Persistence of an extreme male-biased adult sex ratio in a natural population of polyandrous bird

In a number of insects, fishes and birds, the conventional sex roles are reversed: males are the main care provider, whereas females focus on matings. The reversal of typical sex roles is an evolutionary puzzle, because it challenges the foundations of sex roles, sexual selection and parental investment theory. Recent theoretical models predict that biased parental care may be a response to biased adult sex ratios (ASRs). However, estimating ASR is challenging in natural populations, because males and females often have different detectabilities. Here, we use demographic modelling with field data from 2101 individuals, including 579 molecularly sexed offspring, to provide evidence that ASR is strongly male biased in a polyandrous bird with male-biased care. The model predicts 6.1 times more adult males than females (ASR=0.860, proportion of males) in the Kentish plover Charadrius alexandrinus. The extreme male bias is consistent between years and concordant with experimental results showing strongly biased mating opportunity towards females. Based on these results, we conjecture that parental sex-role reversal may occur in populations that exhibit extreme male-biased ASR.     

Genetic variation and reproductive patterns in wetland mosses suggest efficient initial colonization of disturbed sites

To understand colonization processes, it is critical to fully assess the role of dispersal in shaping biogeographical patterns at the gene, individual, population, and community levels. We test two alternative hypotheses (H I and H II) for the colonization of disturbed sites by clonal plants, by analyzing intraspecific genetic variation in one and reproductive traits in two typical fen mosses with separate sexes and intermittent spore dispersal, comparing disturbed, early‐succession (limed) fens and late‐successional rich fens. H I suggests initial colonization of disturbed sites by diverse genotypes of which fewer remain in late‐successional fens and an initially balanced sex ratio that develops into a possibly skewed population sex ratio. H II suggests initial colonization by few genotypes and gradual accumulation of additional genotypes and an initially skewed sex ratio that alters into the species‐specific sex ratio, during succession. Under both scenarios, we expect enhanced sexual reproduction in late‐successional fens due to resource gains and decreased intermate distances when clones expand. We show that the intraspecific genetic diversity, assessed by two molecular markers, in Scorpidium cossonii was higher and the genetic variation among sites was smaller in disturbed than late‐successional rich fens. Sex ratio was balanced in S. cossonii and Campylium stellatum in disturbed fens and skewed in C. stellatum in late‐successional fens, thus supporting H I. In line with our prediction, sex expression incidence was higher in, and sporophytes were confined to, late‐succession compared to disturbed rich fens. Late‐successional S. cossonii sites had more within‐site patches with two or more genotypes, and both species displayed higher sex expression levels in late‐successional than in disturbed sites. We conclude that diverse genotypes and both sexes disperse efficiently to, and successfully colonize new sites, while patterns of genetic variation and sexual reproduction in late‐successional rich fens are gradually shaped by local conditions and interactions over extended time periods.     

蜜蜂性别决定与性比调控机理研究

叙述了 4个主要蜜蜂性别决定机理的假说 :即性位点假说、基因平衡假说、蜜蜂性别决定综合假说和性基因数量决定假说。然后就蜜蜂性比由蜂王操纵 ,或是由工蜂操纵进行了论述 ,并对蜜蜂性比调控机理研究提出了一些建议     

Causes of a male-biased operational sex ratio in the fiddler crab<Emphasis Type="Italic"> Uca crenulata</Emphasis>

This research investigates the causes of a male-biased operational sex ratio in a population of the California fiddler crab, Uca crenulata. Mensurative studies revealed there were almost twice as many adult males as females, mating occurred across half of the days within the breeding season, and females had much longer individual reproductive cycles than males. Therefore, many more males than females were available for mating on each breeding day. Perhaps as a consequence, males spent a large proportion of their time fighting with neighbors and rapidly waving their large claws when females passed by. Electronic Publication     

Seasonal sex ratio and unbalanced investment sex ratio in the Banksia bee Hylaeus alcyoneus

Abstract 1. Hylaeus alcyoneus is an endemic solitary bee common on coastal heaths of Western Australia. The bee is unusual in that males are larger than females. This size dimorphism presents an opportunity to test the theory of resource-dependent sex allocation, in which theory predicts that when resources are low the sex ratio should be biased towards the smaller sex. In most bees, females are larger than males and, in line with theoretical prediction, sex ratios are male biased when resources are scarce.
2. The emerging sex ratio and brood mass from a natural population of H. alcyoneus using trap nests was studied over two seasons (1999, 2000). A switch from a male- to a female-biased sex ratio through the season was found, which was related to a reduced floral resource.
3. Fisherian sex ratio theory predicts that total investment in each sex throughout a season should be equal and that the sex ratio should be biased towards the smaller sex. By measuring the mass of the emerging progeny, the total investment was found to favour males. Possible explanations for this bias in investment are discussed.     

