Egg Sex Ratio Research Articles

Testing the local mate competition rule in a quasi-gregarious parasitoid with facultative superparasitism

Abstract As an important domain of evolutionary ecology, sex allocation theory well explains the evolution of investment into female versus male offspring. Local mate competition (LMC) is a good predictor of sex allocation, where the optimal sex ratio becomes less female-biased and asymptotically approaches 0.5 as the number of foundresses increases. Parasitoid wasps, with haplodiploid sex determination, offer excellent opportunities to test how organisms manipulate their offspring sex ratio in response to environmental variation, and many species have been proved to allocate sex according to predictions under LMC. When hosts are spatially clustered, as in gregarious species, the mating systems of quasi-gregarious parasitoids meet the essential assumptions (female mating before dispersal) of LMC. However, inconsistent with predictions, in the quasi-gregarious species Anastatus disparis (Hymenoptera: Eupelmidae), a strongly female-biased eclosion sex ratio (0.156 ± 0.018 to 0.185 ± 0.016) was observed as the number of females laying eggs in a patch increased. Superparasitism, in which 44.7% of parasitized hosts contained more than one egg but only one adult emerged from each host, was common in A. disparis. However, the egg sex ratio was determined by microsatellites and likely fit the predictions of LMC theory. Male-biased offspring mortality arising from superparasitism during development likely contributes to the shift from the primary sex ratio predicted under LMC to the observed female-biased eclosion sex ratio. Inconsistent with results in gregarious parasitoids, the role of superparasitism in driving sex ratio shifts in quasi-gregarious parasitoids should be incorporated into LMC-based predictions of sex ratios.

How Egg Storage Duration Prior to Incubation Impairs Egg Quality and Chicken Embryonic Development: Contribution of Imaging Technologies.

Storing fertilised eggs prior to incubation is a frequent practice in commercial hatcheries to coordinate activities and synchronise hatchings. However, the conditions used to store eggs can have major impacts on egg quality and the subsequent viability of chicken embryos. While storage temperatures of 16–18°C are classically used in hatcheries, the duration of storage varies from three to more than 10 days. We explored the effect of storage duration (zero, three or 10 days; D0, D3 and D10, respectively) at 16°C, 80% relative humidity (RH) on egg quality (Broiler, Ross 308), using computed tomography (CT) and classical measurements (egg weight, eggshell strength, egg white pH, Haugh units, yolk index and colour). The results revealed that a storage duration of up to 10 days negatively affected some egg quality traits (yolk index and volume, air chamber volume and egg white pH). Eggs stored for three or 10 days were further incubated for 11, 13 or 15 days (37.8°C, 55% RH). Eggs were analysed by magnetic resonance imaging (MRI) and CT to assess the development of the embryo and internal egg changes occurring during incubation. First, data showed that the fertility and sex ratio of eggs were not affected by storage duration. However, the mortality of viable eggs was increased in the D10 group compared to the D3 group. Results of non-invasive imaging technologies revealed that the storage of eggs for 10 days impaired embryo growth as early as 11 days of incubation (decrease in brain and embryo volumes). Collectively, these data provide new evidence that the duration of egg storage negatively affects embryonic growth. They further corroborate that this parameter is likely to be crucial to synchronising embryonic stages and maybe reducing the hatching window, hence limiting the time spent by newborn chicks in hatchers. In addition, our results highlight that CT and MRI imaging technologies are useful non-invasive tools to evaluate egg quality prior to incubation and the impact of storage (or incubation) practices on developmental growth of the embryo.

Effects of periodically repeated high-temperature exposure on the immediate and subsequent fitness of different developmental stages of Propylaea japonica.

