Doctor of Philosophy (PhD)
This dissertation focuses on the molecular underpinnings surrounding the evolution of the biomineralized shells of marine bivalves. Bivalve molluscs synthesize remarkably complex shells from calcium carbonate and an organic matrix of proteins secreted from the dorsal edge of the mantle. Molecular analyses of shell matrix proteins (SMPs) have suggested high rates of gene turnover despite the conserved nature of the shell itself. Here, I used proteomic and transcriptomic data to identify the SMPs and other biomineralization proteins from seven bivalve species that diverged 3-513 Mya. Contrary to previous studies that identified only a few shared biomineralization transcripts across the Bivalvia, I found 69 that were ubiquitous, all validated by proteomic analysis. 37 of these candidates are homologous to annotated genes with functions central to shell deposition, including interacting with polysaccharides, forming extracellular matrix, inhibiting proteases, and binding calcium. My description of these shell-associated proteins marks a 15-fold increase in the size of the known bivalve biomineralization toolbox. These results suggest that bivalve biomineralization is underpinned by a conserved suite of genes and call into question the need to invoke high levels of novel gene recruitment to explain the evolution of SMP diversity. Following the characterization of this toolkit, I used transcriptomic data to describe the gene families expressed in the mantle of marine bivalves and compare rates of gene family changes through time. Gene families tied to biomineralization processes were found to be evolving 50% faster than other mantle genes, potentially contributing to observed variation in bivalve shells across taxa. One gene in particular, dominin, appears to have originated from a duplication of extracellular SOD3 and subsequently adopted novel functionality in binding calcium, coinciding with the origin of the calcified shell.
Duhon, Mark Ira II, "The Evolution of Bivalve Shell Matrix Proteins" (2020). LSU Doctoral Dissertations. 5257.