Unexpectedly low frequencies of diploid males in an inbreeding parasitoid with complementary sex determination

In hymenopterans with single locus complementary sex determination, sex depends on the genotype at one polymorphic locus. Haploids are always male, while diploids are female when heterozygous and male when homozygous at the sex‐determining locus. Brothers and sisters have a 50% chance of sharing a sex allele (i.e. of being ‘matched’), and hence half of all sibling matings are expected to produce diploid males at the expense of females. Nevertheless, observed frequencies of diploid males are often lower than predicted, as diploid males may succumb to pre‐imaginal mortality, or because unmatched mates or sperm enjoy a competitive advantage. We counted diploid males in broods of the parasitoid wasp Cotesia glomerata sampled in the field, and in broods produced through controlled laboratory crosses. Consistently, the frequency of diploid males fell below expectations based upon the estimated occurrence of sibling mating. In the staged broods with diploid males, females made up a disproportionately large share of the diploids. Broods with and without diploid males were of similar size. Hence, the shortage of diploid males cannot be accounted for by differential pre‐imaginal mortality alone. Instead, we postulate the existence of a mechanism that leads to preferential fertilization of eggs by sperm bearing unmatched alleles. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ●● , ●●–●●.     

Effective population size in Hymenoptera with complementary sex determination

Complementary sex determination in the haplodiploid Hymenoptera leads to the production of inviable or effectively sterile diploid males when diploid progeny are homozygous at the sex-determining locus. The production of diploid males reduces the number of females in a population and biases the effective breeding sex ratio in favor of haploid males. This in turn will reduce the effective population size (Ne) of hymenopteran populations with complementary sex determination relative to the expected reductions due to haplodiploidy alone. The effects of diploid male production on Ne in hymenopterans with complementary sex determination when diploid males are either inviable or effectively sterile are assessed theoretically. In both models, low allelic diversity at the sex locus reduces the Ne of hymenopteran populations, and this effect is largest when diploid males are effectively sterile.     

Experimental support for multiple-locus complementary sex determination in the parasitoid Cotesia vestalis

Despite its fundamental role in development, sex determination is highly diverse among animals. Approximately 20% of all animals are haplodiploid, with haploid males and diploid females. Haplodiploid species exhibit diverse but poorly understood mechanisms of sex determination. Some hymenopteran insect species exhibit single-locus complementary sex determination (sl-CSD), where heterozygosity at a polymorphic sex locus initiates female development. Diploid males are homozygous at the sex locus and represent a genetic load because they are inviable or sterile. Inbreeding depression associated with CSD is therefore expected to select for other modes of sex determination resulting in fewer or no diploid males. Here, we investigate an alternative, heretofore hypothetical, mode of sex determination: multiple-locus CSD (ml-CSD). Under ml-CSD, diploid males are predicted to develop only from zygotes that are homozygous at all sex loci. We show that inbreeding for eight generations in the parasitoid wasp Cotesia vestalis leads to increasing proportions of diploid males, a pattern that is consistent with ml-CSD but not sl-CSD. The proportion of diploid males (0.27 ± 0.036) produced in the first generation of inbreeding (mother–son cross) suggests that two loci are likely involved. We also modeled diploid male production under CSD with three linked loci. Our data visually resemble CSD with linked loci because diploid male production in the second generation was lower than that in the first. To our knowledge, our data provide the first experimental support for ml-CSD.     

Ladies last: diel rhythmicity of adult emergence in a parasitoid with complementary sex determination

Protandry, the earlier adult emergence of males, is explained as either an adaptive strategy maximizing male mating opportunities at the same time as minimizing female pre‐reproductive mortality, or as an incidental by‐product of sexual dimorphism fuelled by selection for other life‐history traits. Adult emergence sequences are monitored of broods of the gregarious larval endoparasitoid Cotesia glomerata L. (Hymenoptera: Braconidae) undergoing pupal development under different temperature regimes. As a haplodiploid species with single‐locus complementary sex determination, gender in C. glomerata is determined by the genotype at one sex locus. Haploids are always male, whereas diploids are female when heterozygous but male when homozygous at the sex locus. Sibling mating promotes homozygosity and thus the production of diploid males. Diploid males are produced at the expense of females, and impose a genetic burden on individuals and populations, despite their exceptional fertility in C. glomerata. Emergence of broods is typically completed within 2 days. Irrespective of temperature, males emerge earlier and within a shorter time interval than females, and a majority of the males in a cluster emerge before the first female. The implications of an incomplete temporal segregation of the sexes on the incidence of inbreeding in C. glomerata are discussed in the light of its sex determination mechanism and its patterns of mating, host exploitation and natal dispersal.     