Repeated extreme high temperature occurs frequently in summer. Propylaea japonica is a predominant predator in South-East Asia and has been considered as a successful natural enemy to control aphids. However, how repeated extreme high temperature affects the fitness of P. japonica remains unclear. This study evaluated the immediate and subsequent fitness of P. japonica when egg, larva, pupa, and adult were exposed to repeated high temperatures (39, 41, or 43 °C for 3h exposure duration per day) during several days. The effect of repeated high temperatures on P. japonica fitness was stage-specific: the egg stage was the most sensitive, the larval and pupal stages were moderately resistant and the adult stage was the most resistant to heat. Repeated high temperatures extended the immature developmental time and decreased the sex ratio of eggs treated with these temperatures, compared to control eggs. A temperature of 39 °C had no significant effect on the pre-oviposition period, oviposition period, fecundity (except stress pupa), or longevity compared with the control, but negative carry-over effects above 39 °C on subsequent stages were found. Repeated high temperature for consecutive days not only had a significant effect on the immediate survival and developmental time, but also had deleterious effects on the subsequent development and performance of P. japonica. The present study provides valuable information for understanding and utilizing P. japonica to control aphids in challenging environments. © 2022 Society of Chemical Industry.

Identification of Empoasca onukii (Hemiptera: Cicadellidae) and Monitoring of its Populations in the Tea Plantations of South China.

Tea green leafhoppers (Empoasca spp.) are considered one of the major pests in tea plantations in Asia. They are, however, difficult to monitor due to their size and flying and jumping abilities. In this study, we clarified the identification of the leafhopper species encountered in our study plantations and examined the impacts of sampling methods in estimating population abundance and sex ratio. The natural sex ratio of eggs, nymphs, and adults of tea green leafhopper and the differences between male and female were tested. Despite previous reports that Empoasca vitis (Goethe) was the major leafhopper present in our study area, our results showed that only Empoasca onukii Matsuda was found. Variation in population size over time and bias in sex ratio depending on the sampling methods were found in our monitoring experiments. In general, adult males were more attracted to yellow sticky cards than females. We believe that because female leafhoppers should be the target in pest control, yellow sticky cards may not be the most suitable monitoring or effective control of tea green leafhopper. We demonstrate the importance of understanding the implications of sampling techniques for population estimation and sex ratio bias as well as how temporal variation may affect monitoring results. Precise monitoring should take into consideration the different life histories of male and female.

Do sex ratio and development differ in sexually size-dimorphic shiny cowbirds (Molothrus bonariensis) parasitizing smaller and larger hosts?

Fil: Tuero, Diego Tomas. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Instituto de Ecologia, Genetica y Evolucion de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Ecologia, Genetica y Evolucion de Buenos Aires; Argentina

Maternal testosterone affects the primary sex ratio and offspring survival in zebra finches

Female birds have repeatedly been reported to adjust the primary sex ratio of their offspring to environmental, social and physiological cues. However, the mechanism behind sex adjustment remains unknown. It has been suggested that maternal hormones may constitute an important mediator in this mechanism, as androgen levels differ between eggs bearing male and female embryos. To evaluate whether the level of maternal androgens affects the offspring sex ratio, we injected female zebra finches, Taeniopygia guttata, with testosterone during egg laying. The sex ratio of eggs laid after testosterone administration became significantly male biased, compared to eggs laid by control females that received a vehicle injection. However, sons of testosterone-treated females suffered lower hatching success. In contrast, daughters seemed to benefit from elevated androgen level in terms of future survival prospects. The opposite effects on male and female offspring may constitute an important constraint on maternal androgen allocation to the eggs and reduce the benefits of biasing the sex ratio towards males by increasing the testosterone level.

