Proximate influence on eusocial caste behaviour

Queens and workers of eusocial sweat bee species are morphologically and developmentally similar, which means that each female is capable of behaving as a queen or a worker. However, few females lay eggs and behave as queens, while the majority of females provision the queen’s offspring, rarely lay eggs, and behave as workers. This makes eusocial sweat bee species, such as Lasioglossum laevissimum, excellent models to study the underling environmental (social) and genetic factors that contribute to variation in caste behaviours. My research focused on describing some of the proximate mechanisms that influence caste behaviours in L. laevissimum females. The social environment of a sweat bee colony, specifically the behaviour of a queen, can have a dramatic impact on worker behaviour. Queens suppress worker reproduction by physically bullying their workers. In a nesting aggregation at Brock University, almost half of L. laevissimum nests became queenless, which provided me with a natural experiment to assess the direct influence by queens on worker behaviour. Dissection data showed that a small proportion (17%) of workers developed their ovaries in both queenright and queenless nests. This suggests that L. laevissimum queens exert an early, negative, and strong influence on worker egg-laying behaviour, which lasts after she is gone. Next I assessed the relationship between gene expression and L. laeivsismum caste behaviours. I predicited that queens would express a gene associated with egg-laying, vitellogenin, more than workers, and that workers would express genes associated with foraging, the foraging gene, more than queens. Lasioglossum laevissimum queens had higher vitellogenin expression levels than workers, and females with high ovarian development had high vitellogenin expression, regardless of caste. On the other hand, queens and workers had similar foraging expression levels. Gene expression comparisons between queens and workers highlight two important behavioural characteristics of sweat bee castes. First, in eusocial sweat bees, both queens and workers actively provision brood at some point during the breeding season, which is reflected in their similar foraging expression levels. Secondly, queens lay eggs while a small proportion of workers have queen-like ovarian development, reflected in vitellogenin expression differences between castes

Foraging gene expression patterns in queens, workers, and males in a eusocial insect

Reproductive division of labour is based on biased expression of complementary parental behaviours, brood production (egg-laying) by queens and brood care (in particular, brood provisioning) by workers. In many social insect species, queens provision brood when establishing colonies at the beginning of a breeding season and reproductive division of labour begins with the emergence of workers. In many social insect species, the expression of foraging (for) mRNA is associated with the intensity of foraging behaviour, and therefore brood provisioning. However, only two studies have compared queen and worker for expression levels, and neither accounted for transcript splice variation. In this study, we compare the expression level of the for-α transcript variant across four life stages of the queen caste, two behavioural groups of workers, and males of a eusocial sweat bee Lasioglossum laevissimum (Smith, 1853). Foundresses collected prior to the onset of the foraging season and males had the highest for-α expression levels. All active (post-hibernatory) queens and workers had similar for-α expression levels independent of behaviour. These results suggest that the for gene in L. laevissium acts as a primer before foraging activity, and that caste-specific expression patterns correlate with the timing of foraging activity in queens and workers

Integrating GWAS and Transcriptomics to Identify the Molecular Underpinnings of Thermal Stress Responses in \u3cem\u3eDrosophila melanogaster\u3c/em\u3e

Thermal tolerance of an organism depends on both the ability to dynamically adjust to a thermal stress and preparatory developmental processes that enhance thermal resistance. However, the extent to which standing genetic variation in thermal tolerance alleles influence dynamic stress responses vs. preparatory processes is unknown. Here, using the model species Drosophila melanogaster, we used a combination of Genome Wide Association mapping (GWAS) and transcriptomic profiling to characterize whether genes associated with thermal tolerance are primarily involved in dynamic stress responses or preparatory processes that influence physiological condition at the time of thermal stress. To test our hypotheses, we measured the critical thermal minimum (CTmin) and critical thermal maximum (CTmax) of 100 lines of the Drosophila Genetic Reference Panel (DGRP) and used GWAS to identify loci that explain variation in thermal limits. We observed greater variation in lower thermal limits, with CTmin ranging from 1.81 to 8.60°C, while CTmax ranged from 38.74 to 40.64°C. We identified 151 and 99 distinct genes associated with CTmin and CTmax, respectively, and there was strong support that these genes are involved in both dynamic responses to thermal stress and preparatory processes that increase thermal resistance. Many of the genes identified by GWAS were involved in the direct transcriptional response to thermal stress (72/151 for cold; 59/99 for heat), and overall GWAS candidates were more likely to be differentially expressed than other genes. Further, several GWAS candidates were regulatory genes that may participate in the regulation of stress responses, and gene ontologies related to development and morphogenesis were enriched, suggesting many of these genes influence thermal tolerance through effects on development and physiological status. Overall, our results suggest that thermal tolerance alleles can influence both dynamic plastic responses to thermal stress and preparatory processes that improve thermal resistance. These results also have utility for directly comparing GWAS and transcriptomic approaches for identifying candidate genes associated with thermal tolerance

Vitellogenin expression corresponds with reproductive status and caste in a primitively eusocial bee, Lasioglossum laevissimum

Vitellogenin (vg) expression is consistently associated with variation in insect phenotypes, particularly egg-laying. Primitively eusocial species, such as eusocial sweat bees, have behaviourally totipotent castes, in which each female is capable of high levels of ovarian development. Few studies have investigated vg expression patterns in primitively eusocial insects, and only one study has focused on a primitively eusocial bee. Here we use a primitively eusocial sweat bee, Lasioglossum laevissimum, and Real Time quantitative PCR (RT-qPCR) to investigate the relationship between vg expression, castes, and variation in phenotypes associated with castes differences. These assays showed that females with high ovarian development had the highest levels of vg expression, and that vg expression levels reflected the reproductive status of females first and caste second. This is in contrast to vg expression patterns observed in advanced eusocial queens and workers, which differ in vg expression based on caste and have caste-specific vg expression patterns. Furthermore, future queens (gynes) do not have ovarian development and had similar vg expression levels to early spring foundresses, which do have ovarian development, supporting Vg’s function as a transporter of lipids and amino acids before diapause