Drosophila pseudoobscura that inhabit Mexico. Female flies captured in nature were counted and their sex ratio calculated and been called generation P, then cultured individualy, allowed to leave adult offspring which was quantified in order to get its sex ratio and designated generation F1. sex ratio was calculated using the expression: number of males times 100 divided by the number of females proposed by Darwin (1871). The sex ratio of each population was taken using the average of all the individual counts from each sample. The values found varied among different generations and populations, so for generation P their values varieded 37.4 to 190.4 and in generation F1 from 31.3 up to 96.4 males for each 100 females. According to their geographical distribution four North to South transects were arranged and in them means varied from 60.8 to 81.7 males for each 100 females. All this means that in Mexican population are more females than males, exceptionally more males than females.

Adult Sex Ratio of a Population of the Japanese Salamander, Hynobius tokyoensis (Caudata: Hynobiidae)

Adult sex ratio is considered to be a key factor in understanding sexual selection, mating behavior, life history and population dynamics. Following Fisher (1930), a general consensus was formed that sex ratio tends to be largely balanced. However, gainsaying this, large variations in sex ratio have been reported among populations and species of amphibians. There is little information on sex ratio in natural populations of Japanese hynobiid salamanders. To help fill this gap, we conducted a long-term census of a population of Hynobius tokyoensis from 1976 to 1985 and estimated the adult sex ratio (the proportion of males within the population) at 0.578 from data gathered during eight non-breeding seasons. The bootstrap 95% confidence interval was 0.516–0.641. The study showed that the adult sex ratio was significantly biased toward males in the population studied; it was nearly 1.5:1 (males:females). Age at first reproduction is 1 year later in females, which suggests that the most likely factor driving the unbalanced sex ratio is the differential maturation rate between the sexes.

SEX RATIO EVOLUTION UNDER PROBABILISTIC SEX DETERMINATION

Sex allocation theory has been remarkably successful at explaining the prevalence of even sex ratios in natural populations and at identifying specific conditions that can result in biased sex ratios. Much of this theory focuses on parental sex determination (SD) strategies. Here, we consider instead the evolutionary causes and consequences of mixed offspring SD strategies, in which the genotype of an individual determines not its sex, but the probability of developing one of multiple sexes. We find that alleles specifying mixed offspring SD strategies can generally outcompete alleles that specify pure strategies, but generate constraints that may prevent a population from reaching an even sex ratio. We use our model to analyze sex ratios in natural populations of Tetrahymena thermophila, a ciliate with seven sexes determined by mixed SD alleles. We show that probabilistic SD is sufficient to account for the occurrence of skewed sex ratios in natural populations of T. thermophila, provided that their effective population sizes are small. Our results highlight the importance of genetic drift in sex ratio evolution and suggest that mixed offspring SD strategies should be more common than currently thought.

Adult Sex Ratio of a Population of the Japanese Salamander, Hynobius tokyoensis (Caudata: Hynobiidae)

Adult sex ratio is considered to be a key factor in understanding sexual selection, mating behavior, life history and population dynamics. Following Fisher (1930), a general consensus was formed that sex ratio tends to be largely balanced. However, gainsaying this, large variations in sex ratio have been reported among populations and species of amphibians. There is little information on sex ratio in natural populations of Japanese hynobiid salamanders. To help fill this gap, we conducted a long-term census of a population of Hynobius tokyoensis from 1976 to 1985 and estimated the adult sex ratio (the proportion of males within the population) at 0.578 from data gathered during eight non-breeding seasons. The bootstrap 95% confidence interval was 0.516–0.641. The study showed that the adult sex ratio was significantly biased toward males in the population studied; it was nearly 1.5:1 (males:females). Age at first reproduction is 1 year later in females, which suggests that the most likely factor driving the unbalanced sex ratio is the differential maturation rate between the sexes.

Sex ratio, mating behaviour and Wolbachia infections in a sheetweb spider

The solitary sheetweb spider Pityohyphantes phrygianus has a female-biased primary sex ratio. Earlier studies have indicated that female post-copulatory position may influence the degree of bias, but the adaptive significance of the departure from a Fisherian 1 : 1 sex ratio is not known. In this study we show that there is a significant relationship between abdominal position and offspring sex ratio in undisturbed females. Moreover, female mating behaviour (and thus sex ratio) is correlated with female size, but this relationship may shift from negative to positive depending on the presence of the maternally acquired endosymbiotic bacterium Wolbachia in both sexes. A survey of wild-caught individuals shows that the incidence of Wolbachia infection in natural populations is high. This indicates that Wolbachia may be a significant factor influencing female post-copulatory behaviour and sex ratio in natural populations. We discuss various adaptive reasons for sex-ratio bias and behavioural plasticity in females. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98, 181–186.

Breeding systems in the clam shrimp family Limnadiidae (Branchiopoda, Spinicaudata)

Abstract. Crustaceans in the class Branchiopoda exhibit a wide range of breeding systems, including dioecy (gonochorism), androdioecy, parthenogenesis, cyclic parthenogenesis, and hermaphroditism. The largest subgroup of the Branchiopods, the Diplostraca, is reported to encompass all five of these breeding systems. However, many of these reports are based primarily on simple observations of sex ratios in natural populations. Herein we report the beginnings of a more rigorous approach to breeding system determination in the Diplostraca, starting with the family Limnadiidae. We combine measurements of sex ratio, offspring rearings, and behavior to identify three breeding systems within the Limnadiidae: dioecy, androdioecy, and selfing hermaphroditism. To date, no instances of parthenogenetic reproduction have been identified in this family. Comparisons of breeding system determination via simple population sex ratios with our more controlled studies show that simple sex ratios can be useful when these sex ratios are ∼50% males (=dioecy) or 5–30% males (androdioecy). However, population sex ratios of 0–5% males or 35–45% males necessitate further investigation because estimates in these ranges cannot distinguish selfing hermaphroditism from androdioecy or androdioecy from dioecy, respectively. We conclude by noting that the genetic sex‐determining system outlined for one of these limnadiid species, Eulimnadia texana, provides a parsimonious framework to describe the evolution of the three breeding systems observed within the Limnadiidae.

Sex-ratio distortion driven by migration loads

The significance of migration load in driving the evolution of recipient populations has long been documented in population genetics, but its effects have not been linked to the formation of biased sex ratios in natural populations. In this study, we develop a single-locus model to demonstrate how the migration load can shape the primary and secondary sex ratios in dioecious plants where sexual dimorphism is determined by the sex chromosomes (the XX–XY or similar systems). Our results show that migration load can generate an array of sex ratios (from the female- to male-biased primary/secondary sex ratios), depending on the selection systems at the gametophyte and sporophyte stages and on the sex ratio in the migrating seeds. Ovule abortion and the purging of maladaptive genes from the immigrating pollen at the gametophyte stage can alter the primary sex ratio and indirectly alter the secondary sex ratio. The presence of maladaptive sex-linked genes from the migrating pollen and seeds of males facilitates the outcome of the female-biased secondary sex ratios, while the presence of maladaptive sex-linked genes from the migrating seeds of females can lead to the male-biased secondary sex ratios. The detrimental effects of the Y-chromosome from the migrating pollen and seeds can enhance the formation of female-biased primary and secondary sex ratios. These theoretical predictions highlight an alternative approach to the existing sex-ratio theories for interpreting the formation of biased sex ratios in the populations that are subject to the impacts of maladaptive genes from immigrants.