Nutritional symbionts in insects and extreme genome reduction

Animals with restricted or limited diets often form intimate symbioses with microbes to supply them with nutrients that are missing in their diets. Some of the best-studied examples come from insects that feed exclusively on plant sap. Plant sap is a very poor source of essential amino acids (the amino acids that animals cannot synthesize themselves), and many sap-feeding insects have one or more intracellular bacterial symbionts which provide them with these missing nutrients. For the past several years I, along with my post-doc advisor and collaborator Nancy Moran, have worked on the genomics of symbionts that live in insects in the Auchenorrhyncha. This system is important for a number of reasons, but primarily because i) several Auchenorrhyncha are important agricultural pests, ii) these insects have developed elaborate relationships with a diverse range of bacteria, and iii) these bacteria have the smallest and most compositionally extreme cellular genomes yet described. Bacteria from the Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, and Bacteroidetes (among others) are all found as symbionts in this group of insects. We have sequenced representaives from all four of these bacterial groups, and even a cursory glance at the tree on the left—which places these symbionts in the context of all other sequenced bacterial genomes—shows they are quite unusual. By many measures, these symbionts have the most extreme bacterial genomes known: Zinderia has the most GC-poor genome (13.5%), Hodgkinia has the smallest genome (144 kilobases, 169 genes), and all of them (Hodgkinia, Carsonella, Zinderia, Sulcia, Tremblaya, Buchnera, etc.) have massively increased rates of sequence evolution. These genomes blur the once-clear distinction between autonomous bacteria and cellular organelle, and raise questions about how these bacteria survive with such small gene sets.

My lab will continue to sequence and analyze complete genomes for selected symbionts for different Auchenorrhynchan insects. Additionally, we have started projects addressing the structure and function of several proteins from many of these symbionts. Additional projects involving deep transcriptome sequencing of the host insect are also available for interested students.

Fungal symbionts in beetles

Several beetle species have intracellular microbial symbionts which provision them with nutrients that are missing in their diets. In several cases, these symbionts are fungi, not bacteria. Our lab is just starting up a project to look at the genomic evolution of intracellular fungal symbionts in a couple of beetle species; please email me if you are interested in learning more about this work.