Effects of polyploidization and their evolutionary implications are revealed by heritable polyploidy in the haplodiploid wasp Nasonia vitripennis.

Recurrent polyploidization occurred in the evolutionary history of most Eukaryota. However, how neopolyploid detriment (sterility, gigantism, gene dosage imbalances) has been overcome and even been bridged to evolutionary advantage (gene network diversification, mass radiation, range expansion) is largely unknown, particularly for animals. We used the parasitoid wasp Nasonia vitripennis, a rare insect system with heritable polyploidy, to begin addressing this knowledge gap. In Hymenoptera the sexes have different ploidies (haploid males, diploid females) and neopolyploids (diploid males, triploid females) occur for various species. Although such polyploids are usually sterile, those of N. vitripennis are reproductively capable and can even establish stable polyploid lines. To assess the effects of polyploidization, we compared a long-established polyploid line, the Whiting polyploid line (WPL) and a newly generated transformer knockdown line (tKDL) for fitness traits, absolute gene expression, and cell size and number. WPL polyploids have high male fitness and low female fecundity, while tKDL polyploids have poor male mate competition ability and high fertility. WPL has larger cells and cell number reduction, but the tKDL does not differ in this respect. Expression analyses of two housekeeping genes indicated that gene dosage is linked to sex irrespective of ploidy. Our study suggests that polyploid phenotypic variation may explain why some polyploid lineages thrive and others die out; a commonly proposed but difficult-to-test hypothesis. This documentation of diploid males (tKDL) with impaired competitive mating ability; triploid females with high fitness variation; and hymenopteran sexual dosage compensation (despite the lack of sex chromosomes) all challenges general assumptions on hymenopteran biology. We conclude that polyploidization is dependent on the duplicated genome characteristics and that genomes of different lines are unequally suited to survive diploidization. These results demonstrate the utility of N. vitripennis for delineating mechanisms of animal polyploid evolution, analogous to more advanced polyploid plant models.

Seasonal Adaptation: Geographic Photoperiod-Temperature Patterns Explain Genetic Variation in the Common Vole Tsh Receptor.

The vertebrate photoperiodic neuroendocrine system uses the photoperiod as a proxy to time the annual rhythms in reproduction. The thyrotropin receptor (TSHR) is a key protein in the mammalian seasonal reproduction pathway. Its abundance and function can tune sensitivity to the photoperiod. To investigate seasonal adaptation in mammals, the hinge region and the first part of the transmembrane domain of the Tshr gene were sequenced for 278 common vole (Microtus arvalis) specimens from 15 localities in Western Europe and 28 localities in Eastern Europe. Forty-nine single nucleotide polymorphisms (SNPs; twenty-two intronic and twenty-seven exonic) were found, with a weak or lack of correlation with pairwise geographical distance, latitude, longitude, and altitude. By applying a temperature threshold to the local photoperiod-temperature ellipsoid, we obtained a predicted critical photoperiod (pCPP) as a proxy for the spring onset of local primary food production (grass). The obtained pCPP explains the distribution of the genetic variation in Tshr in Western Europe through highly significant correlations with five intronic and seven exonic SNPs. The relationship between pCPP and SNPs was lacking in Eastern Europe. Thus, Tshr, which plays a pivotal role in the sensitivity of the mammalian photoperiodic neuroendocrine system, was targeted by natural selection in Western European vole populations, resulting in the optimized timing of seasonal reproduction.

Strong variation in frequencies of male and female determiners between neighboring housefly populations.

Sex-determination mechanisms evolve rapidly and vary between species. Occasionally, polymorphic systems are found, like in the housefly. Studying the dynamics and stability of such systems can provide a better understanding of the evolution of sex-determination systems. In the housefly, dominant male-determining loci (M) can lie not only on the Y chromosome (MY ), but also on autosomes (MA ) or the X chromosome (MX ). M enforces male development by inhibiting the female-determining gene transformer (tra). A mutant tra allele, traD , is insensitive to M and is a dominant female determiner. MY prevails at high latitudes and polymorphic M loci together with traD at low latitudes. To get more insight into the stability and frequencies of these sex determiners with mutually exclusive dominance, we investigated 5 regional Spanish populations. We found strong variation among populations. Two populations with hemizygous MIII were found, 2 contained homozygous MX with additional hemizygous MI and MII in 1 population. One population contained homozygous and hemizygous MX with additionally hemizygous MII . All females in populations with homozygous M, had traD , whereas no traD was found in populations without homozygous M. Our results indicate locally stable systems may either harbor a single hemizygous M and no traD , corresponding to a male heterogametic system, or homozygous and/or multiple M and heterozygous traD , reminiscent of a female heterogametic system. They support that M loci can accumulate in the presence of a dominant female determiner. Limited migration between populations may contribute to the stability of these systems.