Genetic evidence for unequal effective population sizes of human females and males

The time to the most recent common ancestor (TMRCA) of the human mitochondria (mtDNA) is estimated to be older than that of the nonrecombining portion of the Y chromosome (NRY). Surveys of variation in globally distributed humans typically result in mtDNA TMRCA values just under 200 thousand years ago (kya), whereas those for the NRY range between 46 and 110 kya. A favored hypothesis for this finding is that natural selection has acted on the NRY, leading to a recent selective sweep. An alternate hypothesis is that sex-biased demographic processes are responsible. Here, we re-examine the disparity between NRY and mtDNA TMRCAs using data collected from individual human populations--a sampling strategy that minimizes the confounding influence of population subdivision in global data sets. We survey variation at 782 bp of the mitochondrial cytochrome c oxidase subunit 3 gene as well as at 26.5 kb of noncoding DNA from the NRY in a sample of 25 Khoisan, 24 Mongolians, and 24 Papua New Guineans. Data from both loci in all populations are best described by a model of constant population size, with the exception of Mongolian mtDNA, which appears to be experiencing rapid population growth. Taking these demographic models into account, we estimate the TMRCAs for each locus in each population. A pattern that is remarkably consistent across all three populations is an approximately twofold deeper coalescence for mtDNA than for the NRY. The oldest TMRCAs are observed for the Khoisan (73.6 kya for the NRY and 176.5 kya for mtDNA), whereas those in the non-African populations are consistently lower (averaging 47.7 kya for the NRY and 92.8 kya for mtDNA). Our data do not suggest that differential natural selection is the cause of this difference in TMRCAs. Rather, these results are most consistent with a higher female effective population size.     

Additional evidence for the genomic imprinting model of sex determination in the haplodiploid wasp Nasonia vitripennis: isolation of biparental diploid males after X-ray mutagenesis

The primary sex-determining signal in the haplodiploid wasp Nasonia vitripennis is not known. In haplodiploid reproduction, unfertilized eggs typically develop into uniparental haploid males and fertilized eggs into biparental diploid females. Although this reproductive strategy is common to all Hymenoptera, sex-determination is not strictly specified by the number of genome copies inherited. Furthermore, primary sex-determining signals differ among haplodiploid species. In the honeybee, for example, the primary signal is the genotype at a single, polymorphic locus: diploid animals that are homozygous develop into males while heterozygotes develop into females. Sex determination in Nasonia cannot be explained by this mechanism. Various lines of evidence show that the inheritance of a paternal genome is required for female sexual development and suggest a genomic imprinting mechanism involving an imprinted gene, expressed only from a paternal copy, that triggers female sexual development. In this model, haploid or diploid uniparental embryos develop into males due to a maternal imprint that silences this locus. The genomic imprinting model predicts that a loss-of-function mutation in the paternal copy of the imprinted gene would result in male sexual development in a biparental diploid embryo. In support of this model, we have identified rare biparental diploid males in the F1 progeny of X-ray mutagenized haploid males. Although uniparental diploid male progeny of virgin triploid females have been previously described, this is the first report of biparental diploid males in Nasonia. Our work provides a new, independent line of evidence for the genomic imprinting model of Nasonia sex determination.     

