Genomic loss of the HSP70cA gene in the vertebrate lineage

Metazoan 70 kDa heat shock protein (HSP70) genes have been classified into four lineages: cytosolic A (HSP70cA), cytosolic B (HSP70cB), endoplasmic reticulum (HSP70er), and mitochondria (HSP70m). Because previous studies have identified no HSP70cA genes in vertebrates, we hypothesized that this gene has lost on the evolutionary path to vertebrates. To test this hypothesis, the present study conducted a comprehensive database search followed by phylogenetic and synteny analyses. The HSP70cA gene was present in invertebrates and animals that belong to subphyla of Chordata, Cephalochordata (lancelets) and Tunicata (tunicates). However, the genomes of early vertebrates in the subphylum Craniata (lamprey, hagfish, elephant shark, and coelacanth) contained only HSP70cB genes, suggesting the loss of the HSP70cA gene in the early period of vertebrate evolution. Synteny analysis using available genomic resources indicated that the synteny around the HSP70 genes was generally conserved between tunicates, but it was largely different between tunicates and lamprey. These results suggest the presence of dynamic chromosomal rearrangement in early vertebrates, which possibly caused the loss of the HSP70cA gene in the vertebrate lineage.