The evolution of sex roles: The importance of ecology and social environment.

Males and females often have different roles in reproduction, although the origin of these differences has remained controversial. Explaining the enigmatic reversed sex roles where males sacrifice their mating potential and provide full parental care is a particularly long-standing challenge in evolutionary biology. While most studies focused on ecological factors as the drivers of sex roles, recent research highlights the significance of social factors such as the adult sex ratio. To disentangle these propositions, here, we investigate the additive and interactive effects of several ecological and social factors on sex role variation using shorebirds (sandpipers, plovers, and allies) as model organisms that provide the full spectrum of sex role variation including some of the best-known examples of sex-role reversal. Our results consistently show that social factors play a prominent role in driving sex roles. Importantly, we show that reversed sex roles are associated with both male-skewed adult sex ratios and high breeding densities. Furthermore, phylogenetic path analyses provide general support for sex ratios driving sex role variations rather than being a consequence of sex roles. Together, these important results open future research directions by showing that different mating opportunities of males and females play a major role in generating the evolutionary diversity of sex roles, mating system, and parental care.

Queen-Worker Conflict over Acceptance of Secondary Queens in Eusocial Insects.

AbstractThe coexistence of multiple reproductives in eusocial insects is widespread, yet the decisions leading to additional queen acceptance are not well understood. Unlike in vertebrates, acceptance decisions are likely controlled by the more numerous helper population rather than the parent reproductive. Yet there are likely to be queen-worker differences in acceptance criteria because workers and queens differ in their relatedness to a secondary queen. We develop a model that examines queen-worker conflict in two scenarios: accepting a queen's sister or daughter. We additionally ask how the mating frequency and split sex ratios affect the outcomes of these conflicts. Our results reveal that conflict over queen acceptance is highest in monandrous mating systems. We identify a "window of conflict" in which a queen is selected to accept her sister but her workers do not. Our result, that polyandry neutralizes conflict over acceptance thresholds, suggests that conflict suppression may be an additional contributor to the maintenance of polyandrous mating systems.

Sexual dimorphism in subterranean amphipod crustaceans covaries with subterranean habitat type.

Sexual dimorphism can evolve in response to sex-specific selection pressures that vary across habitats. We studied sexual differences in subterranean amphipods Niphargus living in shallow subterranean habitats (close to the surface), cave streams (intermediate), and cave lakes (deepest and most isolated). These three habitats differ because at greater depths there is lower food availability, reduced predation, and weaker seasonality. Additionally, species near the surface have a near-even adult sex ratio (ASR), whereas species from cave lakes have a female-biased ASR. We hypothesized (a) a decrease in sexual dimorphism from shallow subterranean habitats to cave lake species because of weaker sexual selection derived from changes in the ASR and (b) an increase in female body size in cave lakes because of stronger fecundity selection on account of oligotrophy, reduced predation, and weaker seasonality. We measured body size and two sexually dimorphic abdominal appendages for all 31 species and several behaviours related to male competition (activity, risk-taking, exploration) for 12 species. Species with an equal ASR that live close to the surface exhibited sexual dimorphism in all three morphological traits, but not in behaviour. The body size of females increased from the surface to cave lakes, but no such trend was observed in males. In cave lake species, males and females differed neither morphologically nor behaviourally. Our results are consistent with the possibility that sexual and fecundity selection covary across the three habitats, which indirectly and directly, respectively, shape the degree of sexual dimorphism in Niphargus species.

Sex-biased sampling may influence Homo naledi tooth size variation.