Doublesex regulates male-specific differentiation during distinct developmental time windows in a parasitoid wasp.

Sexually dimorphic traits in insects are subject to sexual selection, but our knowledge of the underlying molecular mechanisms is still scarce. Here we investigate how the highly conserved gene, Doublesex (Dsx), is involved in shaping sexual dimorphism in the model parasitoid wasp Nasonia vitripennis (Hymenoptera: Pteromalidae). First, we present the revised Dsx gene structure including an alternative transcription start, and two additional male NvDsx transcript isoforms. We show sex-specific NvDsx expression and splicing throughout development, and demonstrate that transient NvDsx silencing in different male developmental stages shifts two sexually dimorphic traits from male to female morphology, with the effect being dependent on the timing of silencing. In addition, we determined the effect of NvDsx on the development of reproductive organs. Transient silencing of NvDsx in early male larvae affects the growth and differentiation of the internal and external reproductive tissues. We did not observe phenotypic changes in females after NvDsx silencing. Our results indicate that male NvDsx is required to suppress female-specific traits and/or to promote male-specific traits during distinct developmental windows. This provides new insights into the regulatory activity of Dsx during male wasp development in the Hymenoptera.

Contribution of opsins and chromophores to cone pigment variation across populations of Lake Victoria cichlids.

Adaptation to heterogeneous sensory environments has been implicated as a key parameter in speciation. Cichlid fish are a textbook example of divergent visual adaptation, mediated by variation in the sequences and expression levels of cone opsin genes (encoding the protein component of visual pigments). In some vertebrates including fish, visual sensitivity is also tuned by the ratio of vitamin A1 /A2 -derived chromophores (i.e., the light-sensitive component of the visual pigment bound to the opsin protein), where higher proportions of A2 cause a more red-shifted wavelength absorbance. This study explores the variation in chromophore ratios across multiple cichlid populations in Lake Victoria, using as a proxy the expression of the gene Cyp27c1, which has been shown to regulate the conversion of vitamin A1 into vitamin A2 in several vertebrates. This study focuses on sympatric Pundamilia cichlids, where species with blue or red male coloration co-occur at multiple islands but occupy different depths and consequently different visual habitats. In the red species, we found higher cyp27c1 expression in populations from turbid waters than from clear waters, but there was no such pattern in the blue species. Across populations, differences between the sympatric species in cyp27c1 expression had a consistent relationship with species differences in opsin expression patterns, but the red/blue identity reversed between clear and turbid waters. To assess the contribution of heritable vs. environmental causes of variation, we tested whether light manipulations induce a change in cyp27c1 expression in the laboratory. We found that cyp27c1 expression was not influenced by experimental light conditions, suggesting that the observed variation in the wild is due to genetic differences. Nonetheless, compared to other cichlid species, cyp27c1 is expressed at very low levels in Pundamilia, suggesting that it may not be relevant for visual adaptation in this species. Conclusively, establishing the biological importance of this variation requires testing of actual A1 /A2 ratios in the eye, as well as its consequences for visual performance.

Absence of complementary sex determination in two Leptopilina species (Figitidae, Hymenoptera) and a reconsideration of its incompatibility with endosymbiont-induced thelytoky.

Complementary sex determination (CSD) is a widespread sex determination mechanism in haplodiploid Hymenoptera. Under CSD, sex is determined by the allelic state of one or multiple CSD loci. Heterozygosity at one or more loci leads to female development, whereas hemizygosity of haploid eggs and homozygosity of diploid eggs results in male development. Sexual (arrhenotokous) reproduction normally yields haploid male and diploid female offspring. Under asexual reproduction (thelytoky), diploidized unfertilized eggs develop into females. Thelytoky is often induced by bacterial endosymbionts that achieve egg diploidization by gamete duplication. As gamete duplication leads to complete homozygosity, endosymbiont-induced thelytokous reproduction is presumed to be incompatible with CSD, which relies on heterozygosity for female development. Previously, we excluded CSD in four Asobara (Braconidae) species and proposed a two-step mechanism for Wolbachia-induced thelytoky in Asobara japonica. Here, we conclusively reject CSD in two cynipid wasp species, Leptopilina heterotoma and Leptopilina clavipes. We further show that thelytoky in L. clavipes depends on Wolbachia titer but that diploidization and feminization steps cannot be separated, unlike in A. japonica. We discuss what these results reveal about the sex determination mechanism of L. clavipes and the presumed incompatibility between CSD and endosymbiont-induced thelytoky in the Hymenoptera.

