Adaptive differences in circadian clock gene expression patterns and photoperiodic diapause induction in <i>Nasonia vitripennis</i>

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.</p

Double nexus-Doublesex is the connecting element in sex determination

In recent years, our knowledge of the conserved master-switch gene doublesex (dsx) and its function in regulating the development of dimorphic traits in insects has deepened considerably. Here, a comprehensive overview is given on the properties of the male- and female-specific dsx transcripts yielding DSXF and DSXM proteins in Drosophila melanogaster, and the many downstream targets that they regulate. As insects have cell-autonomous sex determination, it was assumed that dsx would be expressed in every somatic cell, but recent research showed that dsx is expressed only when a cell is required to show its sexual identity through function or morphology. This spatiotemporal regulation of dsx expression has not only been established in D. melanogaster but in all insect species studied. Gradually, it has been appreciated that dsx could no longer be viewed as the master-switch gene orchestrating sexual development and behaviour in each cell, but instead should be viewed as the interpreter for the sexual identity of the cell, expressing this identity only on request, making dsx the central nexus of insect sex determinatio

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. Here, we explore 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 conversion of Vitamin A1 - into A2 in several vertebrates.We focus 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 versus 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. However, 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. This article is protected by copyright. All rights reserved

Genetic variability of arrhenotokous and thelytokous Venturiacanescens (Hymenoptera)

The ichneumonid wasp Venturia canescens (Hymenoptera) has been studied extensively for foraging behaviour and population dynamics of sexually (arrhenotokous) and parthenogenetically (thelytokous) reproducing individuals. Here we report the development of a set of microsatellite markers for V.canescens and use them to show that arrhenotokous individuals have more genetic variability than thelytokous ones, which are even homozygous for all tested loci. Crosses between arrhenotokous individuals suggested one marker, Vcan071, to be linked with the Complementary Sex Determiner (CSD) locus and one, Vcan109, with the Virus Like Protein (vlp-p40) locus. The genome size of V. canescens was estimated to be 274–279 Mb. We discuss how both reproductive modes can give rise to the observed genetic variability and how the new markers can be used for future genetic studies of V. canescens

Visual system plasticity is differently mediated by cone opsin expression and chromophore composition in closely related cichlid species

Phenotypic plasticity allows organisms to rapidly adjust to environmental changes. Cichlid fish inhabit a wide range of light environments and show a large diversity in visual system properties, which makes them a good model system to address the role of phenotypic plasticity in visual adaptation. Cichlid retinal cone pigments consist of opsin proteins bound to Vitamin A1 or A2-derived chromophores. Plasticity in expression has been shown for cichlid opsin genes, but less is known about the contribution of cyp27c1, the enzyme that converts Vitamin A1 into A2,. Here, we studied both opsin and cyp27c1 expression patterns for three closely related cichlid species from different visual habitats in Lake Victoria, across different light treatments. We found differences in cyp27c1 as well as in opsin expression patterns between the three species. Experimental light treatments affected the developmental trajectory of cyp27c1 expression in one species and opsin expression in all three species. Within each species, we found large individual variation in cyp27c1 expression levels and no consistent association with opsin expression levels. These results indicate that visual system plasticity of even closely related species can be differentially mediated by opsin and cyp27c1 expression, possibly associated with species differences in visual niche

Absence of complementary sex determination in two <i>Leptopilina</i> 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

Hybrid incompatibilities are affected by dominance and dosage in the haplodiploid wasp <em>Nasonia</em>

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.</p

A chimeric gene paternally instructs female sex determination in the haplodiploid wasp <i>Nasonia</i>

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

Genomics of sex allocation in the parasitoid wasp Nasonia vitripennis

This research was funded by the Netherlands Genomics Initiative (Zenith 93511041) and the by the Natural Environment Research Council (NE/J024481/1).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.Publisher PDFPeer reviewe