A frequent source of debate in paleoanthropology concerns the taxonomic unity of fossil assemblages, with many hominin samples exhibiting elevated levels of variation that can be interpreted as indicating the presence of multiple species. By contrast, the large assemblage of hominin fossils from the Rising Star cave system, assigned to Homo naledi, exhibits a remarkably low degree of variation for most skeletal elements. Many factors can contribute to low sample variation, including genetic drift, strong natural selection, biased sex ratios, and sampling of closely related individuals. In this study, we tested for potential sex-biased sampling in the Rising Star dental sample. We compared coefficients of variation for the H. naledi teeth to those for eight extant hominoid samples. We used a resampling procedure that generated samples from the extant taxa that matched the sample size of the fossil sample for each possible Rising Star dental sex ratio. We found that variation at four H. naledi tooth positions-I2, M1, P4, M1-is so low that the possibility that one sex is represented by few or no individuals in the sample cannot be excluded. Additional evidence is needed to corroborate this inference, such as ancient DNA or enamel proteome data, and our study design does not address other potential factors that would account for low sample variation. Nevertheless, our results highlight the importance of considering the taphonomic history of a hominin assemblage and suggest that sex-biased sampling is a plausible explanation for the low level of phenotypic variation found in some aspects of the current H. naledi assemblage.

Why the Shaw-Mohler equation works and when it doesn't.

Fitness gain curves were introduced into the framework of the Shaw-Mohler equation, the foundation of sex allocation theory. I return to the Shaw-Mohler equation to consider how it embodies the rare-sex advantage underlying frequency-dependent selection on the sex ratio. The Shaw-Mohler formulation is based on the numbers of males and females randomly mating in a panmictic population. Gain curves are meant to describe reproductive success through male and female functions in hermaphrodites and have been inserted in place of male and female numbers in the Shaw-Mohler equation. In doing so, gain curves bypass consideration of the implicit mating process in the Shaw-Mohler argument and can lead to anomalies like unequal total male and female fitness in a population. If gain curves truly represent fitness gain, equality of total male and female fitness requires a constant sex allocation of equal resource investment into male and female functions. The blurring of input with fitness outcome has led to misinterpretation of what gain curves mean in reproductive ecology. They can describe a particular reproductive ecology, such as diminishing fitness returns on resource investment, but lack causal efficacy with respect to sex allocation.

Female-biased birth sex ratio in a female dispersal primate suggests local resource competition.

Group living may entail local resource competition (LRC) which can be reduced if the birth sex ratio (BSR) is biased towards members of the dispersing sex who leave the group and no longer compete locally with kin. In primates, the predicted relationship between dispersal and BSR is generally supported although data for female dispersal species are rare and primarily available from captivity. Here, we present BSR data for Phayre's leaf monkeys (Trachypithecus phayrei crepusculus) at the Phu Khieo Wildlife Sanctuary, Thailand (N = 104). In this population, nearly all natal females dispersed, while natal males stayed or formed new groups nearby. The slower reproductive rate in larger groups suggests that food can be a limiting resource. In accordance with LRC, significantly more females than males were born (BSR 0.404 males/all births) thus reducing future competition with kin. This bias was similar in 2-year-olds (no sex-differential mortality). It became stronger in adults, supporting our impression of particularly fierce competition among males. To better evaluate the importance of BSR, more studies should report sex ratios throughout the life span, and more data for female dispersal primates need to be collected, ideally for multiple groups of different sizes and for several years.

Unveiling sex bias and adverse neonatal outcomes in ultrasound estimation of gestational age: A population-based cohort study.

