DISSERTATION RESEARCH: Determining the impact of symbiont metabolic role in symbiont genome decay in Adelgidae (Hemiptera)

Utah State University, Logan UT

Investigators

Abstract

This research will test the assumption that symbiont genome size variation is based on host diet as well as on the age of the symbiotic relationship. Plant-sap feeding insects form close partnerships, or symbioses, with bacteria. These partnerships allow the insects to feed on nutrient-poor plant-sap by supplementing their diets. This association is beneficial for both partners and it comes with consequences. One consequence is that the bacteria typically evolve to have extremely small genomes. In many groups of insects these bacterial partnerships are stable over millions of years. This study will test if the genome size of a bacteria is determined by how long the insect and bacteria have coevolved, or if it requires a more complicated explanation involving the host's diet type. The study will broaden our general understanding of bacterial-insect associations in nature, a topic that may impact agriculture and conservation. Additionally, this project includes efforts to encourage Native American students to pursue education in the sciences by providing them with hands-on laboratory experiences. Previous studies have revealed the extreme reductions in the genomes of symbiotic bacteria and the loss of functions in critical nutrient-provisioning pathways. While recent studies have begun to examine the details of mode and tempo of genome degradation in symbionts, important questions remain. How important is selection in driving smaller genome size? How important is the age of the symbiotic associations? Do the particular metabolic or ecological roles of the symbionts influence the change in genome size? Progress has been made, but studies have been constrained by the fact that these factors are hard to disentangle. This research introduces a novel system that removes several confounding factors to allow a clearer view of what causes decreased genome size in symbionts. The genomes of symbionts will be compared in closely related lineages of adelgid insects that share a common symbiont but differ in the nutrients available through host diet. This will allow the influence of host diet nutrient content to be examined while controlling for the age of the symbiosis. A number of symbiont genomes from within and across adelgid lineages will also be sequenced for a more general survey of the causes of symbiont genome decay. Gene deletions in all core metabolic categories, particularly nutritional provisioning will be quantified. Patterns of genome decay will then be analyzed to detect correlation between decay and age of the association, nutritional role and other factors.