Sex Allocation Conflict in Ants: When the Queen Rules

Insect societies are paramount examples of cooperation, yet they also harbor internal conflicts whose resolution depends on the power of the opponents. The male-haploid, female-diploid sex-determining system of ants causes workers to be more related to sisters than to brothers, whereas queens are equally related to daughters and sons. Workers should thus allocate more resources to females than to males, while queens should favor an equal investment in each sex. Female-biased sex allocation and manipulation of the sex ratio during brood development suggest that workers prevail in many ant species. Here, we show that queens of Formica selysi strongly influenced colony sex allocation by biasing the sex ratio of their eggs. Most colonies specialized in the production of a single sex. Queens in female-specialist colonies laid a high proportion of diploid eggs, whereas queens in male-specialist colonies laid almost exclusively haploid eggs, which constrains worker manipulation. However, the change in sex ratio between the egg and pupae stages suggests that workers eliminated some male brood, and the population sex-investment ratio was between the queens' and workers' equilibria. Altogether, these data provide evidence for an ongoing conflict between queens and workers, with a prominent influence of queens as a result of their control of egg sex ratio.

Discrimination of larval sex and timing of male brood elimination by workers in honeybees (Apis mellifera L.)

To test a hypothesis that workers in social Hymenoptera can identify male broods at early stages of development and eliminate them to avert excessive investments in male brood, the ability of larval sex discrimination and the timing of male brood elimination by honeybee (Apis mellifera L.) workers were determined. A comparison between the egg sex ratio and the sex ratio of 5 day-old larvae showed that male larvae might selectively die and/or be eliminated in colonies with low survival rates of broods. Nurse bees conditioned by the extracts of male larval cuticles responded significantly more frequently to male extracts than female ones extracted from 3 and 4–5 day-old larvae. It is known that during the developmental stage of 4–5 days, the weight sharply increases in male larvae. The nurse bees conditioned by the extracts of male comb cells failed to distinguish between male and female extracts. Honeybee workers can thus discriminate the sex of broods for male elimination based on cuticular substances during the larval periods with the highest growth rates, which may avert excessive investments in male brood.

Subtle manipulation of egg sex ratio in birds

Mothers are predicted to overproduce male or female eggs when the relative fitness gains from one sex are higher and outweigh the costs of manipulation. However, in birds such biases are often difficult to distinguish from differential embryo or chick mortality. Using a molecular technique to identify the sex of early embryos, we aim to determine the effect of maternal nutrition on zebra finch (Taeniopygia guttata) egg sex ratios after 2 days of incubation, which is as close to conception as is currently possible. We found no overall bias in the sex ratio of eggs laid and sex did not differ with relative laying order under any diet regime. However, mothers on a low-quality diet did produce a female bias in small clutches and a slight male bias in large clutches. On a high-quality diet, mothers produced a male bias in small clutches and a female bias in large clutches. Those on a standard diet produced a roughly even sex ratio, irrespective of clutch size. These observed biases in egg sex are partly in line with predictions that, in this species, daughters suffer disproportionately from poor rearing conditions. Thus, when relatively malnourished, mothers should only rear daughters in small broods and vice versa. Sex-ratio patterns in this species therefore appear to be subtle.

Egg sex ratio and paternal traits: using within-individual comparisons

Empirical studies of sex ratios in birds have been limited due to difficulties in determining offspring sex. Since molecular sexing techniques removed this constraint, the last 5 years has seen a great increase in studies of clutch sex ratio manipulation by female birds. Typically these studies investigate variation in clutch sex ratios across individuals in relation to environmental characteristics or parental traits, and often they find no relationships. In this study we also found that clutch sex ratios did not vary in relation to a number of biological and environmental factors for 238 great tit Parus major nests. However, interesting sex ratio biases were revealed when variation in clutch sex ratios was analyzed within individual females breeding in successive years. There was a significant positive relationship between the change in sex ratio of a female's clutch from one year to the next and the relative body condition of her partner. Females mating with males of higher body condition in year x + 1 produced relatively male-biased sex ratios, and the opposite was true for females mated with lower condition males. Within-individual analysis also allowed investigations of sex ratio in relation to partner change. There was no change in sex ratios of females pairing with the same male; however, females pairing with a new male produced clutches significantly more female biased. Comparisons of clutch sex ratios within individuals may be a powerful method for detecting sex ratio variation, and perhaps female birds may indeed manipulate egg sex but require personal contextual experience for such decisions. Copyright 2002.

