Multiple Recombining Loci Encode MaSp1, the Primary Constituent of Dragline Silk, in Widow Spiders (Latrodectus: Theridiidae)

Abstract

Spiders spin a functionally diverse array of silk fibers, each composed of one or more unique proteins. Most of these proteins, in turn, are encoded by members of a single gene family thought to have arisen through duplication and divergence of an ancestral silk gene. Because of its remarkable mechanical properties, orb weaver dragline silk, a composite of 2 proteins (MaSp1 and MaSp2), is the best studied. Here, we demonstrate that multiple loci encode MaSp1 in widow spiders (Latrodectus). Because these copies may be the result of more recent duplication events than those leading to the currently recognized silk gene paralogs, they offer insight into the early evolutionary fate of silk gene duplicates. In addition to 3 presumed functional MaSp1 loci in Latrodectus hesperus (Western black widow) and Latrodectus geometricus (brown widow) genomes, we find a MaSp1 pseudogene in L. hesperus, demonstrating the potential for unrecognized extinction of silk gene paralogs. We also document recombination events among L. hesperus MaSp1 loci and between Latrodectus MaSp1 loci and MaSp2. This result supports the hypothesis that concerted evolution occurs not only within an individual silk gene but also among silk gene paralogs. One of the L. geometricus MaSp1 copies encodes a protein that has diverged in amino acid composition and potentially converged on the secondary structure of MaSp2. Based on the presence of multiple MaSp1 loci and the phylogenetic distribution of MaSp1 versus MaSp2, we propose that MaSp2 is derived from an ancestral MaSp1 duplicate. Finally, divergence has occurred in the upstream flanking sequences of the L. hesperus MaSp1 loci, the region most likely to contain regulatory motifs, providing ample opportunity for differential expression. However, the benefits associated with increased protein production may be the primary mechanism maintaining multiple functional MaSp1 copies in widow genomes.