Population genetic structure and male-biased dispersal in the queenless ant Diacamma cyaneiventre

In this study we investigated the population genetic structure of the queenless ant Diacamma cyaneiventre. This species, lacking winged queens, is likely to have a restricted female dispersal. We used both mitochondrial and microsatellite markers to assess the consequence of such restricted female dispersal at three geographical scales: within a given locality (< 1 km), between localities within a given region (< 10 km) and between regions (> 36 km). Within a locality, a strong population structure was observed for mitochondrial DNA (mtDNA) whereas weak or nonexistent population genetic structure was observed for the microsatellites (around 5% of the value for mtDNA). Male gene flow was estimated to be about 20-30 times higher than female gene flow at this scale. At a larger spatial scale, very strong genetic differentiation for both markers was observed between localities - even within a single region. Female dispersal is nonexistent at these scales and male dispersal is very restricted, especially between regions. The phylogeographical structure of the mtDNA haplotypes as well as the very low genetic diversity of mtDNA within localities indicate that new sites are colonized by a single migration event from adjacent localities, followed by successive colony fissions. These patterns of genetic variability and differentiation agree with what is theoretically expected when colonization events are kin-structured and when, following colonization, dispersion is mainly performed by males.     

Heritable variation of sex ratio in a polychaete worm

Ophryotrocha labronica is a gonochoristic polychaete worm whose sex determining mechanism and sex ratio control are supposed to be polygenic. From a lab population, whose sex ratio (i.e., proportion of males) was 0.5, the estimate of sex ratio heritability by offspring-father regression was 0.54 ± 0.15 and by offspring-mother regression was not significantly different from 0. Estimate of sex ratio repeatability between successive broods of a pair was 0.64 ± 0.33. Since female parents do not contribute in any way to the variability of sex ratio, sex ratio variation seems to be largely a paternal character. On the basis of these estimates we advance the hypothesis that in this species sex is determined by a multilocus genetic system, allowing the combined effects of a female major sex gene (which could give rise to a form of female heterogamety) and masculinizing modifiers. The hypothesis that the male sex has the least canalised sexual differentiation is supported by the observation that some old males developed oocytes.     

PSR (paternal sex ratio) chromosomes: the ultimate selfish genetic elements

PSR (paternal sex ratio) chromosomes are a type of supernumerary (or B) chromosomes that occur in haplodiploid arthropods. They are transmitted through sperm but then cause loss of the paternal chromosomes (except themselves) early in development. As a result, PSR chromosomes convert diploid fertilized eggs (which would normally develop into females) into haploid males that carry a PSR chromosome. Because they act by completely eliminating the haploid genome of their hosts, PSR chromosomes are the most extreme form of selfish or parasitic DNA known. PSR was originally described in the parasitic wasp Nasonia vitripennis (Pteromalidae). A second PSR chromosome has been found in Trichogramma kaykai, an egg parasitoid from a different family of Hymenoptera (Trichogrammatidae). We argue that PSR chromosomes are likely to be widespread in haplodiploid organisms, but have so far gone under reported due to a paucity of population genetic studies in haplodiploids. We describe the two known PSR systems and related phenomena, and models indicating the conditions conducive to increase of PSR like chromosomes in haplodiploids.     

Evidence for sex difference in the CSF/plasma albumin ratio in ~20 000 patients and 335 healthy volunteers

Given sex‐related differences in brain disorders, it is of interest to study if there is a sex difference in the permeability of the blood‐cerebrospinal fluid barrier (BCSFB) and the blood‐brain barrier (BBB). The CSF/serum albumin ratio (Q_Alb) is a standardized biomarker that evaluates the function of these barriers. In previous studies, contradictory results have been reported with respect to sex difference using this quotient, possibly because of small population sizes and heterogeneity with respect to ages. Q_Alb measurements in more than 20 000 patients between 1 and 90 years visiting our hospitals revealed a significant sex difference in all age groups also when excluding patients with pathologically high CSF albumin > 400 mg/L. Similar pattern was found in 335 healthy volunteers in similar age intervals. Although also other factors are likely important, our observation is consistent with lower integrity of the brain barriers in males. If the difference in Q_Alb is caused mainly by a difference in barrier function, this may require different drug doses and strategies for efficient central nervous system (CNS) delivery in males and females, as well as it may indicate differences in brain metabolism. Moreover, our study emphasizes that different reference values should be used both for different ages and sexes.     

