Evolutionary selection pressure and family relationships among connexin genes.

We suggest an extension of connexin orthology relationships across the major vertebrate lineages. We first show that the conserved domains of mammalian connexins (encoding the N-terminus, four transmembrane domains and two extracellular loops) are subjected to a considerably more strict selection pressure than the full-length sequences or the variable domains (the intracellular loop and C-terminal tail). Therefore, the conserved domains are more useful for the study of family relationships over larger evolutionary distances. The conserved domains of connexins were collected from chicken, Xenopus tropicalis, zebrafish, pufferfish, green spotted pufferfish, Ciona intestinalis and Halocynthia pyriformis (two tunicates). A total of 305 connexin sequences were included in this analysis. Phylogenetic trees were constructed, from which the orthologies and the presumed evolutionary relationships between the sequences were deduced. The tunicate connexins studied had the closest, but still distant, relationships to vertebrate connexin 36, 39.2, 43.4, 45 and 47. The main structure in the connexin family known from mammals pre-dates the divergence of bony fishes, but some additional losses and gains of connexin sequences have occurred in the evolutionary lineages of subsequent vertebrates. Thus, the connexin gene family probably originated in the early evolution of chordates, and underwent major restructuring with regard to gene and subfamily structures (including the number of genes in each subfamily) during early vertebrate evolution.