A single QTL with large effect is associated with female functional virginity in an asexual parasitoid wasp.

During the transition from sexual to asexual reproduction, a suite of reproduction-related sexual traits become superfluous, and may be selected against if costly. Female functional virginity refers to asexual females resisting to mate or not fertilizing eggs after mating. These traits appear to be among the first that evolve during transitions from sexual to asexual reproduction. The genetic basis of female functional virginity remains elusive. Previously, we reported that female functional virginity segregates as expected for a single recessive locus in the asexual parasitoid wasp Asobara japonica. Here, we investigate the genetic basis of this trait by quantitative trait loci (QTL) mapping and candidate gene analyses. Consistent with the segregation of phenotypes, we found a single QTL of large effect, spanning over 4.23 Mb and comprising at least 131 protein-coding genes, of which 15 featured sex-biased expression in the related sexual species Asobara tabida. Two of the 15 sex-biased genes were previously identified to differ between related sexual and asexual population/species: CD151 antigen and nuclear pore complex protein Nup50. A third gene, hormone receptor 4, is involved in steroid hormone mediated mating behaviour. Overall, our results are consistent with a single locus, or a cluster of closely linked loci, underlying rapid evolution of female functional virginity in the transition to asexuality. Once this variant, causing rejection to mate, has swept through a population, the flanking region does not get smaller owing to lack of recombination in asexuals.

A chimeric gene paternally instructs female sex determination in the haplodiploid wasp Nasonia.

Various primary signals direct insect sex determination. In hymenopteran insects, the presence of a paternal genome is needed to initiate female development. When absent, uniparental haploid males develop. We molecularly and functionally identified the instructor sex-determination gene, wasp overruler of masculinization (wom), of the haplodiploid wasp Nasonia vitripennis This gene contains a P53-like domain coding region and arose by gene duplication and genomic rearrangements. Maternal silencing of wom results in male development of haploid embryos. Upon fertilization, early zygotic transcription from the paternal wom allele is initiated, followed by a timely zygotic expression of transformer (tra), leading to female development. Wom is an instructor gene with a parent-of-origin effect in sex determination.

Next-generation biological control: the need for integrating genetics and genomics.

Biological control is widely successful at controlling pests, but effective biocontrol agents are now more difficult to import from countries of origin due to more restrictive international trade laws (the Nagoya Protocol). Coupled with increasing demand, the efficacy of existing and new biocontrol agents needs to be improved with genetic and genomic approaches. Although they have been underutilised in the past, application of genetic and genomic techniques is becoming more feasible from both technological and economic perspectives. We review current methods and provide a framework for using them. First, it is necessary to identify which biocontrol trait to select and in what direction. Next, the genes or markers linked to these traits need be determined, including how to implement this information into a selective breeding program. Choosing a trait can be assisted by modelling to account for the proper agro-ecological context, and by knowing which traits have sufficiently high heritability values. We provide guidelines for designing genomic strategies in biocontrol programs, which depend on the organism, budget, and desired objective. Genomic approaches start with genome sequencing and assembly. We provide a guide for deciding the most successful sequencing strategy for biocontrol agents. Gene discovery involves quantitative trait loci analyses, transcriptomic and proteomic studies, and gene editing. Improving biocontrol practices includes marker-assisted selection, genomic selection and microbiome manipulation of biocontrol agents, and monitoring for genetic variation during rearing and post-release. We conclude by identifying the most promising applications of genetic and genomic methods to improve biological control efficacy.

Genomics of sex allocation in the parasitoid wasp Nasonia vitripennis.

BACKGROUND: Whilst adaptive facultative sex allocation has been widely studied at the phenotypic level across a broad range of organisms, we still know remarkably little about its genetic architecture. Here, we explore the genome-wide basis of sex ratio variation in the parasitoid wasp Nasonia vitripennis, perhaps the best studied organism in terms of sex allocation, and well known for its response to local mate competition. RESULTS: We performed a genome-wide association study (GWAS) for single foundress sex ratios using iso-female lines derived from the recently developed outbred N. vitripennis laboratory strain HVRx. The iso-female lines capture a sample of the genetic variation in HVRx and we present them as the first iteration of the Nasonia vitripennis Genome Reference Panel (NVGRP 1.0). This panel provides an assessment of the standing genetic variation for sex ratio in the study population. Using the NVGRP, we discovered a cluster of 18 linked SNPs, encompassing 9 annotated loci associated with sex ratio variation. Furthermore, we found evidence that sex ratio has a shared genetic basis with clutch size on three different chromosomes. CONCLUSIONS: Our approach provides a thorough description of the quantitative genetic basis of sex ratio variation in Nasonia at the genome level and reveals a number of inter-related candidate loci underlying sex allocation regulation.

