Doctor of Philosophy (PhD)
Biological Sciences and Museum of Natural Science
Evolutionary biology seeks to understand the forces that have generated and shaped patterns of diversity. In this dissertation, I investigated phylogenetic relationships and the evolution of three traits—migration, nest-type, and geographic distribution—in the swallow family (Hirundinidae), a globally distributed group of about 85 species of aerial insectivorous birds. I first applied a dataset of several thousand ultraconserved element (UCE) loci to reconstructing the phylogeny of the New World genus Tachycineta. Three methods of phylogenetic inference produced topologically discordant trees, but post-inference analyses provided evidence that the tree produced by maximum likelihood analysis of a concatenated data matrix is the most plausible phylogenetic hypothesis. The topological conflicts were driven by a tiny proportion of sites, which influenced the other, coalescent-based, phylogenetic inferences. I then used UCE data to reconstruct the entire phylogeny of Hirundinidae. This resolved deep divergences left unresolved by previous studies. The improved tree enabled estimations of character states of geographic range, nest type, and migration in swallow ancestors, suggesting that originally swallows were African burrow-excavators, and that they diversified mainly during the mid to late Miocene. Nest type in swallows is highly conserved, and innovations in nest architecture were important in swallow diversification, with transitions from burrow excavation to mud-nest construction and secondary-cavity adoption associated with the family’s two major radiations. Seasonal migratory behavior, however, is not conserved. The evolution of migration is scattered across the tree, and reconstructions of most nodes indicated a high probability of ancestors being partially migratory. Finally, I used differential gene expression to investigate the molecular basis of avian migration by sequencing brain and liver transcriptomes of Tree Swallows (Tachycineta bicolor) congregating at a stopover in Louisiana before flying across the Gulf of Mexico on their southern migration. Several hundred transcripts were differentially expressed in brain and liver tissues among individuals displaying variable migratory readiness (based on accumulated fat). Metabolic and mitochondrial Gene Ontology terms were enriched in transcripts upregulated in birds with the lowest fat scores, suggesting that Tree Swallows undergo a period of intense metabolic activity likely related to weight gain immediately after arriving at the stopover.
Brown, Clare Elisabeth, "Phylogeny and Evolution of Swallows (Hirundinidae) With a Transcriptomic Perspective on Seasonal Migration" (2019). LSU Doctoral Dissertations. 4823.