Pre-ovulation control of hatchling sex ratio in the Seychelles warbler.

Females of some bird species have a high degree of control over the sex ratio of their offspring at laying. Although several mechanisms have been put forward to explain how females might control the sex of their eggs, virtually nothing is known. As females are the heterogametic sex in birds, adjustment of the clutch sex ratio could arise either by pre- or post-ovulation control mechanisms. The Seychelles warbler (Acrocephalus sechellensis) exhibits extreme adaptive egg sex ratio bias. Typically, warblers produce only single-egg clutches, but by translocating pairs to vacant habitat of very high quality, most females were induced to produce two-egg clutches. Overall, females skewed clutch sex ratios strongly towards daughters (86.6%). This bias was evident in the first egg, but critically, also in the second eggs laid a day apart, even when all absent, unhatched, or unsexed second eggs were assumed to be male. Although a bias in the first egg may arise through either pre- or post-ovulation mechanisms, the skew observed in second eggs could only arise through pre-ovulation control. Post-ovulation adjustment may also contribute to skewed hatchling sex ratios, but as sex-biased release of gametes is likely to be a more efficient process of control, pre-ovulation mechanisms may be the sole means of adjustment in this species. High fitness differentials between sons and daughters, as apparent in the Seychelles warblers, may be necessary for primary sex ratio adjustment to evolve.

Primary Sex-Ratio Variation in Two Brood Parasitic Birds: Brown-Headed Cowbird (Molothrus Ater) and Common Cuckoo (Cuculus Canorus)

Abstract We examined primary sex-ratios of two brood parasitic species, Brown-headed Cowbird (Molothrus ater) and the Common Cuckoo (Cuculus canorus), to determine whether there was any evidence of primary sex-ratio manipulation as has been demonstrated in other species of birds. Despite good reasons for why female brood parasites should manipulate the primary sex-ratio of their young, we found a lack of evidence for a bias in the sex-ratio of eggs produced at the population level, with respect to the host species parasitized or time of breeding season, or in terms of the sex ratio of eggs produced by individual females. Thus, this study provides another example in birds of little evidence for sex-ratio variation in relation to environmental factors.

Laying date and laying sequence influence the sex ratio of crimson rosella broods

We examine the patterns of sex allocation in crimson rosellas Platycercus elegans, a socially monogamous Australian parrot. Overall, 41.8% of nestlings were male, a significant female bias. However underlying this population-level bias were non-random patterns of sex allocation within broods. Broods produced early in the season were female-biased, but the proportion of males in a brood increased as the breeding season progressed. Female rosellas may obtain greater fitness benefits from early-fledging daughters than sons because daughters can breed as 1-year-olds whereas sons do not breed until they are at least 2 years old. Laying date and laying sequence also interacted to influence the sex ratio of eggs. The sex of early-laid eggs strongly followed the brood level pattern, whereas the sex of middle- and late-laid eggs did not change significantly as the season progressed. Nevertheless, late-laid eggs were very unlikely to be male at the end of the season. We argue these differing seasonal patterns reflect the relative costs and benefits to producing early-hatched males and females at different times of the season. Female rosellas appear to maximise the probability that daughters are able to breed early but to minimise competitive asymmetries within the brood. In particular, late-hatched male chicks are disadvantaged if their oldest sibling is male, explaining the dearth of broods containing late-hatched males at the end of the breeding season.