Latitudinal Variation in Circadian Rhythmicity in Nasonia vitripennis.

Many physiological processes of living organisms show circadian rhythms, governed by an endogenous clock. This clock has a genetic basis and is entrained by external cues, such as light and temperature. Other physiological processes exhibit seasonal rhythms, that are also responsive to light and temperature. We previously reported a natural latitudinal cline of photoperiodic diapause induction in the parasitic wasp Nasonia vitripennis in Europe and a correlated haplotype frequency for the circadian clock gene period (per). To evaluate if this correlation is reflected in circadian behaviour, we investigated the circadian locomotor activity of seven populations from the cline. We found that the proportion of rhythmic males was higher than females in constant darkness, and that mating decreased rhythmicity of both sexes. Only for virgin females, the free running period (τ) increased weakly with latitude. Wasps from the most southern locality had an overall shorter free running rhythm and earlier onset, peak, and offset of activity during the 24 h period, than wasps from the northernmost locality. We evaluated this variation in rhythmicity as a function of period haplotype frequencies in the populations and discussed its functional significance in the context of local adaptation.

Life-history traits of the Whiting polyploid line of the parasitoid Nasonia vitripennis.

In hymenopterans, males are normally haploid (1n) and females diploid (2n), but individuals with divergent ploidy levels are frequently found. In species with 'complementary sex determination' (CSD), increasing numbers of diploid males that are often infertile or unviable arise from inbreeding, presenting a major impediment to biocontrol breeding. Non-CSD species, which are common in some parasitoid wasp taxa, do not produce polyploids through inbreeding. Nevertheless, polyploidy also occurs in non-CSD Hymenoptera. As a first survey on the impacts of inbreeding and polyploidy of non-CSD species, we investigate life-history traits of a long-term laboratory line of the parasitoid Nasonia vitripennis (Walker) (Hymenoptera: Pteromalidae) ('Whiting polyploid line') in which polyploids of both sexes (diploid males, triploid females) are viable and fertile. Diploid males produce diploid sperm and virgin triploid females produce haploid and diploid eggs. We found that diploid males did not differ from haploid males with respect to body size, progeny size, mate competition, or lifespan. When diploid males were mated to many females (without accounting for mating order), the females produced a relatively high proportion of male offspring, possibly indicating that these males produce less sperm and/or have reduced sperm functionality. In triploid females, parasitization rate and fecundity were reduced and body size was slightly increased, but there was no effect on lifespan. After one generation of outbreeding, lifespan as well as parasitization rate were increased, and a body size difference was no longer apparent. This suggests that outbreeding has an effect on traits observed in an inbred polyploidy background. Overall, these results indicate some phenotypic detriments of non-CSD polyploids that must be taken into account in breeding.

Natural and Artificial Selection for Parasitoid Resistance in Drosophila melanogaster Leave Different Genetic Signatures.

Adaptation of complex traits depends on standing genetic variation at multiple loci. The allelic variants that have positive fitness effects, however, can differ depending on the genetic background and the selective pressure. Previously, we interrogated the Drosophila melanogaster genome at the population level for polymorphic positions and identified 215 single nucleotide polymorphisms (SNPs) that had significantly changed in frequency after experimental evolution for increased parasitoid resistance. In the current study, we follow up on 11 of these SNPs as putative targets of the experimental selection process (Jalvingh et al., 2014). We study the patterns of genetic variation for these SNPs in several European field populations. Furthermore, we associate the genetic variation of these SNPs to variation in resistance against the parasitoid Asobara tabida, by determining the individual phenotype and SNP genotype for 144 individuals from four Selection lines and four non-selected Control lines and for 400 individuals from 12 Field lines that differ in parasitoid resistance. For the Selection lines we additionally monitored the changes in allele frequencies throughout the five generations of experimental selection. For three genes, mbl (Zn-finger protein), mthl4 (G-protein coupled receptor) and CG17287 (protein-cysteine S-palmitoyltransferase) individual SNP genotypes were significantly associated with resistance level in the Selection and Control lines. Additionally, the minor allele in mbl and mthl4 were consistently and gradually favored throughout the five generations of experimental evolution. However, none of these alleles did appear to be associated to high resistance in the Field lines. We suggest that, within field populations, selection for parasitoid resistance is a gradual process that involves co-adapted gene complexes. Fast artificial selection, however, enforces the sudden cumulating of particular alleles that confer high resistance (genetic sweep). We discuss our findings in the context of local adaptation.