BACKGROUND: Gestational age estimation by second-trimester ultrasound biometry introduces systematic errors due to sex differences in early foetal growth, consequently increasing the risk of adverse neonatal outcomes. Ultrasound estimation earlier in pregnancy may reduce this bias. OBJECTIVES: To investigate the distribution of sex ratio by gestational age and estimate the risk of adverse outcomes in male foetuses born early-term and female foetuses born post-term by first- and second-trimester ultrasound estimations. METHODS: This population-based study compared two cohorts of births with gestational age based on first- and second-trimester ultrasound in the Medical Birth Registry of Norway between 2016 and 2020. We used a log-binomial regression model to estimate adjusted relative risk (RR) with 95% confidence interval (CI) for Apgar score <7 at 5 min, umbilical artery pH <7.05, neonatal intensive care unit (NICU) admission and respiratory morbidity in relation to foetal sex. RESULTS: The sex ratio at birth in gestational weeks 36-43 showed less male predominance in pregnancies estimated in first compared to second trimester. Any adverse outcome was registered in 627 of 4470 male infants born in gestational weeks 37-38 and 618 of 6406 females born ≥41 weeks. Male infants born in weeks 37-38 had lower risk of NICU admission (RR 0.76, 95% CI 0.58, 0.99), Apgar score <7 at 5 min (RR 0.63, 95% CI 0.28, 1.41) and respiratory morbidity (RR 0.68, 95% CI 0.37, 1.25) in first- compared to second-trimester estimations. Female infants estimated in first trimester born ≥41 weeks had lower risk of umbilical artery pH <7.05, NICU admissions and respiratory morbidity; however, CIs were wide. CONCLUSIONS: Early ultrasound estimation of gestational age may reduce the excess risk of adverse neonatal outcomes and highlight the role of foetal sex and the timing of ultrasound assessment in the clinical evaluation of preterm and post-term pregnancies.

Sex ratio at birth across 100 years in Sweden and risk of cardiovascular disease and all-cause mortality - a national register study.

The human sex ratio at birth (SRB) undergoes temporary changes around a mean proportion of 0.51 male births. SRB has been well studied for historical, geographical, and secular trends, but until now not linked to health outcomes in the total population, e.g. for cardiovascular disease (CVD) or mortality during follow-up of birth cohorts. We used linkage analysis based on national registers in Sweden that cover all births from 1900 to 2016. SRB at birth was calculated by every 10-year birth cohort in all survivors living in 1997 for a follow-up analysis of risk of CVD and mortality with data from national registers. When the highest quartile of SRB was used as reference, a slightly increased risk of fatal CVD (HR 1.03 (95% confidence intervals, CI): 1.02-1.04), non-fatal CVD (HR 1.01; 95%CI: 1.01-1.02) and mortality (HR 1.02; 95%CI, 1.01-1.03) was found after full adjustments in men belonging to the lowest SRB quartile. A similar pattern was also found for fatal CHD in women. in the lowest SBR quartile compared to the highest, HR 1.03 (95%CI: 1.02-1.05). In conclusion, in birth cohorts with a relatively lower than expected number of males born, long-term adverse health effects were observed with slightly increased cardiovascular risk and total mortality at the population level. This could indicate that men belonging to so-called "culled cohorts" in a developed country during the 20th century are characterized by a slightly increased risk that could reflect negative early life influences and environmental exposures in pregnant women resulting in selective loss of male embryos or fetuses. In a public health perspective SRB could be of some importance to monitor as an aspect of birth statistics linked to relatively minor population health effects.

Bud development, flower phenology and life history of holoparasitic Rafflesia cantleyi.

Despite being the world's largest single-flower, Rafflesia's biology and life history are still poorly understood due to its cryptic growth strategy on Tetrastigma vines. Previous studies have been mostly short-term, contrary to Rafflesia's long development period before blooming. Bud development and flower phenology of R. cantleyi was studied in a dipterocarp forest in Lata Jarum, Peninsular Malaysia. Seven populations, consisting of 247 buds, were monitored fortnightly for 65 months in two discrete studies between 2009 and 2018. The bud size distribution of R. cantleyi is dynamic, progressively changing from small flower buds to larger buds before flowering. Buds < 5.0 cm across had the slowest growth and highest mortality rates, while those > 15.0 cm across demonstrated accelerated growth. The bud growth profiles of the same site clustered distinctively regardless of sex with successful blooming rate that varied greatly between sites, prompting speculation about their relatedness to the sites' physical attributes. We reported the first female-dominated population in Rafflesia's life history. Rafflesia cantleyi's blooming rate at Lata Jarum is moderate to high, with non-seasonal flowering phenology as evident by the lack of synchronisation and consistency between flowering and local rainfall patterns. Based on the field data of the present study and the published information of other Rafflesia species, R. cantleyi's life cycle was estimated between 4.0 and 5.3 years. Our findings further explain Rafflesia's biology and life history and highlight the gap in knowledge of the natural habitats on the endoparasite's growth and fate potentially for future conservation and study.

