Condition Sex Ratio Research Articles

Trends in female-selective abortion among Asian diasporas in the United States, United Kingdom, Canada and Australia.

Preference for sons and smaller families and, in the case of China, a one-child policy, have contributed to missing girl births in India and China over the last few decades due to sex-selective abortions. Selective abortion occurs also among Indian and Chinese diaspora, but their variability and trends over time are unknown. We examined conditional sex ratio (CSR) of girl births per 1000 boy births among second or third births following earlier daughters or sons in India, China, and their diaspora in Australia, Canada, United Kingdom (UK), and United States (US) drawing upon 18.4 million birth records from census and nationally representative surveys from 1999 to 2019. Among Indian women, the CSR in 2016 for second births following a first daughter favoured boys in India (866), similar to those in diaspora in Australia (888) and Canada (882). For third births following two earlier daughters in 2016, CSRs favoured sons in Canada (520) and Australia (653) even more than in India (769). Among women in China outside the one-child restriction, CSRs in 2015 for second order births somewhat favoured more girls after a first son (1154) but more heavily favoured boys after a first daughter (561). Third-birth CSRs generally fell over time among diaspora, except among Chinese diaspora in the UK and US. In the UK, third-birth CSRs fell among Indian but not among other South Asian diasporas. Selective abortion of girls is notable among Indian diaspora, particularly at higher-order births.

Sex-specific patterns of senescence in artificial insect populations varying in sex-ratio to manipulate reproductive effort

BackgroundThe disposable soma theory of ageing assumes that organisms optimally trade-off limited resources between reproduction and longevity to maximize fitness. Early reproduction should especially trade-off against late reproduction and longevity because of reduced investment into somatic protection, including immunity. Moreover, as optimal reproductive strategies of males and females differ, sexually dimorphic patterns of senescence may evolve. In particular, as males gain fitness through mating success, sexual competition should be a major factor accelerating male senescence. In a single experiment, we examined these possibilities by establishing artificial populations of the mealworm beetle, Tenebrio molitor, in which we manipulated the sex-ratio to generate variable levels of investment into reproductive effort and sexual competition in males and females.ResultsAs predicted, variation in sex-ratio affected male and female reproductive efforts, with contrasted sex-specific trade-offs between lifetime reproduction, survival and immunity. High effort of reproduction accelerated mortality in females, without affecting immunity, but high early reproductive success was observed only in balanced sex-ratio condition. Male reproduction was costly on longevity and immunity, mainly because of their investment into copulations rather than in sexual competition.ConclusionsOur results suggest that T. molitor males, like females, maximize fitness through enhanced longevity, partly explaining their comparable longevity.

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

ABSTRACTThe 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.

Same-sex sexual behaviour as a by-product of reproductive strategy under male–male scramble competition

A variety of hypotheses have been suggested to explain the evolution of same-sex sexual behaviour (SSB) in male insects. Males may be poor at discriminating other males from females, inexperienced, or may need to learn how to distinguish females. Alternatively SSB could be a by-product of a plastic reproductive strategy to succeed in scramble competition. Such a strategy could be adaptive if the cost of missing the chance to mate with available females exceeds the cost of mounting single males instead (i.e. SSB). We tested predictions derived from these hypotheses by experimentally varying the sex ratio and thus the intensity of scramble competition in the water strider Tenagogerris euphrosyne. We allowed males to experience female-biased or male-biased sex ratio treatments for a week, and then compared their expression of SSB under standard conditions. We also measured daily mating success of males during the treatment and related this to SSB frequency under assay conditions. Males kept in male-biased sex ratio conditions exhibited more SSB, mounting males and females equally often under assay conditions. Males kept under female-biased conditions only mounted males one-quarter as often as they mounted females in the assay. Male mating experience under treatment conditions did not influence the expression of SSB in the assay. In a second experiment we found that males that were more likely to exhibit SSB enjoyed higher initial mating success in a subsequent mating assay in a male-biased sex ratio. Our results indicate that plasticity in reproductive strategy not only increases mating opportunity but also confers a reproductive benefit. Taken together, SSB in T. euphrosyne is a highly labile trait responsive to sex ratio that delivers increased mating success under intense scramble competition.