Adaptive Differences in Circadian Clock Gene Expression Patterns and Photoperiodic Diapause Induction in Nasonia vitripennis.

Day length (photoperiod) and temperature oscillate daily and seasonally and are important cues for season-dependent behavior. Larval diapause of the parasitoid Nasonia vitripennis is maternally induced following a certain number of days (switch point) of a given critical photoperiod (CPP). Both the switch point and the CPP follow a latitudinal cline in European N. vitripennis populations. We previously showed that allelic frequencies of the clock gene period correlate with this diapause induction cline. Here we report that circadian expression of four clock genes-period (per), cryptochrome-2 (cry-2), clock (clk), and cycle (cyc)-oscillates as a function of photoperiod and latitude of origin in wasps from populations from the extremes of the cline. Expression amplitudes are lower in northern wasps, indicating a weaker, more plastic clock. Northern wasps also have a later onset of activity and longer free-running rhythms under constant conditions. RNA interference of per caused speeding up of the circadian clock, changed the expression of other clock genes, and delayed diapause in both southern and northern wasps. These results point toward adaptive latitudinal clock gene expression differences and to a key role of per in the timing of photoperiodic diapause induction of N. vitripennis.

Maternal provision of transformer-2 is required for female development and embryo viability in the wasp Nasonia vitripennis.

In insect sex determination a primary signal starts the genetic sex determination cascade that, in most insect orders, is subsequently transduced down the cascade by a transformer (tra) ortholog. Only a female-specifically spliced tra mRNA yields a functional TRA-protein that forms a complex with TRA2, encoded by a transformer-2 (tra2) ortholog, to act as a sex specific splicing regulator of the downstream transcription factors doublesex (dsx) and fruitless (fru). Here, we identify the tra2 ortholog of the haplodiploid parasitoid wasp N. vitripennis (Nv-tra2) and confirm its function in N. vitripennis sex determination. Knock down of Nv-tra2 by parental RNA interference (pRNAi) results in complete sex reversal of diploid offspring from female to male, indicating the requirement of Nv-tra2 for female sex determination. As Nv-tra2 pRNAi leads to frequent lethality in early developmental stages, maternal provision of Nv-tra2 transcripts is apparently also required for another, non-sex determining function during embryogenesis. In addition, lethality following Nv-tra2 pRNAi appears more pronounced in diploid than in haploid offspring. This diploid lethal effect was also observed following Nv-tra pRNAi, which served as a positive control in our experiments. As diploid embryos from fertilized eggs have a paternal chromosome set in addition to the maternal one, this suggests that either the presence of this paternal chromosome set or the dosage effect resulting from the diploid state is incompatible with the induced male development in N. vitripennis caused by either Nv-tra2 or Nv-tra pRNAi. The role of Nv-tra2 in activating the female sex determination pathway yields more insight into the sex determination mechanism of Nasonia.

Male sex in houseflies is determined by Mdmd, a paralog of the generic splice factor gene CWC22.

Across species, animals have diverse sex determination pathways, each consisting of a hierarchical cascade of genes and its associated regulatory mechanism. Houseflies have a distinctive polymorphic sex determination system in which a dominant male determiner, the M-factor, can reside on any of the chromosomes. We identified a gene, Musca domesticamale determiner (Mdmd), as the M-factor. Mdmd originated from a duplication of the spliceosomal factor gene CWC22 (nucampholin). Targeted Mdmd disruption results in complete sex reversal to fertile females because of a shift from male to female expression of the downstream genes transformer and doublesex The presence of Mdmd on different chromosomes indicates that Mdmd translocated to different genomic sites. Thus, an instructive signal in sex determination can arise by duplication and neofunctionalization of an essential splicing regulator.

QTL analysis of the photoperiodic response and clinal distribution of period alleles in Nasonia vitripennis.

