Recruitment and diversification of an ecdysozoan family of neuropeptide hormones for black widow spider venom expression

Abstract

Venoms have attracted enormous attention because of their potent physiological effects and dynamic evolution, including the convergent recruitment of homologous genes for venom expression. Here we provide novel evidence for the recruitment of genes from the Crustacean Hyperglycemic Hormone (CHH) and arthropod Ion Transport Peptide (ITP) superfamily for venom expression in black widow spiders. We characterized latrodectin peptides from venom gland cDNAs from the Western black widow spider (Latrodectus hesperus), the brown widow (Latrodectus geometricus) and cupboard spider (Steatoda grossa). Phylogenetic analyses of these sequences with homologs from other spider, scorpion and wasp venom cDNAs, as well as CHH/ITP neuropeptides, show latrodectins as derived members of the CHH/ITP superfamily. These analyses suggest that CHH/ITP homologs are more widespread in spider venoms, and were recruited for venom expression in two additional arthropod lineages. We also found that the latrodectin 2 gene and nearly all CHH/ITP genes include a phase 2 intron in the same position, supporting latrodectin's placement within the CHH/ITP superfamily. Evolutionary analyses of latrodectins suggest episodes of positive selection along some sequence lineages, and positive and purifying selection on specific codons, supporting its functional importance in widow venom. We consider how this improved understanding of latrodectin evolution informs functional hypotheses regarding its role in black widow venom as well as its potential convergent recruitment for venom expression across arthropods.

Keywords: AIC; Akaike Information Criterion; BEB; Bayes Empirical Bayes; CHH; CPP; Clade Posterior Probability; Crustacean Hyperglycemic Hormone; DNA complementary to RNA; EST; Expressed Sequence Tag(s); FEL; FUBAR; Fast Unbiased Bayesian AppRoximation; GTR+I+G; Generalized Time Reversible plus Gamma plus Invariant model; ICK; ITP; Inhibitor Cystine Knot; Ion Transport Peptide; LRT; Latrodectin; Latrodectus; MEME; ML; Mixed Effects Model of Episodic Diversifying Selection; Molecular evolution; NCBI; National Center for Biotechnology Information; Phylogeny; RTA; RTA clade; Random Taxon Additions; Retrolateral Tibial Apophysis clade; SLAC; TPM3uf+I+G; Venom; aa; amino acid(s); base pair(s); bp; cDNA; d(N); d(S); double strand(ed); ds; fixed effects likelihood; likelihood ratio test; maximum likelihood; nonsynonymous substitutions per nonsynonymous site; single-likelihood ancestor counting; synonymous substitutions per synonymous site; three parameter with unequal base frequencies plus gamma plus Invariant model; α-Latrotoxin LMWPs; α-latrotoxin associated low molecular weight proteins.