Summary:

Humans impact ecosystems in a multitude of ways, increasingly exposing contemporary organisms to abiotic and biotic stressors. Environmental stress has strong negative impacts on biological diversity, as species can go locally extinct, if they are unable to migrate to a more benign habitat or to overcome the stressor via plastic or evolutionary adaptation. Several factors are thought to constrain adaptive evolution, such as gene flow, lack of genetic variation and genetic or functional trade-offs. Despite trade-offs being postulated as playing a central role in evolutionary theory, interactions between abiotic and biotic stress resistances have rarely been investigated in stress adaptation. This project aims to test for trade-offs between abiotic and biotic stress resistances in the well-established Drosophila melanogaster study system. Selection experiments on stress resistance provides an opportunity to study evolutionary constraints resulting from genetic trade-offs between traits. Lines that have been selected for different abiotic stressors (heat, cold and desiccation) will be tested for costs and benefits, whereas performance will be tested in outdoor cages under different biotic stressors (competition, predation, parasites) and under different climatic conditions (hot, moderate and cold days). These experiments provide a test of whether trade-offs between abiotic and biotic stress resistances are potential constraints to stress adaptation, which is crucial to better understand the evolutionary potential of contemporary populations.

Summary:

The maternally inherited intracellular bacteria Wolbachia are ubiquitous amongst arthropod hosts. Although well known for their ability to induce various host reproductive manipulations, Wolbachia have also been shown to increase host fitness by protecting against infectious microbes or increasing fecundity; and they are expected to evolve towards mutualism in natural host populations.

Recent data from our lab indicates the rapid sequential spread of two Wolbachia variants (wAu and wRi), only one of which induces significant reproductive parasitism in the form of cytoplasmic incompatibility, in Australian populations of Drosophila simulans over approximately 20 years. In each case analyses suggest that these dynamics are best understood as Fisherian waves of favourable variants, involving net host fitness benefits of a non-trivial magnitude.

Despite their leading roles in genetics, the suite of naturally occurring pathogens which significantly impact field populations of Drosophila melanogaster and D. simulans is not well characterized. Moreover specific infections (such as DMelSV) that are maintained for study in laboratory populations may have relatively benign effects in nature.

We have found that each of the wAu and wRi Wolbachia variants currently persist, at quite different infection frequencies, amongst geographically isolated populations of D. simulans within Australia. If these Wolbachia variants have the effect of promoting host fitness by protecting against infectious microbes, a comparison of these different host populations has potential to reveal pathogens that are ecologically significance and provide some indication of the intensity of the selective pressure they exert on a genetically tractable model host.