In seasonal environments, organisms synchronize their life cycle with the annual cycle of environmental factors. In many insect species, this includes a diapause response: a timed dormant stage that allows to survive harsh winter conditions. Previously, we have shown that larval diapause in the parasitic wasp Nasonia vitripennis is induced by the mother upon exposure to a threshold number of short photoperiods (named switch point) and diapause response follows a latitudinal cline in natural populations. Here, we present a QTL analysis using two lines derived from the extremes of this clinal distribution: a northern line from Oulu, Finland and a southern line from Corsica, France. A genomic region on chromosome 1 and one on chromosome 5 were found to be associated with photoperiodic diapause induction. Interestingly, these regions contain the putative clock genes period, cycle (chromosome 1) and cryptochrome (chromosome 5). An analysis of period polymorphisms in seven European populations showed a clinal distribution of two main haplotypes that correlate with the latitudinal cline for diapause induction.

The spatio-temporal partitioning of sperm by males of the prospermatogenic parasitoid Nasonia vitripennis is in line with its gregarious lifestyle.

Male fitness depends on the number of lifetime progeny of their mates and could be constrained by the chance of finding a mate, lifespan and temporal patterns of sperm production and allocation. Here, we used the parasitic wasp Nasonia vitripennis with a two-week lifespan and a gregarious lifestyle, to analyze how the reproductive system is organized to allocate spermatozoa over consecutive matings. Results show that spermatogenesis is synchronized and completed one day before emergence so that males emerge with a full sperm complement. We also found a regulation of spermatozoa transfer between testis and seminal vesicles that allows males to partition small ejaculates over multiple matings. Overall, this study shows that for N. vitripennis, male fertilization potential is determined (1) at the pupal stage, when spermatogenesis takes place to generate a complete life-long stock, (2) on emergence, when transport of spermatozoa from testes to seminal vesicles is initiated and (3) in adulthood, during which spermatozoa are partitioned over successive copulations. Such life history-traits are consistent with the gregarious lifestyle of N. vitripennis.

Quantitative Trait Locus Analysis of Mating Behavior and Male Sex Pheromones in Nasonia Wasps.

A major focus in speciation genetics is to identify the chromosomal regions and genes that reduce hybridization and gene flow. We investigated the genetic architecture of mating behavior in the parasitoid wasp species pair Nasonia giraulti and Nasonia oneida that exhibit strong prezygotic isolation. Behavioral analysis showed that N. oneida females had consistently higher latency times, and broke off the mating sequence more often in the mounting stage when confronted with N. giraulti males compared with males of their own species. N. oneida males produce a lower quantity of the long-range male sex pheromone (4R,5S)-5-hydroxy-4-decanolide (RS-HDL). Crosses between the two species yielded hybrid males with various pheromone quantities, and these males were used in mating trials with females of either species to measure female mate discrimination rates. A quantitative trait locus (QTL) analysis involving 475 recombinant hybrid males (F2), 2148 reciprocally backcrossed females (F3), and a linkage map of 52 equally spaced neutral single nucleotide polymorphism (SNP) markers plus SNPs in 40 candidate mating behavior genes revealed four QTL for male pheromone amount, depending on partner species. Our results demonstrate that the RS-HDL pheromone plays a role in the mating system of N. giraulti and N. oneida, but also that additional communication cues are involved in mate choice. No QTL were found for female mate discrimination, which points at a polygenic architecture of female choice with strong environmental influences.

Hybrid incompatibilities are affected by dominance and dosage in the haplodiploid wasp Nasonia.

Study of genome incompatibilities in species hybrids is important for understanding the genetic basis of reproductive isolation and speciation. According to Haldane's rule hybridization affects the heterogametic sex more than the homogametic sex. Several theories have been proposed that attribute asymmetry in hybridization effects to either phenotype (sex) or genotype (heterogamety). Here we investigate the genetic basis of hybrid genome incompatibility in the haplodiploid wasp Nasonia using the powerful features of haploid males and sex reversal. We separately investigate the effects of heterozygosity (ploidy level) and sex by generating sex reversed diploid hybrid males and comparing them to genotypically similar haploid hybrid males and diploid hybrid females. Hybrid effects of sterility were more pronounced than of inviability, and were particularly strong in haploid males, but weak to absent in diploid males and females, indicating a strong ploidy level but no sex specific effect. Molecular markers identified a number of genomic regions associated with hybrid inviability in haploid males that disappeared under diploidy in both hybrid males and females. Hybrid inviability was rescued by dominance effects at some genomic regions, but aggravated or alleviated by dosage effects at other regions, consistent with cytonuclear incompatibilities. Dosage effects underlying Bateson-Dobzhansky-Muller (BDM) incompatibilities need more consideration in explaining Haldane's rule in diploid systems.