Density-dependent mating behaviors reduce male mating harassment in locusts.

Male mating harassment may occur when females and males do not have the same mating objectives. Communal animals need to manage the costs of male mating harassment. Here, we demonstrate how desert locusts in dense populations reduce such conflicts through behaviors. In transient populations (of solitarious morphology but gregarious behavior), we found that nongravid females occupied separate sites far from males and were not mating, whereas males aggregated on open ground (leks), waiting for gravid females to enter the lekking sites. Once a male mounted a gravid female, no other males attacked the pair; mating pairs were thereby protected during the vulnerable time of oviposition. In comparison, solitarious locusts displayed a balanced sex ratio in low-density populations, and females mated irrespective of their ovarian state. Our results indicate that the mating behaviors of desert locusts are density dependent and that sex-biased behavioral group separation may minimize the costs of male mating harassment and competition.

Primary sex ratio in euploid embryos of consanguine couples after IVF/ICSI.

PURPOSE: To assess the primary sex ratio (males-to-females at time of conception) in blastocysts from consanguine couples undergoing IVF/ICSI treatments and its correlation with chromosomal constitution. METHOD: A total of 5135 blastocysts were analyzed by preimplantation-genetic testing for aneuploidy (PGT-A) with next-generation sequencing (NGS) from November 2016 to December 2020. From those, a total of 1138 blastocysts were from consanguine couples (CS) and 3997 from non-consanguine couples (NCS). Only blastocysts presenting normal sex chromosome constitution with or without autosomal aneuploidies were included. Primary sex ratio (PSR) of biopsied blastocysts was compared between CS and NCS couples. RESULTS: Expanded blastocysts derived from CS had 47.7% XY versus 52.3% XX constitutions, presenting a PSR of 0.91. In NCS, 48.9% of expanded blastocysts were XY and 51.2% XX, with a less pronounced PSR of 0.95. When stratifying embryos by ploidy, euploid embryos from CS had the lowest PSR (0.87) with 46.6% XY versus 53.4% XX blastocysts (OR 0.89, 95% CI 0.70-1.14; NS), but it did not achieve statistical significance. The lower PSR seemed rather related to euploid embryos from first-degree cousins (PSR = 0.80 versus 0.98 in second-degree cousins, NS). Euploid embryos from NCS presented a PSR of 0.96, with 49.1% XY versus 50.9% XX blastocysts (OR 0.98, 95% CI 0.79-1.22; NS). Significant differences in prevalence of euploidy of specific chromosomes were encountered between CS and NCS. CONCLUSIONS: The primary sex ratio was generally similar in expanded blastocysts from consanguine and non-consanguine couples, with a slight decrease in primary sex ratio of euploid blastocysts from consanguine couples.

Culture media affect sex after IVF treatment-a detailed analysis of explanatory variables.