PRIMARY SEX-RATIO VARIATION IN TWO BROOD PARASITIC BIRDS: BROWN-HEADED COWBIRD (MOLOTHRUS ATER) AND COMMON CUCKOO (CUCULUS CANORUS)

Abstract We examined primary sex-ratios of two brood parasitic species, Brown-headed Cowbird (Molothrus ater) and the Common Cuckoo (Cuculus canorus), to determine whether there was any evidence of primary sex-ratio manipulation as has been demonstrated in other species of birds. Despite good reasons for why female brood parasites should manipulate the primary sex-ratio of their young, we found a lack of evidence for a bias in the sex-ratio of eggs produced at the population level, with respect to the host species parasitized or time of breeding season, or in terms of the sex ratio of eggs produced by individual females. Thus, this study provides another example in birds of little evidence for sex-ratio variation in relation to environmental factors.

Nutritional factors affecting the egg sex ratio adjustment by a honeybee queen

To determine whether and how honeybee (Apis mellifera L.) queens control the proportion of male reproductives, experiments were done with colonies under two nutritional conditions at two seasons. During the reproductive season the proportion of male eggs laid by queens under insufficiently-food-supplied conditions was lower than that under sufficiently-food-supplied conditions. The smaller proportion of male egg production could not be accounted for by cannibalization of male eggs by workers. The workers' allocation to male cell construction did not differ between sufficiently- and insufficiently-food-supplied conditions. During the non-reproductive season, however, queens showed much reduced or nearly no production of male eggs, even if the colonies were sufficiently supplied with food. These results suggest that the honeybee queen adjusts the egg sex ratio by referring to both the nutritional resources and their own intrinsic seasonal factors.

Nestling sex ratio variation in the cooperatively breeding acorn woodpecker ( Melanerpes formicivorus )

We examined sex ratio variation in the cooperatively breeding acorn woodpecker (Melanerpes formicivorus) over a 27-year period in central coastal California. The sex ratio of eggs is even or possibly biased toward females. However, hatching asynchrony and brood reduction differentially favor the slightly larger males such that the sex ratio when young become self-sufficient is 54.0% males, slightly but significantly different from 50:50. This bias, or at least the differential mortality that produces it, does not appear to be explainable by any of the major hypotheses for an overall sex bias. These include the repayment model, local resource competition, and sexual size dimorphism, all of which predict a female bias or a male bias significantly smaller than that observed. On an individual level, we found no evidence for facultative sex ratio manipulation related to annual differences in the acorn crop, apparent seasonal declines in food availability, or differences in territory quality. We also found no evidence for a non-binomial distribution of sexes within broods despite the potential fitness benefits of creating large coalitions of same-sexed siblings. Males inherit their natal territories more frequently than do females, and we speculate that birds may invest more in sons either to increase the probability that they will be able to pass along their territory to their descendants or to facilitate their sons' ability to compete as future cobreeders. Alternatively, acorn woodpeckers may be unable to fully compensate for the ancillary bias caused by differential juvenile survivorship, leading to unequal investment in the sexes contrary to the prediction of Fisher. Opposite to what has been found in other birds and mammals, the faster growth and larger size of male nestlings appears to confer a survival benefit rather than a cost.

Sex ratio adjustment in relation to paternal attractiveness in a wild bird population.

When the relative fitness of sons and daughters differs, sex-allocation theory predicts that it would be adaptive for individuals to adjust their investment in different sexes of offspring. Sex ratio adjustment by females in response to the sexual attractiveness of their mate would be an example of this. In vertebrates the existence of this form of sex ratio adjustment is controversial and may be confounded with sex-biased mortality, particularly in sexually size-dimorphic species. Here we use PCR amplification of a conserved W-chromosome-linked gene to show that the sex ratio within broods of a natural population of sexually size-monomorphic collared flycatchers Ficedula albicollis is related to the size of their father's forehead patch, a heritable secondary sexual character implicated in female choice. Experimental manipulations of paternal investment, which influence the size of his character in future breeding attempts, result in corresponding changes in the sex ratio of offspring born to males in those breeding attempts. In contrast, manipulations of maternal investment have no effect on future sex ratios, and there is no relationship between variables predicting the reproductive value of the brood and nestling sex ratio. Analysis of recruitment of offspring reveals similar patterns of sex ratio bias. The results suggest that female collared flycatchers be able to adjust the sex ratio of eggs ovulated in response to the phenotype of their mate. This finding is most consistent with "genetic quality" models of sexual selection.