Evolution of individual variation in behaviour and behavioural plasticity under scramble competition

In scramble competition mating systems, individual behaviours related to male–male competition are expected to be plastic across sociosexual environments. However, the evolutionary forces that shape individual differences in behavioural plasticity remain poorly understood. We measured behaviour associated with scramble competition in male water striders, Tenagogerris euphrosyne, in two social contexts. Male T. euphrosyne tend to mount other individuals or pairs indiscriminately and then dismount if their target is either another male or a mating pair. We measured the speed with which a focal male dismounted (i.e. his sensitivity to having made an inappropriate mounting) in each of these contexts. We quantified temporal consistency of dismount speed within each context, as well as individual variation in behavioural plasticity across contexts. We then estimated the effect of the behaviour and of plasticity on individual fitness in different sex ratio conditions. We found that individuals differed in their recognition sensitivity traits, and also in how plastic their sensitivity (i.e. dismount speed) was to context (mating status or sex). When we measured the mating success of each male within experimental groups kept at four different sex ratios, recognition sensitivity traits in both contexts interacted to influence mating success, and there were significant differences in linear selection on sensitivity traits between sex ratio treatments. We discuss how insensitive males evolve, and how individual variation in behavioural plasticity could be maintained by selection or environmental variability.

Growth, Mortality, and Biochemical Composition inOctopus vulgarisReared Under Different Conditions of Sex Ratio

AbstractOctopus vulgarisis a potential candidate to diversify marine aquaculture. Even though mortality under rearing conditions has been related to reproductive processes, the benefit of sex segregation remains unclear. In order to clarify this, wild octopuses (862 ± 101 g) were reared in floating cages under three proportions of male : female sex ratio: 1:1, 1:0, and 4:1 (n = 30 per treatment). The experimental period lasted 2 mo and octopuses were fed on bogue,Boops boops(aquaculture by‐product). Higher growth rates were observed in octopuses reared under 1:1 and 1:0 (1.8%/d) in comparison with those reared under 4:1 conditions (1.6%/d). Regarding sexes, a lower growth was detected in females reared under 4:1 (1.1%/d) in comparison with females (1.8%/d) and males (1.7–1.9%/d) reared under 1:1 conditions. Survival was 97, 97, and 90% in sex ratio 1:1, 1:0, and 4:1, respectively. Sexual maturity data showed that males were all mature, while most females were still maturing (60–84%) at the end of the rearing period. High lipid content in bogue (44% dw) did not reflect on octopus muscle. This tissue showed a similar biochemical composition irrespective of the sex ratio condition, with a high protein (87% dw), a low lipid (5% dw), and a high n‐3HUFAcontent (42%).

Adult sex ratio affects divorce rate in the monogamous endoparasite Schistosoma mansoni

“Divorce” (mate switching) rate is known to vary largely both between and within socially monogamous species. Although the adult sex ratio can have an important influence on mating patterns, very few studies have investigated the influence of sex ratio on divorce rate in monogamous species, and even less so from an experimental point of view. In addition, most studies on the causes and consequences of divorce have been performed on vertebrate species, whereas data for invertebrate monogamous species remain scarce. Schistosoma mansoni is a monogamous endoparasite with a complex life cycle characterized by asexual reproduction in the intermediate host and sexual reproduction in the definitive host. In the wild, populations of S. mansoni inside their definitive hosts are characterized by a male-biased sex ratio. We studied the influence of experimentally varying the adult sex ratio on divorce rate in S mansoni, using controlled infections of hosts with clonal populations. The more male-biased the sex ratio was, the more the divorce rate increased, whereas no such effect was observed under a female-biased sex ratio. In this study and for the first time, we showed, by handling the sex ratio, that the divorce rate increases in adult male-biased sex ratio conditions in a monogamous species.

