Temporal Analysis of Effective Population Size and Mating System in a Social Wasp

Novel insights into paternity skew in a polyandrous social wasp

Females of many species are polyandrous. However, polyandry can give rise to conflict among individuals within families. We examined the level of polyandry and paternity skew in the common eastern yellowjacket wasp, Vespula maculifrons, in order to gain a greater understanding of conflict in social insects. We collected 10 colonies of V. maculifrons and genotyped workers and prereproductive queens at highly variable microsatellite markers to assign each to a patriline. Genotypic data revealed evidence of significant paternity skew among patrilines. In addition, we found that patrilines contributed differentially to caste production (worker vs. queen), suggesting an important role for reproductive conflict not previously discovered. We also investigated if patterns of paternity skew and mate number varied over time. However, we found no evidence of changes in levels of polyandry when compared to historical data dating back almost 40 years. Finally, we measured a suite of morphological traits in individuals from the most common and least common patrilines in each colony to test if males that showed highly skewed reproductive success also produced offspring that differed in phenotype. Our data revealed weak correlation between paternity skew and morphological phenotype of offspring sired by different males, suggesting no evidence of evolutionary tradeoffs at the level investigated. Overall, this study is the first to report significant paternity and caste-associated skew in V. maculifrons, and to investigate the phenotypic consequences of skew in a social wasp. Our results suggest that polyandry can have important consequences on the genetic and social structure of insect societies.

Genetic and environmental effects on morphological traits of social phenotypes in wasps

Many species exhibit distinct phenotypic classes, such as sexes in dioecious species or castes in social species. The evolution of these classes is affected by the genetic architecture governing traits shared between phenotypes. However, estimates of the genetic and environmental factors contributing to phenotypic variation in distinct classes have rarely been examined. We studied the genetic architecture underlying morphological traits in phenotypic classes in the social wasp Vespula maculifrons. Our data revealed patriline effects on a few traits, indicating weak genetic influences on caste phenotypic variation. Interestingly, traits exhibited higher heritability in queens than workers. This result suggests that genetic variation has a stronger influence on trait variation in the queen caste than the worker caste, which is unexpected because queens typically experience direct selection. Moreover, estimates of heritability for traits were correlated between the castes, indicating that variability in trait size was governed by similar genetic architecture in the two castes. However, we failed to find evidence for a significant relationship between caste dimorphism and caste correlation, as would be expected if trait evolution was constrained by intralocus genetic conflict. Our analyses also uncovered variation in the allometric relationships for traits. These analyses suggested that worker traits were proportionally smaller than queen traits for most traits examined. Overall, our data provide evidence for a strong environmental and moderate genetic basis of trait variation among castes. Moreover, our results suggest that selection previously operated on caste phenotype in this species, and phenotypic variation is now governed primarily by environmental differences.

Pervasive relaxed selection in termite genomes

Genetic changes that enabled the evolution of eusociality have long captivated biologists. More recently, attention has focussed on the consequences of eusociality on genome evolution. Studies have reported higher molecular evolutionary rates in eusocial hymenopteran insects compared with their solitary relatives. To investigate the genomic consequences of eusociality in termites, we analysed nine genomes, including newly sequenced genomes from three non-eusocial cockroaches. Using a phylogenomic approach, we found that termite genomes have experienced lower rates of synonymous substitutions than those of cockroaches, possibly as a result of longer generation times. We identified higher rates of non-synonymous substitutions in termite genomes than in cockroach genomes, and identified pervasive relaxed selection in the former (24-31% of the genes analysed) compared with the latter (2-4%). We infer that this is due to reductions in effective population size, rather than gene-specific effects (e.g. indirect selection of caste-biased genes). We found no obvious signature of increased genetic load in termites, and postulate efficient purging of deleterious alleles at the colony level. Additionally, we identified genomic adaptations that may underpin caste differentiation, such as genes involved in post-translational modifications. Our results provide insights into the evolution of termites and the genomic consequences of eusociality more broadly.

Social structure of perennial Vespula squamosa wasp colonies

Many social species show variation in their social structure in response to different environmental conditions. For example, colonies of the yellowjacket wasp Vespula squamosa are typically headed by a single reproductive queen and survive for only a single season. However, in warmer climates, V. squamosa colonies sometimes persist for multiple years and can grow to extremely large size. We used genetic markers to understand patterns of reproduction and recruitment within these perennial colonies. We genotyped V. squamosa workers, pre-reproductive queens, and males from perennial colonies in the southeastern United States at 10 polymorphic microsatellite loci and one mitochondrial DNA locus. We found that V. squamosa from perennial nests were produced by multiple reproductives, in contrast to typical annual colonies. Relatedness of nestmates from perennial colonies was significantly lower than relatedness of nestmates from annual colonies. Our analyses of mitochondrial DNA indicated that most V. squamosa perennial colonies represented semiclosed systems whereby all individuals belonged to a single matriline despite the presence of multiple reproductive females. However, new queens recruited into perennial colonies apparently mated with non-nestmate males. Notably, perennial and annual colonies did not show significant genetic differences, supporting the hypothesis that perennial colony formation represents an instance of social plasticity. Overall, our results indicate that perennial V. squamosa colonies show substantial changes to their social biology compared to typical annual colonies and demonstrate variation in social behaviors in highly social species.