Sex ratio regulation in the waspEupelmus vuilleti (crwf.), an ectoparasitoid on bean weevil larvae (Hymenoptera: Pteromalidae)

In Africa, the seeds and/or pods ofVigna unguiculata andVigna radiata (Papilionacea) are attacked in fields and storage structures by bruchid beetlesCallosobruchus maculatus andBruchidius atrolineatus, on which parasitoid communities can develop. One of these parasitoids is the solitary ectoparasitoidEupelmus vuilleti (Eupelmidae). The storage conditions ofV. unguiculata andV. radiata favor the overlapping of all host stages during several months. These conditions suggest that female parasitoids would vary the sex ratio of their offspring according to the different sizes or developmental stages of hosts. The sex ratio ofE. vuilleti was strongly related to the developmental stage of the hostC. maculatus. Under our experimental conditions, where superparasitism is rare, the proportion of daughters varied between 5 and 25% on the third larval stage but reached 70 to 90% on the pupae. The increase in the proportion of daughters was also observed in the absence of superparasitism. In this case, there was an absolute coincidence between the sex ratio of eggs laid and that of emerged adults. Manipulation of the sex of the egg by the females seems to take place at the time of the egg's fertilization. The relation between host weight and egg sex showed that the male eggs are preferentially laid on lighter host larvae and the female eggs on heavier ones.

Biological Notes on the Mealybug Dysmicoccus obesus (Homoptera: Pseudococcidae) on Loblolly Pine in Southern Arkansas

Dysmicoccus obesus (Lobdell) was first found in Arkansas in 1983 living in crevices, under bark scales of loblolly pines (Pinus taeda L.). Most mealybugs (77%) were found on the bole between 0–90 cm of the ground, and they showed slight preferences for the northern and southern bole exposures. Crematogaster ants were observed tending the mealybugs. Three broods per year were detected, with adults produced in May, July and September. Dysmicoccus obesus probably overwinter off the tree as immatures. Cytogenetic observations showed D. obesus was sexual and the sex ratio of eggs was 1:1. Males were never found. Mean fecundity was 300 ± 15 (SE). The documented occurrence of D. obesus from 10 southern and southeastern states suggests its distribution is probably throughout the range of its loblolly pine host. Records from Maryland indicate the mealybug also feeds on Virginia pine (P. virginiana Mill.).

Reacting mouse sperm with monoclonal H-Y antibodies does not influence sex ratio of eggs fertilized in vitro

The fertility of Y-bearing mouse sperm was examined after reacting cauda epididymal sperm with monoclonal H-Y antibodies and protein A sensitized sheep red blood cells. Treated sperm were used for the in vitro fertilization of mouse oocytes which subsequently produced live offspring. There was no significant shift in the sex ratio in favor of females, suggesting that X- and Y-bearing sperm may share the surface antigen. Additional studies were directed toward ascertaining whether haploid expression, as measured by the presence of H-Y antigen, occurs in epididymal sperm or during their capacitation in vitro. Ligation of the corpus epididymus, preventing subsequent transport of sperm to the cauda region, resulted in a linear decrease in H-Y positive cauda sperm. By 17 days after ligation, no positively reacting sperm were observed. Incubation of cauda epididymal sperm for 3 h in capacitating medium eliminated positive reaction by the capacitated sperm to the H-Y antiserum. Furthermore, the percentage of H-Y-positive sperm from different regions of the male reproductive tract appeared to decrease during their transport from the testis to the epididymus and vas deferens. We suggest that H-Y antigen appears on the sperm surface during association with testicular constituents and is removed during epididymal transport and capacitation. No evidence of haploid expression by epididymal mouse sperm was found.