Sexual conflict over the duration of copulation in Drosophila montana: why is longer better?

BackgroundConflicts of interest between the sexes are increasingly recognized as an engine driving the (co-)evolution of reproductive traits. The reproductive behaviour of Drosophila montana suggests the occurrence of sexual conflict over the duration of copulation. During the last stages of copulation, females vigorously attempt to dislodge the mounting male, while males struggle to maintain genital contact and often successfully extend copulations far beyond the females' preferred duration.ResultsBy preventing female resistance, we show that females make a substantial contribution towards shortening copulations. We staged matings under different sex ratio conditions, and provide evidence that copulation duration is a form of male reproductive investment that responds to the perceived intensity of sperm competition as predicted by game theoretical models. Further, we investigated potential benefits to persistent males, and costs to females coerced into longer matings. While males did not benefit in terms of increased progeny production by protracting copulation, female remating was delayed after long first copulations.ConclusionCopulation time is a trait subject to sexual conflict. Mating durations exceeding female optima serve males as a form of 'extended mate guarding': by inducing mating refractoriness in the female, a male extends the time over which its sperm is exclusively used to sire progeny and reduces the likelihood of the female being reinseminated by a competitor.

Zur Ökophysiologie, Sexualität und Populationsgenetik litoraler Gammaridea — ein Überblick

Comparative investigations on the physiological capacities in the euryhaline amphipodsGammarus locusta, G. oceanicus, G. salinus, G. zaddachi andG. duebeni were reviewed. In order to assess the adaptations of these species to the abiotic conditions of their environment, the following criteria were examined: oxygen consumption in relation to ambient salinity and temperature levels, respiratory responses following osmotic stress, resistance capacities to oxygen deficiency, resistance to aerial exposure and the simultaneous presence of hydrogen sulphide. Covering the range from marine to typically brackish-water inhabitants, the 5 species show adaptive responses in the above-mentioned order. Respiration is less intensely modified by external factors, and oxygen consumption decreases. Accompanied by faster rates of acclimation to new steady states of performance, resistance capacities increase. The significance of the findings obtained is discussed in relation to the environmental requirements of the amphipods considered. Based on breeding experiments, the sex-determining systems reported thus far inGammarus species are outlined. As demonstrated inG. duebeni, a more or less pronounced influence of external factors such as photoperiod may become effective. A preponderance of males was noted when offspring were raised under long-day photoperiods, whereas females prevailed under short-day conditions. In terms of the critical daylength, the light per day was estimated as being between 13 and 14 h (Elbe estuary population). Feminizing microporidians (Octosporea effeminans, Thelohania herediteria), which are transovarially transmitted, can interfere with the system of sex determination and sex differentiation of the host. As reflected in variousG. duebeni populations, they cause a maternally transferred sex-ratio condition by the production of all-female broods, thereby mimicking extrachromosomal inheritance. An increase of the salinity level to 25–30‰ results in a disappearance ofO. effeminans. In both microsporidians, long exposure to low temperatures (≤4°C) produces eggs which are not all parasitized. Furthermore, intersexuality can be induced by changing environmental factors. Microsporidian species have no influence on sex differentiation inG. duebeni celticus, G. salinus, G. locusta andG. pulex. Patterns of relative electrophoretic mobilities of proteins and the distribution of allele frequencies at polymorphic gene loci can be utilized for species diagnosis and for the evaluation of the relationships between different taxa, particularly at and below the species level. As exemplified by studies on several gammarids from marine, brackish and freshwater environments, inter- and infraspecific gene-enzyme variation is described. Electrophoretic investigations on natural populations of the euryhaline amphipodsG. zaddachi, G. salinus, G. tigrinus and others from different geographic areas provided evidence of considerable biochemical genetic variation. InTalitrus saltator- andTalorchestia deshayesii-populations the extent of variability based on micro-and macrogeographic aspects is illustrated. The large-scale genetic divergence is demonstrated by comparison of samples obtained from the Baltic, North, Atlantic and northern Mediterranean Seas.