Causes and consequences of small population size for a specialist parasitoid wasp

The parasitoid wasp Cotesia melitaearum lives in extremely small extinction-prone populations in the Åland islands of southwest Finland. Intensive observational data from two generations, a laboratory competition experiment, and 8 years of survey data were used to measure the causes, extent and consequences of small population size for this parasitoid. In the spring generations of 1999 and of 2000 we observed 21 out of 23 and 26 populations respectively, ranging in size from 2 to 103 parasitoid cocoons. Within these populations the fraction of individuals surviving to adulthood decreased with increasing parasitoid population size. The largest source of mortality was predation (44%) followed by parasitism (20%) and unknown causes (10%). In the field about 30% of the host butterfly larvae are parasitized by a competing parasitoid, Hyposoter horticola. A laboratory competition experiment showed that C. melitaearum eggs died when laid in post-diapause host larvae occupied by H. horticola. Consequently one-third of the progeny of the over-wintering generation of C. melitaearum from the field die as a result of larval competition. The survey of host and parasitoid population dynamics over 8 years showed that extinction of local host butterfly populations occupied by the parasitoid was not associated with current parasitoid population size. Over the same period small parasitoid populations were more likely to become extinct than large populations. However, parasitoid population size was not related to parasitoid extinction when the host also became extinct. These data suggest that the parasitoid populations are kept small through the action of natural enemies and competitors, some of which are density dependent. Local populations are so small that they become extinct frequently and rarely measurably affect the population dynamics of their host. It is likely that this parasitoid persists in Åland because of the spatial asynchrony of local population dynamics.     

Fixation probabilities and effective population numbers in diploid populations with overlapping generations

A finite diploid population, observed at times t = 0, 1, 2,…, is studied. An individual is said to be in age group i at time t if its age is between i and i + 1 units at that time, where i ? 1. It is assumed that the number of individuals in a particular age-sex class is the same for every t and that the probability that a male offspring was produced by a mating of a male in age group i and a female in age group j is p_ij^m (with a corresponding probability p^f_ij for a female offspring), regardless of when the individual is born. The probability of ultimate fixation of an allele A₁ and the inbreeding effective number, for large populations, is calculated under the further assumptions that A₁ is neutral and that mating is random, given the ages of the mates.     

No evidence that sex and transposable elements drive genome size variation in evening primroses

Genome size varies dramatically across species, but despite an abundance of attention there is little agreement on the relative contributions of selective and neutral processes in governing this variation. The rate of sex can potentially play an important role in genome size evolution because of its effect on the efficacy of selection and transmission of transposable elements (TEs). Here, we used a phylogenetic comparative approach and whole genome sequencing to investigate the contribution of sex and TE content to genome size variation in the evening primrose (Oenothera) genus. We determined genome size using flow cytometry for 30 species that vary in genetic system and find that variation in sexual/asexual reproduction cannot explain the almost twofold variation in genome size. Moreover, using whole genome sequences of three species of varying genome sizes and reproductive system, we found that genome size was not associated with TE abundance; instead the larger genomes had a higher abundance of simple sequence repeats. Although it has long been clear that sexual reproduction may affect various aspects of genome evolution in general and TE evolution in particular, it does not appear to have played a major role in genome size evolution in the evening primroses.     

No evidence for color or size preference in either sex of a dichromatic stream fish, Percina roanoka

Sexual dimorphism is hypothesized to be the result of differential selection pressures between the sexes. Dimorphic traits can serve as indicators of mate quality, altering mate preferences in the opposite sex in favor of a conspicuous trait. Common indicators of mate quality include color and size, with traditional assumptions and evidence predicting a preference for more colorful and/or larger sized mates in many species. Both male and female preferences for more colorful and larger mates within a species are rarely examined simultaneously, however. We examined a sexually dichromatic freshwater fish, Percina roanoka and found that male coloration is positively correlated with size, suggesting color may function as an indicator of viability. We tested preferences for coloration and size in both sexes in a dichotomous mate choice setup in which only visual signals were exchanged. Neither females nor males exhibited a color or size preference in individuals of the opposite sex. Visual cues alone therefore appear to be insufficient to elicit a significant preference in both sexes of this species. Male coloration in P. roanoka does not appear to be driven solely by female preference.     

Inbreeding in a natural population of Euodynerus foraminatus (Hymenoptera: Vespidae), a solitary wasp with single-locus complementary sex determination

The solitary wasp Euodynerus foraminatus has single-locus complementary sex determination (sl-CSD), which is normally incompatible with inbreeding because it increases the production of sterile or inviable diploid males. Previous field observations of E. foraminatus have suggested that high levels of sibling mating are present in this species. However, conclusions about inbreeding and its genetic consequences could be flawed if based solely upon behavioural observations. Through microsatellite DNA genotyping of 102 E. foraminatus females in southwest Michigan, we estimate that between 55% and 77% of the matings in this population take place between siblings, but the frequency of diploid males is lower than expected. Our data suggest that a mixture of inbreeding and outbreeding persists in E. foraminatus despite the presence of sl-CSD.     