PURPOSE: The purpose of this study was to provide a detailed analysis of clinical and laboratory factors associated with skewed secondary sex ratio (SSR) after ART. METHOD: Retrospective cohort study of embryos resulting in live births, from frozen and fresh single blastocyst transfers. Embryos were cultured in either G-TL (n = 686) or Sage media (n = 685). Data was analyzed using a multivariate logistic regression model and a mixed model analysis. RESULTS: Significantly more male singletons were born after culture in Sage media compared to G-TL media (odds ratio (OR) 1.34, 95% CI (1.05, 1.70), P = 0.02). Inner cell mass grade B vs A (OR 1.36 95% CI (1.05, 1.76), P = 0.02) and one previous embryo transfer (OR 1.49, 95% CI (1.03, 2.16), P = 0.03) were associated with a significantly higher probability of male child at birth. Factors associated with a reduced probability of male child were expansion grade 3 vs 5 (OR 0.66, 95% CI (10.45, 0.96), P = 0.03) and trophectoderm grade B vs A (OR 0.57, 95% CI (0.44, 0.74), P = 0.00). Male embryos developed significantly faster in Sage media compared to G-TL media for the stages of blastocyst (- 1.12 h, 95% CI (- 2.12, - 0.12)), expanded blastocyst (- 1.35 h, 95% CI (- 2.34, - 0.35)), and hatched blastocyst (- 1.75 h, 95% CI (- 2.99, - 0.52)). CONCLUSION: More male children were born after culture in Sage media compared to G-TL media. Male embryo development was affected by culture media. Our observations suggest that culture media impact male embryo quality selectively, thus potentially favoring the selection of male embryos.

Oviposition behavior of the quasi-gregarious parasitoid, Ooencyrtus kuvanae (Hymenoptera: Encyrtidae).

The parasitoid wasp, Ooencyrtus kuvanae (Howard) (Hymenoptera: Encyrtidae), is a natural enemy of the spongy moth, a significant forest pest in North America. We investigated the oviposition behavior of O. kuvanae females on spongy moth egg masses by (i) presenting female parasitoids with a single spongy moth egg mass that was replaced every day, 2nd day, 4th day, 8th day, or 16th day (which is the total length of the oviposition period) and (ii) presenting female parasitoids with 1, 2, 4, or 8 egg masses at a time. Offspring developmental length ranged from 18 to 24 days. On average, male offspring exhibited faster developmental times, emerging approximately 1 day ahead of females. The amount of time that adult females spent on an egg mass affected the number of parasitized eggs. Specifically, more offspring emerged in the 4-, 8-, and 16-day treatments than in scenarios involving daily or every second-day egg mass replacement. The percentage of male offspring decreased as the number of egg masses presented to females increased. Interestingly, the total number of female offspring remained constant, but the number of male offspring decreased with an increase in the number of egg masses and time spent by the parent within a patch. The observed sexual dimorphism in development time, the influence of resource availability on offspring sex ratios, and flexible oviposition patterns illustrate the adaptability of O. kuvanae in response to varying conditions. These insights have implications for our understanding of parasitoid-host interactions and their potential role in biological control strategies.

Gender Control of Mouse Embryos by Activation of TLR7/8 on X Sperm via Ligands dsRNA-40 and dsRNA-DR.

Gender control technologies are promising for enhancing the production efficiency of the farm animal industry, and preventing sex-linked hereditary diseases in humans. It has been shown that the X sperm of mammalian animals specifically expresses X-chromosome-derived toll-like receptor 7/8 (TLR7/8), and the activation of TLR7/8 on the X sperm by their agonist, R848, can separate X and Y sperm via the specific inhibition of X sperm motility. The use of R848-preselected sperm for fertilization resulted in sex-ratio-skewed embryos or offspring. In this study, we aimed to investigate whether two other TLR7/8 ligands, double-stranded RNA-40 (dsRNA-40) and double-stranded RNA-DR (dsRNA-DR), are also effective in the separation of mouse X and Y sperm and the subsequent generation of gender-ratio-skewed in vitro fertilization (IVF) embryos. Our results indicated that cholesterol modification significantly enhances the transfection of dsRNA-40 and dsRNA-DR into sperm cells. dsRNA-40 and dsRNA-DR incubation with mouse sperm could separate X and Y sperm by the specific suppression of X sperm motility by decreasing its ATP level and mitochondrial activity. The use of a dsRNA-40- or dsRNA-DR-preselected upper layer of sperm, which predominantly contains high-motility Y sperm, for IVF caused a male-biased sex ratio shift in resulting embryos (with 65.90-74.93% of embryos being male). This study develops a simple new method for the efficient separation of mammalian X and Y sperm, enabling the selective production of male or female progenies.

