Comparative Genomics Reveals 13 Different Isoforms of Mytimycins (A-M) in Mytilus galloprovincialis.

Magalí Rey-Campos,Antonio Figueras,Marco Gerdol,Alberto Pallavicini,Beatriz Novoa

doi:10.3390/ijms22063235

Abstract

Mytimycins are cysteine-rich antimicrobial peptides that show antifungal properties. These peptides are part of the immune network that constitutes the defense system of the Mediterranean mussel (Mytilus galloprovincialis). The immune system of mussels has been increasingly studied in the last decade due to its great efficiency, since these molluscs, particularly resistant to adverse conditions and pathogens, are present all over the world, being considered as an invasive species. The recent sequencing of the mussel genome has greatly simplified the genetic study of some of its immune genes. In the present work, we describe a total of 106 different mytimycin variants in 16 individual mussel genomes. The 13 highly supported mytimycin clusters (A–M) identified with phylogenetic inference were found to be subject to the presence/absence variation, a widespread phenomenon in mussels. We also identified a block of conserved residues evolving under purifying selection, which may indicate the “functional core” of the mature peptide, and a conserved set of 10 invariable plus 6 accessory cysteines which constitute a plastic disulfide array. Finally, we extended the taxonomic range of distribution of mytimycins among Mytilida, identifying novel sequences in M. coruscus, M. californianus, P. viridis, L. fortunei, M. philippinarum, M. modiolus, and P. purpuratus.

Highlights

Mytimycins are cysteine-rich antimicrobial peptides isolated for the first time from Mytilus edulis in 1996 [1]
A total of 106 different nucleotide sequences encoding mytimycins were found in the 16 mussel genome assemblies, i.e., all the mytimycin variants that were present in the analyzed genomes
In the past few years, the genomic and transcriptomic study of Mytilus galloprovincialis has revealed an enormous amount of inter-individual genetic diversity in this bivalve of great economic and ecological interest [6,13]

Summary

Introduction

Mytimycins are cysteine-rich antimicrobial peptides isolated for the first time from Mytilus edulis in 1996 [1]. The advance of massive sequencing technologies has provided a strong contribution in unveiling this scenario, in particular thanks to the release of the Mytilus galloprovincialis genome sequence [13], which revealed the presence of widespread gene presence/absence variation for the first time in a metazoan. This dispensable nature of 25% of the mussel coding genes undoubtedly offers a source of enormous genetic variability. A striking example of this molecular diversity is represented by another class of cysteine-rich AMPs, i.e., myticins, which show a great level of inter-individual variability, with very little overlap among individuals (120 different isoforms were described in only 16 individuals) [14]

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