Low effective population size and survivorship in a grassland grouse

Assessments of census size (N _c) and effective population size (N _e) are necessary for the conservation of species exhibiting population declines. We examined two populations (Oklahoma and New Mexico) of the lesser prairie-chicken (Tympanuchus pallidicinctus), a declining lek-breeding bird, in which one population (Oklahoma) has larger clutch size and more nesting attempts per year but lower survival caused by human changes to the landscape. We estimated demographic and genetic estimates of N _e for each population and found that both populations have low N _e estimates with a risk of inbreeding depression. Although Oklahoma females produce a larger number of offspring, the proportion of females successfully reproducing is not higher than in New Mexico. Higher reproductive effort has likely reached a physiological limit in Oklahoma prairie-chickens but has not led to a higher N _e or even a larger N _c than New Mexico. We propose that future conservation efforts focus on maximizing survivorship and decreasing the variance in reproductive success because these factors are more likely than increasing reproductive output alone to yield population persistence in lek-breeding species.     

Host size selection and sex allocation in Leptomastix dactylopii, a parasitoid of Planococcus citri

Host size selection and sex allocation in Leptomastix dactylopii Howard were studied (1) to determine whether invulnerable size classes exist, (2) to establish whether the size range of hosts attacked differs from its potential competitor Leptomastidea abnormis (Girault). It was found that L. dactylopii attacks mainly the larger host sizes (i.e. 3rd and 4th instars and reproducing females). Proportionally more female parasitoids were reared from larger hosts (i.e. 4th instars and reproducing females), while from size class 3 (i.e. mainly 3rd and smaller 4th instars) predominantly male wasps were reared. The adaptive meaning of the host selection behaviour of L. dactylopii is discussed.

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Zusammenfassung Es wurde untersucht, wie die Schlupfwespe, Leptomastix dactylopii Howard zwischen den verschiedenen Grössenklassen ihres Wirtes, der Schmierlaus Planococcus citri (Risso), wählt.Der Parasitoid bevorzugt das vierte Stadium des Wirtes, aber parasitiert auch Wirte im 3. Stadium und adulte Weibchen mit Eisack. Wirte in ersten und zweiten Stadium wurden nie befallen. Weibliche Parasitoide entstanden vor allem bei Wirten im 4. Stadium, aber auch bei adulten Weibchen, männliche nahezu nur bei Wirten im 3. Stadium. Wirte im 4. Stadium und adulte Weibchen, die von L. dactylopii befallen worden waren, produzierten noch eine anzahl Eier bevor sie starben. Darum sind diese Wirte teilweise unangreifbar. Diese Eigenschaft kann sich möglicherweise auf die Stabilität der Parasitoid-Wirt-Interaktionen auswirken. L. dactylopii befällt die älteren Stadien und der potentiëlle Konkurrent Leptomastidae abnormia die jüngeren Stadien. Solch ein Unterschied kann Koexistenz beider Parasitoidarten möglich machen.

     

No evidence for inbreeding avoidance in a great reed warbler population

Inbreeding depression may drive the evolution of inbreedingavoidance through dispersal and mate choice. In birds, manyspecies show female-biased dispersal, which is an effectiveinbreeding avoidance mechanism. In contrast, there is scarceevidence in birds for kin discriminative mate choice, whichmay, at least partly, reflect difficulties detecting it. First,kin discrimination may be realized as dispersal, and this isdifficult to distinguish from other causes of dispersal. Second,even within small, isolated populations, it is often difficultto determine the potential candidates available to a femalewhen choosing a mate. We sought evidence for inbreeding avoidancevia kin discrimination in a breeding population of great reedwarblers (Acrocephalus arundinaceus) studied over 17 years.Inbreeding depression is strong in the population, suggestingthat it would be adaptive to avoid relatives as mates. Detaileddata on timing of settlement and mate search movements madeit possible to identify candidate mates for each female, andlong-term pedigrees and resolved parentage enabled us to estimaterelatedness between females and their candidate mates. We foundno evidence for kin discrimination: mate choice was random withrespect to relatedness when all mate-choice events were considered,and, after correction for multiple tests, also in all breedingyears. We suggest that dispersal is a sufficient inbreedingavoidance mechanism in most situations, although the lack ofkin discriminative mate choice has negative consequences forsome females, because they end up mating with closely relatedmales that lowers their fitness.     

