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Molecular Evolution of Silk Genes in Mesothele and Mygalomorph Spiders, With Implications for the Early Evolution and Functional Divergence of Silk

  • Author(s): Starrett, James Richard
  • Advisor(s): Hayashi, Cheryl Y
  • et al.
Abstract

The evolution of adaptively significant gene families is an important subject in the field of evolutionary genetics. For spiders, the gene families encoding silk proteins have received considerable attention due to the high performance capabilities of the fibers they produce. However, silk gene research has largely focused on spiders of the infraorder Araneomorphae, leaving much of the phylogenetic diversity of spiders unsampled for their silk genes. Here, I sample silk genes from spiders of the suborder Mesothelae and infraorder Mygalomorphae, which are distantly related to araneomorph spiders. Phylogenetic analyses of the spidroin genes indicate that numerous duplications occurred in the spidroin gene family after opisthotheles (mygalomorphs plus araneomorphs) split from mesotheles. However, while mesotheles appear to possess a single spidroin gene, they possess numerous copies of genes homologous to Egg Case Proteins, which are currently only known from one araneomorph species. Together these results indicate that the common ancestor of extant spiders possessed a diversity of silk genes. In addition to higher level species sampling of silk genes, I sequence repetitive and carboxy terminal regions of spidroins from all species of the trapdoor spider genus, Aliatypus. Gene tree analyses and tests of selection suggest that contrasting evolutionary forces influenced the different regions of the spidroin gene. I also investigate the expression of silk transcripts from the tarsi and silk glands of tarantulas. Tarantulas exude silk-like secretions from their tarsi, which is hypothesized to increase surface adhesion. I discovered that while known spidroin silk genes are not expressed in the tarsi, novel silk-like genes are expressed in tarantula silk glands and tarsi. Gene families of adaptive significance may also show phylogenetic signal. Here, I sample hemocyanin gene family sequences from a phylogenetically diverse sample of spider species and infer gene trees and species trees. Phylogenetic analyses reveal that despite instances of lineage specific duplication and loss of hemocyanin paralogs, hemocyanins have phylogenetic utility for most spider groups. This dissertation shows the importance of research on gene family evolution, the roles of gene families in adaptation, and the utility of gene families in phylogenetics.

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