Using Drosophila melanogaster to understand how microbes affect host behavior
Animals live in a world teeming with microbial life. Co-existing over evolutionary time, both microbes and animals have evolved methods to influence the other to maximize their respective fitness. It has been the focus of my doctoral work to study how microbes affect animal behavior using the model species Drosophila melanogaster.
Like all other animals so-far encountered, the fruit fly digestive system is occupied by microbes consisting of mostly bacteria. Contrary to my expectations, I did not encounter fly behaviors that changed by varying their associated gut bacteria. In order to understand what changes were taking place in flies with manipulated gut flora, I assayed gene expression in dissected guts or in whole flies. I discovered that associating flies from the embryonic stage with zero, one or three bacterial taxa resulted in the same transcriptional profile in the adult gut, suggesting that adult guts are buffered against the bacteria that passage through them. However, associating flies with the yeast Saccharomyces cerevisiae, whether alone or in combination with these bacteria, was sufficient to recapitulate the transcriptional profile observed in guts of conventionally-reared flies. This suggests that yeast, not bacteria, are key in mediating the gut transcriptional response of adult flies. In contrast, transcription within the entire adult animal varied with exposure to different microbial populations, suggesting that different bacterial taxa can influence fly hosts prior to adulthood.
Yeast, Saccharomyces cerevisiae, is a key food source for D. melanogaster both in the wild and in the laboratory. Previous work by former Eisen laboratory member Kelly Schiabor had demonstrated a correlation between attraction to yeast grown under natural (sugar-replete, nitrogen-limiting) conditions (YVN) and chimeric variant in the gene encoding a component of fly olfaction, odorant receptor 22 (Or22). In conjunction with Alli Quan, I tested the hypothesis that the chimeric Or22 allele in D. melanogaster mediates sensitivity of flies to YVN and therefore makes these flies better suited to detect yeast in the environment. Through sequence analysis, bidirectional crosses between chimeric and non-chimeric lines and ultimately, replacement of a non-chimeric Or22 allele with a chimeric allele within a non-chimeric background, we found that Or22 alone does not mediate sensitivity to YVN. Still, the signs of selection at the Or22 locus suggest that this receptor confers some adaptive function in wild flies.
Entomophthora muscae is a fungal pathogen that infects, alters the behavior of, and then kills dipterans. Predominantly reported in house flies and other large Muscoidea, critically-ill flies summit, extend their proboscis, and raise their wings up and away from their dorsal abdomen in the moments prior to death, dying in an elevated position that appears to benefit fungal dispersal and therefore fitness. Despite being described over 160 years ago, we know little of E. muscae biology, especially the molecular means through which it alters host behavior. Serendipitously, I discovered a strain of E. muscae (CNE1) that infects wild Drosophila species, including D. melanogaster. I have isolated this strain in the laboratory both in vivo, through active propagation between healthy fruit flies, and in vitro, in liquid culture. By observing the isolated fungal culture with modern technology, I have been able to corroborate and add to the series of observations describing how this bizarre organism infects, grows and abandons spent hosts. Crucially, by having isolated a strain that naturally infects a model organism, I have been able to begin testing a variety of molecular hypothesis as to which host machinery is necessary for observed phenotypes, demonstrating that specific neurons and genes are not involved in mediating end-of-life behaviors. With stable E. muscae CNE1 culture, I have been able to show that the fungus first travels to the brain and central nervous system in infected flies before proliferating uncontrollably in the body cavity. I have also, with the aid of Michaels Bronski and Eisen, sequenced the genome of the isolate, and, as a consequence, have been able to assay gene expression of both host and fungus over the course of infection. Ultimately, this work represents only the beginning of what is possible in the E. muscae CNE1-D. melanogaster system, not just for understanding the molecular basis of host behavioral manipulation, but also for studying varied aspects of host-microbe interactions.