No evidence for association between DRD3 and COMT with schizophrenia in a Malay population

Molecular components of the dopamine D3 receptor (DRD3) may play an important role in the pathophysiology of schizophrenia. Previous studies have demonstrated an association between DRD3 Ser9Gly and cathechol-o-methyltransferase (COMT, SNP = rs165656) polymorphisms and schizophrenia but the results were inconclusive. We investigated this apparent association between Ser9Gly (A/G) polymorphism and an intronic SNP (dbSNP or rs165656) in 261 Malay patients diagnosed with schizophrenia and 216 controls, using PCR-RFLP. The genotype distribution of the polymorphism DRD3 Ser9Gly was in Hardy-Weinberg equilibrium (HWE) for patients (P = 0.1251) and out of HWE for controls (P = 0.0137). However, both healthy controls and schizophrenia patients were out of HWE for the polymorphism COMT rs165656. Based on allele and genotype frequencies in both groups, we found no significant association of DRD3 Ser9Gly polymorphisms and COMT (rs165656) with schizophrenia in Malays. Further studies should examine the association between other dopamine-related genes and the behavioral phenotypes of schizophrenia.     

Evaluating methods for estimating local effective population size with and without migration

Effective population size is a fundamental parameter in population genetics, evolutionary biology, and conservation biology, yet its estimation can be fraught with difficulties. Several methods to estimate N_e from genetic data have been developed that take advantage of various approaches for inferring N_e. The ability of these methods to accurately estimate N_e, however, has not been comprehensively examined. In this study, we employ seven of the most cited methods for estimating N_e from genetic data (Colony2, CoNe, Estim, MLNe, ONeSAMP, TMVP, and NeEstimator including LDNe) across simulated datasets with populations experiencing migration or no migration. The simulated population demographies are an isolated population with no immigration, an island model metapopulation with a sink population receiving immigrants, and an isolation by distance stepping stone model of populations. We find considerable variance in performance of these methods, both within and across demographic scenarios, with some methods performing very poorly. The most accurate estimates of N_e can be obtained by using LDNe, MLNe, or TMVP; however each of these approaches is outperformed by another in a differing demographic scenario. Knowledge of the approximate demography of population as well as the availability of temporal data largely improves N_e estimates.     

cloncase: Estimation of sex frequency and effective population size by clonemate resampling in partially clonal organisms

Inferring reproductive and demographic parameters of populations is crucial to our understanding of species ecology and evolutionary potential but can be challenging, especially in partially clonal organisms. Here, we describe a new and accurate method, cloncase , for estimating both the rate of sexual vs. asexual reproduction and the effective population size, based on the frequency of clonemate resampling across generations. Simulations showed that our method provides reliable estimates of sex frequency and effective population size for a wide range of parameters. The cloncase method was applied to Puccinia striiformis f.sp. tritici, a fungal pathogen causing stripe/yellow rust, an important wheat disease. This fungus is highly clonal in Europe but has been suggested to recombine in Asia. Using two temporally spaced samples of P. striiformis f.sp. tritici in China, the estimated sex frequency was 75% (i.e. three‐quarter of individuals being sexually derived during the yearly sexual cycle), indicating strong contribution of sexual reproduction to the life cycle of the pathogen in this area. The inferred effective population size of this partially clonal organism (N_c = 998) was in good agreement with estimates obtained using methods based on temporal variations in allelic frequencies. The cloncase estimator presented herein is the first method allowing accurate inference of both sex frequency and effective population size from population data without knowledge of recombination or mutation rates. cloncase can be applied to population genetic data from any organism with cyclical parthenogenesis and should in particular be very useful for improving our understanding of pest and microbial population biology.