An inclusive fitness model for the sex ratio in a partially sibmating population with inbreeding cost

Summary We construct an inclusive fitness model to find the evolutionarily stable sex ratios in a partially sibmating diploid or haplodiploid population. We assume a constant rate of sibmating with inbred offspring incurring a fitness penalty which, under haplodiploidy, is only suffered by females. We construct a one-locus genetic model for the same problem and observe that when selection is weak it gives the same numerical results as the inclusive fitness model.     

Secondary sex ratio variation in the Cayo Santiago macaque population

Secondary sex ratios (SSR) were calculated from 1,385 offspring delivered by 372 females in the Cayo Santiago population of free-ranging rhesus monkeys (Macaca mulatta) from 1976 through 1984. The SSR for the entire colony ranged from 0.86 to 1.46 males per female (combined total: 1.08), but no significant difference was observed (P > .05). SSR values were compared among the troops for each year. The SSR differed significantly among the six social groups (P < .05) only in 1978. The annual SSR of each troop was compared over 9 years. Significant variation was found only in group O. The annual SSR was significantly skewed (P < .05, males > females) for three troops in 3 separate years. The SSR did not vary according to troop rank. No significant difference was found among the 17 matrilines of the population, but comparison of matrilines within each social group revealed a significant difference in the SSR (P < .02) for the three matrilines in group I. This was due to the significantly skewed SSR (P = .0080, females > males) of the DM genealogy in that troop. SSR values were not related to matrilineal rank. Individual dominance rank did not bias the SSR. Complete reproductive histories for 266 females showed no evidence of significantly skewed SSR values. Age-related effects on the SSR were examined by using cross-sectional and cohort-based analyses. The SSR did not vary significantly (P > .05) with maternal age, but it was significantly skewed (P < .05) toward males at the ages of 5 and 9 years. Parity had no significant effect (P > .05) on SSR values. Wide variation occurred in the SSR of the Cayo Santiago population. Rank-related adjustment of the SSR at the level of the troop, matriline, or individual, as reported in short-term studies of other primate social groups, may reflect normal annual variation in the SSR evident only from longitudinal observations of large multigroup primate populations.     

About the sex ratio in connection with early embryonic mortality in man

The problem of the functioning specificity of sex chromosomes during the early stages of embryogenesis in man and the associated problem of the sex ratio in spontaneous and induced abortions, as well as in newborns, remains open. We have conducted a cytogenetic examination of 342 spontaneous abortions divided into three clinical groups on the basis of the severity of the developmental disturbances of the embryo: spontaneous abortionssensu stricto with a developed embryo without any significant intrauterine delay of development (n=100), nondeveloping pregnancies (n=176), and anembryonic fetuses (n=66). The frequency of chromosomal mutations in these groups was 22.0, 48.3, and 48.5%, respectively. Statistical analysis has demonstrated significant differences between the studied groups in the frequencies of the normal and abnormal karyotypes: the major contributions to these differences were associated with autosomal trisomy, triploidy, and the 46.XY karyotype. The presence of 46.XY may reflect the specific genetic mechanisms of the prenatal mortality of embryos with the normal karyotype, associated with sex and/or with the imprinting of X-chromosomes. The sex ratio in spontaneous abortions with the normal karyotype was as follows: 0.77 for spontaneous abortions with well-developed embryos without any significant intrauterine delay of development; 0.60 for nondeveloping pregnancies; and 0.31 for anembryonic fetuses. An analysis of DNA from the embryos and their parents has demonstrated a low probability of contamination of cell cultures with mother cells as a possible source of the prevalence of embryos with the 46.XX karyotype among spontaneous abortions. Nondeveloping pregnancies and anembryonic fetuses showed statistically significant differences in the sex ratio from the control group consisting of medical abortions (1,11). Differences in the sex ratio were due to an increasingly lower proportion of embryos with karyotype 46.XY (relative to the expected one) among the fetuses with an increased severity of developmental disturbances. The statistical “chances ratio” index also provided evidence that embryos with the 46.XY karyotype had a higher propensity to produce a well-formed fetus as compared with the female embryos. We propose that the expression of genes of the maternal X-chromosome in XY embryos supports a more stable development during early embryogenesis as compared with XX embryos. In the latter case, normal development is coupled with the operation of an additional mechanism for compensation of the dose of X-linked genes. Operation of this mechanism increases the probability of disturbances in female embryos. A higher viability of XY embryos during the early stages of ontogenesis in man appears to explain their underrepresentation in samples of spontaneously aborted embryos and appears to be the major factor responsible for the deviation of the sex ratio from the theoretically expected value.     

