Consequences of Host Life Cycles for Symbiont Genome Evolution

Aphids are small insects that feed exclusively on plant sap, a notoriously low source of nutrients due to the high sugar content and low amino acid content. To make up for these deficiencies in nutrition, aphids harbor Buchnera aphidicola, a bacterial endosymbiont that resides in a specialized organ called the bacteriome. B. aphidicola provides essential amino acids and vitamins for the aphid in exchange for a safe place to live. Over the course of the symbiosis (established 160 million years ago), B. aphidicola has lost much of its genome, including essential genes for cell envelope synthesis, DNA replication and regulation, and genes essential to the symbiosis, such as genes that synthesize essential amino acids and vitamins. These gene losses can be detrimental to the future of the symbiosis between aphids and B. aphidicola. Previous research has reported variation in B. aphidicola genome size and content among different aphid lineages. To understand factors that contribute to such variation, I studied how aphid food and life cycles affect patterns of evolution and selection on the B. aphidicola genome. In Chapter 1, I compared seventy-two B. aphidicola genomes across four aphid ecological categories (tree, herbaceous, host-alternating, and galling) and found that the tree and galling ecologies exhibited a substantial effect on genome size and content. For Chapter 2, I investigated how galling and the loss of host-alternation in aphids impacts patterns of genome evolution and selection in two ancestrally host-alternating and galling lineages - Hamamelistes and Pemphigus. In Chapter 3, I described a new species of aphid, named Hamamelistes blackmani Dederich & von Dohlen, that exclusively feeds on two species of Fothergilla