Deadly combinations:Hybrid incompatibilities in the parasitic wasp genus <i>Nasonia</i>

Invloed man zijn en DNA-hoeveelheid bij soortvorming parasitaire wespen Sinds Darwin’s The Origin of Species is er veel onderzoek gedaan naar de processen die een rol spelen bij soortvorming. Het promotieonderzoek van Tosca Koevoets richt zich op het vroege stadium in het proces van soortvorming. Wanneer groepen niet meer in contact staan met elkaar, zullen ze langzaam ten gaan veranderen ten opzichte van elkaar, totdat zij verschillende soorten zijn geworden. Uiteindelijk zullen deze soorten genetisch zo veranderd zijn, dat het combineren van DNA van de verschillende soorten niet meer mogelijk is, wat tot uiting komt in allerlei problemen in de hybride nakomelingen. Deze problemen blijken in het beginstadium van soortvorming anders te zijn in mannen dan in vrouwen. Om te onderzoeken waar dit door komt, maakte Koevoets gebruik van parasitaire wespensoorten uit het geslacht Nasonia. Wespen hebben geen geslachtschromosomen, maar vrouwen ontstaan uit bevruchte eitjes en mannen uit onbevruchte eitjes. Vrouwen hebben dus twee keer zoveel DNA als mannen. Ook bij wespen bleken mannen kwetsbaarder voor hybridisatie dan vrouwen. Deze kwetsbaarheid werd zelfs nog groter onder stressomstandigheden. Dit werd veroorzaakt doordat mannen minder genetische variatie hebben, maar ook doordat ze überhaupt minder DNA hebben. Via een genetische techniek kunnen we er voor zorgen dat bevruchte eitjes (dus eigenlijk bestemd om vrouw te worden) zich tot man ontwikkelen. Daardoor kon onderzocht worden wat belangrijker is: man zijn of de hoeveelheid DNA. Natuurlijk lag het antwoord in het midden; beide facetten bleken belangrijk. Dit onderzoek toont aan dat genetische interacties, de hoeveelheid DNA en de genetische variatie belangrijke aspecten zijn bij het ontstaan van soorten

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

Recombination and its impact on the genome of the haplodiploid parasitoid wasp Nasonia

Homologous meiotic recombination occurs in most sexually reproducing organisms, yet its evolutionary advantages are elusive. Previous research explored recombination in the honeybee, a eusocial hymenopteran with an exceptionally high genome-wide recombination rate. A comparable study in a non-social member of the Hymenoptera that would disentangle the impact of sociality from Hymenoptera-specific features such as haplodiploidy on the evolution of the high genome-wide recombination rate in social Hymenoptera is missing. Utilizing single-nucleotide polymorphisms (SNPs) between two Nasonia parasitoid wasp genomes, we developed a SNP genotyping microarray to infer a high-density linkage map for Nasonia. The map comprises 1,255 markers with an average distance of 0.3 cM. The mapped markers enabled us to arrange 265 scaffolds of the Nasonia genome assembly 1.0 on the linkage map, representing 63.6% of the assembled N. vitripennis genome. We estimated a genome-wide recombination rate of 1.4-1.5 cM/Mb for Nasonia, which is less than one tenth of the rate reported for the honeybee. The local recombination rate in Nasonia is positively correlated with the distance to the center of the linkage groups, GC content, and the proportion of simple repeats. In contrast to the honeybee genome, gene density in the parasitoid wasp genome is positively associated with the recombination rate; regions of low recombination are characterized by fewer genes with larger introns and by a greater distance between genes. Finally, we found that genes in regions of the genome with a low recombination frequency tend to have a higher ratio of non-synonymous to synonymous substitutions, likely due to the accumulation of slightly deleterious non-synonymous substitutions. These findings are consistent with the hypothesis that recombination reduces interference between linked sites and thereby facilitates adaptive evolution and the purging of deleterious mutations. Our results imply that the genomes of haplodiploid and of diploid higher eukaryotes do not differ systematically in their recombination rates and associated parameters.Publisher PDFPeer reviewe

Functional and Evolutionary Insights from the Genomes of Three Parasitoid Nasonia Species

We report here genome sequences and comparative analyses of three closely related parasitoid wasps: Nasonia vitripennis, N. giraulti, and N. longicornis. Parasitoids are important regulators of arthropod populations, including major agricultural pests and disease vectors, and Nasonia is an emerging genetic model, particularly for evolutionary and developmental genetics. Key findings include the identification of a functional DNA methylation tool kit; hymenopteran-specific genes including diverse venoms; lateral gene transfers among Pox viruses, Wolbachia, and Nasonia; and the rapid evolution of genes involved in nuclear-mitochondrial interactions that are implicated in speciation. Newly developed genome resources advance Nasonia for genetic research, accelerate mapping and cloning of quantitative trait loci, and will ultimately provide tools and knowledge for further increasing the utility of parasitoids as pest insect-control agents.