Early-life food stress hits females harder than males in insects: A meta-analysis of sex differences in environmental sensitivity.

Fitness consequences of early-life environmental conditions are often sex-specific, but corresponding evidence for invertebrates remains inconclusive. Here, we use meta-analysis to evaluate sex-specific sensitivity to larval nutritional conditions in insects. Using literature-derived data for 85 species with broad phylogenetic and ecological coverage, we show that females are generally more sensitive to food stress than males. Stressful nutritional conditions during larval development typically lead to female-biased mortality and thus increasingly male-biased sex ratios of emerging adults. We further demonstrate that the general trend of higher sensitivity to food stress in females can primarily be attributed to their typically larger body size in insects and hence higher energy needs during development. By contrast, there is no consistent evidence of sex-biased sensitivity in sexually size-monomorphic species. Drawing conclusions regarding sex-biased sensitivity in species with male-biased size dimorphism remains to wait for the accumulation of relevant data. Our results suggest that environmental conditions leading to elevated juvenile mortality may potentially affect the performance of insect populations further by reducing the proportion of females among individuals reaching reproductive age. Accounting for sex-biased mortality is therefore essential to understanding the dynamics and demography of insect populations, not least importantly in the context of ongoing insect declines.

The Role of Maternal Preconception Adiposity in Human Offspring Sex and Sex Ratio.

We evaluated relationships between preconception adiposity and human offspring sex and sex ratio. Using data from a prospective preconception cohort nested within a randomized controlled trial based at 4 US clinical sites (2006-2012), we used logistic regression to estimate odds ratios (ORs) and 95% confidence intervals (CIs) for male:female sex ratio, and log-identity regression to estimate risk differences (RDs) and 95% CIs for male and female livebirth according to preconception adiposity measures. Inverse-probability weights accounted for potential selection bias. Among 603 women attempting pregnancy, there were meaningful reductions in sex ratio for the highest category of each adiposity measure. The lowest sex ratios were observed for obesity (body mass index of ≥30, calculated as weight (kg)/height (m)2, OR = 0.48, 95% CI: 0.26, 0.88) relative to normal body mass index, and the top tertiles (tertile 3) of serum leptin (OR = 0.50, 95% CI: 0.32, 0.80) and skinfold measurements (OR = 0.50, 95% CI: 0.32, 0.79) relative to the lowest tertiles. Reductions were driven by 11-15 fewer male livebirths per 100 women (for obesity, RD = -15, 95% CI: -23, -6.7; for leptin tertile 3, RD = -11, 95% CI: -20, -3.2; and for skinfolds tertile 3, RD = -11, 95% CI: -19, -3.3). We found that relationships between preconception adiposity measures and reduced sex ratio were driven by a reduction in male births.

Five Decades of Data Yield No Support for Adaptive Biasing of Offspring Sex Ratio in Wild Baboons (Papio cynocephalus).

AbstractOver the past 50 years, a wealth of testable, often conflicting hypotheses have been generated about the evolution of offspring sex ratio manipulation by mothers. Several of these hypotheses have received support in studies of invertebrates and some vertebrate taxa. However, their success in explaining sex ratios in mammalian taxa-especially in primates-has been mixed. Here, we assess the predictions of four different hypotheses about the evolution of biased offspring sex ratios in the baboons of the Amboseli basin in Kenya: the Trivers-Willard, female rank enhancement, local resource competition, and local resource enhancement hypotheses. Using the largest sample size ever analyzed in a primate population (n=1,372 offspring), we test the predictions of each hypothesis. Overall, we find no support for adaptive biasing of sex ratios. Offspring sex is not consistently related to maternal dominance rank or biased toward the dispersing sex, nor is it predicted by group size, population growth rates, or their interaction with maternal rank. Because our sample size confers power to detect even subtle biases in sex ratio, including modulation by environmental heterogeneity, these results suggest that adaptive biasing of offspring sex does not occur in this population.