Brood sex ratio in the Kentish plover

How and why do the mating opportunities of males and femalesdiffer in natural population of animals? Previously we showedthat females have higher mating opportunities than males inthe Kentish plover Charadrius alexandrinus. Both parents incubatethe eggs, and males provide more brood care than females; thusit is not obvious why the females find new mates sooner thanthe males. In this study we investigated whether the sex-biasedmating opportunities stem from biased offspring sex ratios.We determined the sex of newly hatched, precocial chicks usingCHD gene markers. Among fully sexed broods, 0.461 ± 0.024(SE) of chicks (454 chicks in 158 broods) were male, and thissex ratio was not significantly different from unity. The proportionof males at hatching decreased significantly over the breedingseason, which occurred consistently in all 3 years of the study.Large chicks were more likely to be males than females. Neitherparental age nor body size of male and female parents was relatedto brood sex ratio. We also sexed a number of chicks that werecaught after they left their nest (range of estimated ages 0–17days) and found that the proportion of males increased withbrood age. This relationship remained highly significant whencontrolling statistically for hatching date. As brood size decreaseddue to mortality after the chicks left their nest, these resultssuggest that the mortality of daughters was higher than thatof the sons shortly after hatching. Taken together, our resultsshow that the female-biased mating opportunities in the Kentishplover are not due to biased brood sex ratio at hatching but,at least in part, are due to female-biased chick mortality soonafter hatching.     

Colony-level sex ratio selection in the eusocial Hymenoptera

We present an inclusive fitness model on worker-controlled sex investments in eusocial Hymenoptera which expands the existing theory for random mating populations as formulated by Trivers and Hare (1976) and Benford (1978). We assume that relatedness asymmetry is variable among colonies — owing to multiple mating, worker reproduction and polygyny — and that workers are able to assess the relatedness asymmetry in their own colony. A simple marginal value argument shows that “assessing” workers maximize their inclusive fitness by specializing on the production of the sex to which they are relatively more related than the average worker in the population is related to that sex. The model confirms our earlier verbal argument on this matter (Boomsma and Grafen, 1990) and gives further quantitative predictions of the optimal sex ratio of relatedness-asymmetry classes for both infinite and finite, random mating populations. It is shown that in large populations all but one of the relatedness-asymmetry classes should specialize on the production of one sex only. The remaining, balancing class is selected to compensate any bias induced by the other class(es) such that the population sex ratio reflects the relatedness asymmetry of that balancing class. In the absence of worker-reproduction, the sex ratio compensation by the balancing-class is generally close to 100%, unless the population is very small. In the Discussion we address explicitly the likelihood of our relatedness-assessment hypothesis and other assumptions made in the model. The relationship of our model with previous theory on sex allocation in eusocial Hymenoptera is worked out in the Appendix.     

Nestling sex ratio is associated with both male and female attractiveness in rock sparrows

According to theory, in species in which male variance in reproductive success exceeds that of the females, sons are more costly to produce; females mated with high quality males or those in better condition should produce more sons. In monogamous species, however, the variance in the reproductive success of the two sexes is often similar and mate choice is often mutual, making predictions regarding sex allocation more difficult. In the rock sparrow Petronia petronia, both males and females have a sexually selected yellow patch on the breast, whose size correlates with individual body condition. We investigated whether the brood sex ratio co‐varies with the size of the yellow patch of the father and the mother in a sample of 173 broods (818 chicks) over 8 breeding seasons. While the size of the yellow patch of the mother and the father did not predict per se a deviation from the expected 1:1 sex ratio, brood sex ratios were predicted by the interaction of male and female yellow patch size. This result is surprising, as the ornament is sexually selected by both males and females as an indicator of quality in both sexes and should therefore be inherited by all offspring irrespective of their sex. It indirectly suggests that other sex‐specific traits associated with patch size (e.g. polygyny in males and fecundity in females) may explain the sex allocation bias observed in rock sparrows. Thus, female individual quality alone, as expressed through the size of the yellow patch, was not associated with the biases in sex ratios reported in this study. Our results rather suggest that sex allocation occurs in response to male attractiveness in interaction with female attractiveness. In other words, females tend to preferentially allocate towards the sex of the parent with more developed ornament within the pair.     

