Genomics and Transcriptomics of the green mussel explain the durability of its byssus

Abstract

Mussels, which occupy important positions in marine ecosystems, attach tightly to underwater substrates using a proteinaceous holdfast known as the byssus, which is tough, durable, and resistant to enzymatic degradation. Although various byssal proteins have been identified, the mechanisms by which it achieves such durability are unknown. Here we report comprehensive identification of genes involved in byssus formation through whole-genome and foot-specific transcriptomic analyses of the green mussel, Perna viridis. Interestingly, proteins encoded by highly expressed genes include proteinase inhibitors and defense proteins, including lysozyme and lectins, in addition to structural proteins and protein modification enzymes that probably catalyze polymerization and insolubilization. This assemblage of structural and protective molecules constitutes a multi-pronged strategy to render the byssus highly resistant to environmental insults.

Figure 1

Green mussels, Perna viridis, attached to a transparent acrylic board, using the byssus, consisting of threads and plaques.

Figure 2

Maximum likelihood (ML) trees of mollusks and bivalves constructed using orthogroups developed from whole-genome sequences. (a) phylogenetic tree of mollusks, constructed using 805 genes selected from genome sequences. Tribolium castaneum and Drosophila melanogaster were used as outgroups. (b) phylogenetic tree of bivalves constructed using 153 genes selected from genome sequences. Lottia gigantea and Haliotis discus hannai were used as an outgroup. All nodes have 100% bootstrap support after 100 replications.

Figure 3

Classification of 81 Highly Expressed genes in the Foot (HEFs) in Perna viridis. Numbers are gene numbers in each category.

Figure 4

Strategy of the green mussel to form a tough, durable byssus, suggested by whole-genome and transcriptome analyses. Major genes expressed exclusively in the foot were classified into two major categories: genes encoding structural proteins (collagens, TMPs, and foot proteins) and processing proteins, including modification and polymerization enzymes and transport proteins, and those encoding proteins that protect the byssus from attack by bacteria and proteinases.