Introgression of a synthetic sex ratio distortion transgene into different genetic backgrounds of Anopheles coluzzii.

The development of genetically modified mosquitoes (GMM) and their subsequent field release offers innovative approaches for vector control of malaria. A non-gene drive self-limiting male-bias Ag(PMB)1 strain has been developed in a 47-year-old laboratory G3 strain of Anopheles gambiae s.l. When Ag(PMB)1 males are crossed to wild-type females, expression of the endonuclease I-PpoI during spermatogenesis causes the meiotic cleavage of the X chromosome in sperm cells, leading to fertile offspring with a 95% male bias. However, World Health Organization states that the functionality of the transgene could differ when inserted in different genetic backgrounds of Anopheles coluzzii which is currently a predominant species in several West-African countries and thus a likely recipient for a potential release of self-limiting GMMs. In this study, we introgressed the transgene from the donor Ag(PMB)1 by six serial backcrosses into two recipient colonies of An. coluzzii that had been isolated in Mali and Burkina Faso. Scans of informative Single Nucleotide Polymorphism (SNP) markers and whole-genome sequencing analysis revealed a nearly complete introgression of chromosomes 3 and X, but a remarkable genomic divergence in a large region of chromosome 2 between the later backcrossed (BC6) transgenic offspring and the recipient paternal strains. These findings suggested to extend the backcrossing breeding strategy beyond BC6 generation and increasing the introgression efficiency of critical regions that have ecological and epidemiological implications through the targeted selection of specific markers. Disregarding differential introgression efficiency, we concluded that the phenotype of the sex ratio distorter is stable in the BC6 introgressed An. coluzzii strains.

Is drift 'directional'? Unequal breeding sex ratio revisited.

Unequal breeding sex ratio can significantly reduce effective population size, allowing a rare neutral allele to jump to a high frequency through genetic drift. However, this one-way alteration to allele frequency appears inconsistent with the concept that drift is non-directional. Based on binomial sampling distribution, this study developed a method to directly and exhaustively measure drift by calculating the mean deviation of change in allele frequency, then applied it to cases of unequal breeding sex ratio. The result shows that, under those cases, (1) the mean deviation can always be divided into two halves that are equal in size but opposite in direction; (2) each half consists of one or several categories represented by various allele proportions in the rare sex; (3) this proportion is another factor that determines the outcome of drift, in addition to effective population size and allele frequency; (4) drift is non-directional on a global scale, but whether an allele will drift up or down can be predicted based on the above factors. This method enables us to dissect every component of the expected change in allele frequency caused by drift and to find out the combined effect of population size, allele frequency and allele proportion in the rarer sex under neutrality but unequal breeding sex ratio.

Sex-specific paternal age effects on offspring quality in Drosophila melanogaster.

Advanced paternal age has been repeatedly shown to modulate offspring quality via male- and/or female-driven processes, and there are theoretical reasons to expect that some of these effects can be sex-specific. For example, sex allocation theory predicts that, when mated with low-condition males, mothers should invest more in their daughters compared to their sons. This is because male fitness is generally more condition-dependent and more variable than female fitness, which makes it less risky to invest in female offspring. Here, we explore whether paternal age can affect the quality and quantity of offspring in a sex-specific way using Drosophila melanogaster as a model organism. In order to understand the contribution of male-driven processes on paternal age effects, we also measured the seminal vesicle size of young and older males and explored its relationship with reproductive success and offspring quality. Older males had lower competitive reproductive success, as expected, but there was no difference between the offspring sex ratio of young and older males. However, we found that paternal age caused an increase in offspring quality (i.e., offspring weight), and that this increase was more marked in daughters than sons. We discuss different male- and female-driven processes that may explain such sex-specific paternal age effects.