Sex ratio selection and multi-factorial sex determination in the housefly: a dynamic model

Sex determining (SD) mechanisms are highly variable between different taxonomic groups and appear to change relatively quickly during evolution. Sex ratio selection could be a dominant force causing such changes. We investigate theoretically the effect of sex ratio selection on the dynamics of a multi-factorial SD system. The system considered resembles the naturally occurring three-locus system of the housefly, which allows for male heterogamety, female heterogamety and a variety of other mechanisms. Sex ratio selection is modelled by assuming cost differences in the production of sons and daughters, a scenario leading to a strong sex ratio bias in the absence of constraints imposed by the mechanism of sex determination. We show that, despite of the presumed flexibility of the SD system considered, equilibrium sex ratios never deviate strongly from 1 : 1. Even if daughters are very costly, a male-biased sex ratio can never evolve. If sons are more costly, sex ratio can be slightly female biased but even in case of large cost differences the bias is very small (<10% from 1 : 1). Sex ratio selection can lead to a shift in the SD mechanism, but cannot be the sole cause of complete switches from one SD system to another. In fact, more than one locus remains polymorphic at equilibrium. We discuss our results in the context of evolution of the variable SD mechanism found in natural housefly populations.     

Age, sex ratio and timing of the catch of kelts and ascending Atlantic salmon in the subarctic River Teno

By 15 June, 82% of the catch of Atlantic salmon Salmo salar kelts had been taken from the middle part of River Teno, northern Scandinavia. The median date of capture was 4 June for males and 8 June for females. Salmon of 1–4 sea–winters (SW) of both sexes survived spawning to return to sea as kelts. Among males, 1 SW kelts were caught earliest in the spring and 3 SW latest, but among females 4 SW were earliest, then 3 SW and finally 1 and 2 SW. There were 17 river and sea–age combinations among the kelts compared with 23 among the ascending salmon. The smolt age distribution and the mean smolt age differed significantly only between female 2 SW ascending salmon (3·97 years) and kelts (4·14 years). The proportion of 1 SW females was higher and that of 3 SW males lower among kelts than among ascending salmon. The proportion of males among 1 SW ascending salmon was 80% but among kelts only 57%. Similarly, the proportion of males among 3 SW fish was 21% for ascending salmon but only 7% for kelts. Hence overwinter mortality was higher among males. Male and female kelts of 1 and female kelts of 2 SWhad a greater mean length than ascending salmon in corresponding groups indicating a better survival of larger fish within an age group. Grilse ascend rivers after most kelts have left, but the main catch of ascending 2–3 SW salmon takes place concurrently with kelts leaving the river, inadvertently targeting kelts in the fishery.     

Environmental condition related reproductive strategies and sex ratio in hydras

Kaliszewicz, A. and Lipińska, A. 2011. Environmental condition related reproductive strategies and sex ratio in hydras. —Acta Zoologica (Stockholm) 00 :1–7. Temperature and food supply appeared to affect sex ratio, sex composition and percentage of sexual individuals in three Hydra species: Hydra vulgaris, Hydra circumcincta and Hydra viridissima. We found three sexes present: females, males and hermaphrodites depending on environmental conditions. Hydra vulgaris appeared to be a species with a temperature‐dependent sex determination (TSD). The males and hermaphrodites were present only under rising temperatures, whereas females were observed exclusively at lowering temperatures. Hydras reproduced asexually at constant room temperature. Unlimited food affected sex ratios and induced the presence of males in H. circumcincta at lowering temperatures. Thus, H. circumcincta may be recognised as another Hydra species in which sex is determined by environmental factors (ESD). Under rising temperatures, the number of hermaphroditic individuals was higher when food supply was unlimited in all three species, indicating that hermaphrodites may need more energy to produce both male and female gonads. Both temperature changes and food supply positively affected asexual reproductive strategies in hydras, especially budding rates. Hydra circumcincta appeared to be less agile than other hydras and able to self‐fertilise. It is likely that self‐fertilisation is an adaptation to the low probability of meeting a mate belonging to the other clone.     

Operational sex ratio influences the opportunity for sexual selection in guppies

The opportunity for sexual selection was greater when the operational sex ratio (OSR) in guppies Poecilia reticulata was biased towards males. This could be due to an increase in both male-male competition and female mate choice under male-biased OSR.