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Current research projects in the lab

1. Evolution and ecology of defensive symbiosis

Virtually all insects are infected with symbiotic bacteria that are passed on from mothers to their offspring, often in the egg cytoplasm. These bacterial endosymbionts play diverse and important roles in the ecology of their hosts, such as providing essential amino acids and vitamins, or protecting against natural enemies. Others have evolved to manipulate the reproduction of their hosts in order to increase the frequency of infected females (since only females can transmit these symbionts), for example by killing males early in development. Our recent work on insect inherited symbionts has focused on a strain of Spiroplasma that protects its host, the woodland fly Drosophila neotestacea, from a common and virulent nematode parasite. Symbiont-free flies are sterilized by nematodes, whereas fertility is fully restored in flies that harbour Spiroplasma. In fact, the benefit conferred by Spiroplasma is so great (because nematode parasite infections are so common), that Spiroplasma-infected flies are replacing uninfected ones, and the infection is currently spreading across North America. We are interested in the ecological and evolutionary consequences of Spiroplasma spread, with a long-term goal of understanding how the costs and benefits of protection conferred by symbionts compare with host-encoded immunity. We are also interested in the mechanism of protection, and have recently identified a toxin encoded by Spiroplasma, called a ribosome-inactivating protein, that appears to target nematodes.

2. Parasites of Drosophila: Nematodes and Trypanosomatids

We have a long-term interest in the ecology and evolution of associations between parasites and their hosts, and in understanding how infectious organisms that share the same host affect each other. We are using Drosophila flies and their parasites and symbionts as a model to address these questions. Drosophila flies are incredibly diverse and speciose (over 1500 described species!), with a broad range of ecologies and life histories. We have been studying mycophagous species, which are among the most common insect visitors to mushrooms in temperate forests, and which host a wide array of parasites. Nematode infections are particularly common and diverse, with parasites exhibiting striking variation in host range and virulence. We have also begun to characterize trypanosomatid parasites of Drosophila. Trypanosomatids are ubiquitous parasites of insects, and include the causal agents of human diseases, such as sleeping sickness and leishmaniasis, that are vectored by blood-feeders. Although it has long been known that many trypanosomatid species infect Drosophila, virtually nothing is known about their ecology and interactions with their hosts. We are also interested in understanding the genetic basis of insect resistance (and susceptibility) to nematodes and trypanosomatids, taking advantage of the wealth of tools and knowledge gathered from D. melanogaster.

3. Sex-ratio distorters

Like microbial symbionts, selfish genetic elements (i.e. genes that increase their transmission at the expense of the organism) are a pervasive, yet often cryptic force in host ecology and evolution. One of the best places to observe and study both symbionts and selfish genetic elements in action is in the context of sex ratio distortion. Indeed, a number of symbiont lineages that are transmitted maternally have independently evolved the ability to cause female sex ratio biases. Some of the best examples of sex ratio distorting selfish genetic elements are X chromosomes that kill sperm that carry a Y chromosome; these are common in Drosophila and other flies. We are interested in the ecological and genetic consequences of sex ratio distortion. How do sex ratio distorters persist and what prevents their hosts from going extinct given the sex ratio bias they produce? When and how does the host evolve to counteract distortion? We are also interested in understanding the mechanism of distortion. We have been studying an extraordinary sex ratio distortion polymorphism in a booklouse, Liposcelis sp. (booklice are the closest free-living relatives of parasitic lice) found in the Chiricahua Mountains in southern Arizona. Although obligately sexual, some females produce exclusively daughters. We are hard at work to understand both the mechanism of distortion, as well as how it persists in nature.

Liposcelis booklouse
(credit: C. Hodson)
Nematode-infected Drosophila
(credit: P. Hamilton)