Phylogenetic analysis of β-defensin-like genes of Bothrops, Crotalus and Lachesis snakes

Abstract

Defensins are components of the vertebrate innate immune system; they comprise a diverse group of small cationic antimicrobial peptides. Among them, β-defensins have a characteristic β-sheet-rich fold plus six conserved cysteines with particular spacing and intramolecular bonds. They have been fully studied in mammals, but there is little information about them in snakes. Using a PCR approach, we described 13 β-defensin-like sequences in Bothrops and Lachesis snakes. The genes are organized in three exons and two introns, with exception of B.atrox_defensinB_01 which has only two exons. They show high similarities in exon 1, intron 1 and intron 2, but exons 2 and 3 have undergone accelerated evolution. The theoretical translated sequences encode a pre-β-defensin-like molecule with a conserved signal peptide and a mature peptide. The signal peptides are leucine-rich and the mature β-defensin-like molecules have a size around 4.5 kDa, a net charge from +2 to +11, and the conserved cysteine motif. Phylogenetic analysis was done using maximum parsimony, maximum likelihood and Bayesian analyses, and all resulted in similar topologies with slight differences. The genus Bothrops displayed two separate lineages. The reconciliation of gene trees and species tree indicated eight to nine duplications and 23 to 29 extinctions depending on the gene tree used. Our results together with previously published data indicate that the ancestral β-defensin-like gene may have three exons in vertebrates and that their evolution occurred according to a birth-and-death model.