A Distinctive and Host-Restricted Gut Microbiota in Populations of a Cactophilic Drosophila Species.

Almost all animals possess gut microbial communities, but the nature of these communities varies immensely. For example, in social bees and mammals, the composition is relatively constant within species and is dominated by specialist bacteria that do not live elsewhere; in laboratory studies and field surveys of , however, gut communities consist of bacteria that are ingested with food and that vary widely among individuals and localities. We addressed whether an ecological specialist in its natural habitat has a microbiota dominated by gut specialists or by environmental bacteria. is a species that is endemic to the Sonoran Desert and is restricted to decaying tissues of two giant columnar cacti, (cardón cactus) and (saguaro cactus). We found that the microbiota differs strikingly from that of the cactus tissue on which the flies feed. The most abundant bacteria in the flies are rare or completely absent in the cactus tissue and are consistently abundant in flies from different cacti and localities. Several of these fly-associated bacterial groups, such as the bacterial order and the genera and , have been identified in prior surveys of insects from the orders Hymenoptera, Coleoptera, Lepidoptera, and Diptera, including several species. Although the functions of these bacterial groups are mostly unexplored, species studied in bees are known to break down plant polysaccharides and use the resulting sugars. Thus, these bacterial groups appear to be specialized to the insect gut environment, where they may colonize through direct host-to-host transmission in natural settings. Flies in the genus have become laboratory models for microbiota research, yet the bacteria commonly used in these experiments are rarely found in wild-caught flies and instead represent bacteria also present in the food. This study shows that an ecologically specialized species possesses a distinctive microbiome, composed of bacterial types absent from the flies' natural food but widespread in other wild-caught insects. This study highlights the importance of fieldwork-informed microbiota research.