Insect adaptation to novel environments

Adaptation to new habitats is expected to impose strong selection for divergence; we aim to understand the factors that promote or hinder adaptation. We use the red flour beetle (Tribolium castaneum) to analyze the behavioral, ecological, and evolutionary aspects of adaptation to novel environments. In most cases we use new dietary resources (different flours) as new habitats, imposing strong selection on population growth and offspring development. However, we sometimes also use other selective agents – such as pathogens – to understand the evolution of specific traits such as immune function.

The evolutionary ecology of insect immune function

Insects do not have cellular components of vertebrate adaptive immunity, and are hence thought to depend primarily on innate immunity for defense against pathogens and parasites. Although innate immunity is less complicated, we do not understand the evolution, variability, mechanisms, and consequences of insect immune responses. Imroze and Arun are using flour beetles infected with their natural pathogen, Bacillus thuringiensis, to test hypotheses about the ecology and evolution of insect immune function. Recently, they showed that while post-infection survival declined in older beetles, components of innate immunity either improved or did not change with age (Khan et al 2016, JAE). Interactions between age, sex and mating differentially affect various immune components, potentially causing the observed contrast between immune components and post-infection survival. These results are exciting because they identify new factors that affect immune senescence, and their relative impact on the organism’s fitness.

Although insects do not have adaptive immunity, many species do exhibit a form non-cellular memory called ‘immune priming’. Individuals “primed” with a non-lethal dose of a pathogen are more likely to survive a subsequent lethal infection with the same pathogen. However, the priming effect seems to be variable across insect species, pathogens, and the severity of infection. What governs this variability? To answer this, Imroze and Arun quantified the survival benefit of immune priming across 10 wild-collected flour beetle populations, at different life stages. They found substantial variability (Khan, Prakash et al 2016, Ecology and Evolution); recent results indicate that this variation in survival benefit is associated with reproductive benefits of priming (Khan et al, in prep). We now plan to analyze the mechanistic basis of immune priming in the flour beetle, and understand the molecular basis of the observed variability across stages.

Testing models of adaptation with gene flow

A major research focus in evolutionary biology and ecology is to understand how populations diverge from each other, particularly when they are exchanging genes. Theory predicts that the effects of gene flow on adaptation depend on relative population size and the strength of selection, but the effects of the timing of migration remain empirically unexplored. Aparna is experimentally testing the impacts of temporal variation in migration in flour beetle populations  adapting to a novel habitat. The project will help us understand the impacts of gene flow on the course of adaptation, with implications for understanding the limits of species distributions.

Factors affecting larval resource choice

Animals often need to evaluate alternative habitats, foods, or mates; and their choice often has major consequences for their survival or reproduction. In the context of habitat choice, prior experience has a major impact: typically, adults are more likely to accept  habitats that they experienced as juveniles. However, the impact of prior experience on food choices is relatively poorly understood. Specifically, it is not clear whether the impact of prior experience is contingent upon spatial or temporal overlap separation of the alternative foods, how long the impact lasts, and what may be the mechanistic basis of the impact of prior experience. Vrinda and Swastika addressed these questions using flour beetle larvae. They found that prior exposure to a novel (suboptimal) resource increases larval acceptance of the resource, but the effect is resource-specific. Vrinda is conducting more experiments to determine whether the change in larval preference has any adaptive significance.