Interaction between genetic, external and parasitic factors in sex determination of the crustacean amphipodGammarus duebeni

Sex determination inGammarus duebeni duebeni was analyzed by breeding experiments conducted with individuals obtained from various populations. Sex ratio of progeny depends on genetic factors and photoperiod. Generally, a preponderance of males was noted when offspring were raised under long-day photoperiods, whereas females prevailed under short-day photoperiods (Bulnheim, 1967, 1969). Based on these previous studies the critical daylength, the transition point at which the photoperiodic response switches, was estimated in specimens from a population of the Elbe estuary as ranging between 13 and 14 hours of light per day at 15° C. The susceptibility to photoperiod is assumed to depend on the balance between a system of male and female determining genetic factors that act on a polygenic basis. From various coastal brackish-water populations thelygenic (all-female) strains could be selected. This maternally inherited sex-ratio condition is caused by the transovarially transferred microsporidiansOctosporea effeminans andThelobania herediteria which, independently of each other, exert a feminizing influence on the host's offspring. Either perfectly or imperfectly thelygenic females may occur. The latter produce eggs that are not all infected; hence their progeny are bisexual. As a consequence of the sex-determining influence of the microsporidians males are generally not parasitized. However, some males associated withO. effeminans were found in a strain derived from a population at Bornholm (Baltic Sea). Also, males infested with one or both of the two parasite species occur in some populations ofG. d. celticus, indicating that in these sex determination is not governed by the microsporidians concerned. The feminizing influence of the parasites may be affected by environmental factors. An increase of the ambient salinity level to 25–30 ‰ results in a disappearance ofO. effeminans in the eggs released by infectedG. d. duebeni females. Thus, the normally acting switch mechanism of sex determination is re-established. In addition, long exposures to low temperatures (≦4° C) may have an adverse effect on the vegetative stages of both microsporidians. Owing to this, infected females may produce eggs which are not all parasitized. Consequently, mixed progeny may arise. Studies on the incidence rates of the two microsporidians. comprising 18 populations indicate that the parasites are widespread in their geographical range. Most populations studied in the Baltic Sea area and brackish-water habitats of the German North Sea coast are associated with either one or both of the two microsporidian parasites. Observations on the sex ratio of a population from the Elbe estuary, performed over several years, revealed considerable seasonal fluctuations. The percentages of infected females, however, remained at a fairly constant level during the study period. The results obtained are discussed with reference to the mechanisms of sex determination and differentiation as revealed in other amphipod species, in particular to the interaction of genetic and non-genetic factors and the occurrence of monogeny phenomena.

Spirochaete-mediated abnormal sex-ratio (SR) condition in Drosophila: a second virus associated with spirochaetes and its use in the study of the SR condition.

SUMMARYA class of abnormal sex-ratio (SR) condition in Drosophila characterized by the elimination of male zygotes is caused by infection with maternally transmitted SR-spirochaetes. A given spirochaete strain, WSR, was shown to have an associated DNA virus, spv-2, in a latent condition. This virus is induced to multiply when another DNA virus, previously designated spv-1, infects WSR spirochaetes; spv-1 is normally associated with another strain of SR-spirochaetes, NSR, and is in a manifest state. Spv-2 infects and lyses NSR, thus eliminating spirochaetes from fly hosts which, in these experiments, were D. melanogaster.The elimination of WSR through lysis by spv-1 results in the ‘curing’ of the SR-condition in the host flies. However, the elimination of NSR by spv-2 does not lead to the immediate elimination of the SR-condition in the host. A hypothesis is presented in favour of the view that a substance, androcidin, is produced by SR-spirochaetes and that this substance is actually responsible for the death of male zygotes. NSR may produce a more potent androcidin than WSR.

