Stress Adapted Mollusca and Nematoda Exhibit Convergently Expanded Hsp70 and AIG1 Gene Families

We recently sequenced the genome of the first subterrestrial metazoan, the nematode Halicephalobus mephisto. A central finding was a dramatic expansion of genes encoding avrRpt2 induced gene (AIG1), and 70 kDa heat shock (Hsp70) domains. While the role of Hsp70 in thermotolerance is well established, the contribution of AIG1 is much more poorly characterized, though in plants some members of this family are heat-induced. Hypothesizing that this dual domain expansion may constitute a general biosignature of thermal stress adaptation, here we examine a number of genomes, finding that expansion of both AIG1 and Hsp70 is common in bivalves. Phylogenetic analysis reveals that the bivalve-specific Hsp70 protein expansion groups with H. mephisto sequences. Our identification of the same gene expansions in bivalves and a nematode implies that this biosignature may be a general stress adaptation strategy for protostomes, particularly those organisms that cannot escape their stressful environments. We hypothesize that the two families play largely complementary mechanistic roles, with Hsp70 directly refolding heat-denatured proteins while AIG1 promotes cellular and organismal survival by inhibiting apoptosis.