Mosaic Evolution of Silk Genes in Aliatypus Trapdoor Spiders (Mygalomorphae, Antrodiaetidae)

Spider silk genes are composed mostly of repetitive sequence that is flanked by non-repetitive terminal regions. Inferences about the evolutionary processes that influenced silk genes have largely been made from analyses using distantly related taxa and ancient silk gene duplicates. These studies have relied on comparisons across the conserved non-repetitive terminal regions to determine orthologous and paralogous relationships, as well as the influence of selection on silk genes. While the repetitive region heavily influences silk fiber mechanical properties, few molecular evolutionary analyses have been conducted on this region due to difficulty in determining homology. Here, we sample internal repetitive and carboxy terminal regions from all extant species of the trapdoor spider genus, Aliatypus. Aliatypus spiders are highly dispersal limited and rely on their silk lined burrow for protection. We determine positional homology across species for the carboxy terminal regions and relative positional homology for the internal repetitive regions. Gene trees based on each of these regions are in good agreement with the Aliatypus species tree, which indicates we sampled single spidroin orthologs in each species. In addition, we find that purifying selection and concerted evolution have acted to conserve Aliatypus spidroin internal repetitive regions. In contrast, selection testing identifies evidence of sites that evolved under positive selection and amino acid replacements that result in radical physicochemical changes in the carboxy terminal region. These findings indicate that comparison of spidroin orthologs across a comprehensive sample of congenerics reveal molecular evolutionary patterns obscured from studies using higher-level sampling of silk encoding genes.