The sex ratio condition in Drosophila bifasciata; Its experimental transmission to several species of Drosophila

The following evidence has been accumulated in this study of the sex ratio ( SR) condition in Drosophila bifasciata: A distinct cytopathology is present in dying males. An infectious material is isolable from the ovaries of SR females. This material, injected into normal healthy hosts, significantly alters the sex ratios of the injected flies and their progenies. Therefore, in view of these transfer-experiment results and the distinctive cytopathology previously described, it is postulated that the SR condition in D. bifasciata is due to an infectious agent exhibiting many viral characteristics: submicroscopic size, intracellular site, nucleopathic effects when multiplying, contagious spread by cell lysis, and affinity for neural and epidermal tissues. The SR agent resembles the sigma virus of Drosophila melanogaster with a stabilized strain, SR, and non-stabilized strains, “ia” and “ib.” Most of the normal flies tested appear to be refractory; the transferred condition is difficult to establish and maintain.

The expression of maternally-transmitted sex ratio condition (SR) in two strains of Drosophila melanogaster.

A study was made ofwillistoni-SR infections, transmitted for many generations through the maternal line, in two wild-type strains ofD. melanogaster. As in the species of origin, the SR agent is essentially lethal only to males. The twomelanogaster strains differ from each other in the time required for the effects of SR to be expressed, both in terms of the first appearance of SR broods and in the developmental stages at which death of males occurs. Host genotype within a species is thus an important factor in the expression of SR. Host age and breeding temperature are also of consequence in determining the degree to which SR is expressed. In any given instance, the specific expression of SR is determined by complex interactions between host, SR agent, and the environment.

An Unusual Sex Ratio in White-Tailed Deer

The breeding ages, sex ratios, and densities of white-tailed deer (Odocoileus virginianus) are described for a 746-acre enclosure at Albany, Georgia, where, over a 3-year period, 30 fawns were reared in the ratio of four bucks per doe. The nearly even sex ratio among adults invites consideration of the theory that the first service of a buck each year produces predominantly male offspring, while subsequent breedings produce predominantly female offspring. A 746-acre enclosure at the Marine Corps Supply Center, Albany, Georgia, was stocked in 1959 and 1960 with white-tailed deer. When all deer were killed in 1963 to terminate the first segment of the study, it was found that a very unusual sex ratio of four bucks per doe existed among fawns. A complete description of deer densities, sex ratios, and ages of the population for 3 years under study in Georgia is presented here. POPULATION CONDITIONS Prehunt populations within the enclosure varied from 32 deer in 1960 and 1961 to 16 deer in 1963. Posthunt populations varied from 21 deer in 1961 to 10 deer in 1963. The rut was apparently completed by mid-November of each year. Twelve to 17 deer were removed annually in December with 48 man-days of hunting. In December, 1963, every deer was killed and the herd reconstructed for each year of the study. The breeding and fawning season populations and fawns reared during the last 3 years of the study are presented in Table 1. Only bucks 11/2 years old and older were considered capable of breeding, while every doe, regardless of age, was considered a breeder. The single exception, a fawn which did not ovulate, is not included in the table. Fawns born in 1959 and 1960 were not considered because they were conceived under unknown density and sexratio conditions prior to release in the enclosure. Breeding season ages of the 31 adult bucks and the 23 fawning does are given in Table 2. Sixteen additional does bred but were killed in hunts that followed the

Interference between "Sex-Ratio" Agents of Drosophila willistoni and Drosophila nebulosa

Interference between two "sex-ratio" agents-that is, treponemalike spirochetes-of different origin, one from Drosophila willistoni and the other from D. nebulosa, was demonstrated by experiments in vivo and in vitro. When the two sex-ratio agents were combined in females of the Oregon-R strain of D. melanogaster, expression of the sex-ratio condition was temporarily interrupted. Several lines of evidence indicate that a substance produced by the sex-ratio agent of D. nebulosa may kill or incapacitate the sex-ratio agent of D. willistoni.