Many invertebrates benefit from the metabolic inputs of specialized symbiotic bacteria, which contribute nutrients, structural components, and defenses against environmental threats. Insect symbioses show dramatic variation. At one extreme, ancient, maternally inherited symbionts that provision essential nutrients to hosts have tiny and static genomes that endure ongoing, irreversible losses of genes and functional capacities overevolutionary time. These are exemplified by Buchnera aphidicola in aphids and by other nutritional symbionts of insects ranging from bloodfeeding flies to cockroaches. In these systems, gene losses by symbionts can be partially compensated by acquisition of new symbionts or, to a limited degree, by coadaptation of hosts. More transient associations with heritable facultative symbionts can provide temporarily useful products, such as defenses against natural enemies that change over time.

At the other extreme, gut symbionts exist within more diverse microbial communities in which related strains may have frequent recombination and gene transfer resulting in highly dynamic genomes and the potential for acquisition of novel capabilities in response to new environmental pressures. The gut symbionts of honey bees and bumble bees are of special interest because the sociality of these insects enables reliable inter-individual transmission and has given rise to a distinctive, specialized microbiota. These bee-associated bacterial species are likely to have evolved with their hosts for 80 million years. In contrast to the reduced genomes and clonality of anciently acquired maternally inherited symbionts, species of honey bee gut symbionts occur as complexes of closely related, recombining strains that vary in gene content and functional capabilities. This enables them to respond to novel environmental conditions such as changes in food or environmental toxins. Initial genome-based studies of the bee gut microbiota provide evidence for roles in defense and digestion and support the view that gene uptake by symbionts is pivotal in determining their effects on hosts.

Sponsored by the UM Museum of Zoology Donald W. Tinkle Memorial Fund