Evolution of two Rh blood group-related genes of the amphioxus species Branchiostoma floridae.

We determined cDNAs of two genes that belong to the Rhesus (Rh) blood group gene family in an amphioxus species (Branchiostoma floridae) and designated them Rh-related-1 (RhR-1) and Rh-related-2 (RhR-2). RhR-1 and RhR-2 consisted of 10 and 11 exons, respectively. 3' UTR sequences of RhR-1 were shorter (220-272 bp) than those of RhR-2 (1,505-1,650 bp). CDS lengths were 1,344 and 1,476 bp for RhR-1 and RhR-2, respectively, and the average nucleotide difference between their CDS regions was 0.33. The corresponding regions of Rh genes from exons 2 to 7 were relatively conserved among the chordate species examined in this study. Length difference numbers were in multiples of three, which implies that codon frames were conserved among them, and the same exon/intron boundary phases were observed in those regions. This region was used for the phylogenetic analyses. RhR-1 and RhR-2 formed a cluster on the phylogenetic tree of the Rh gene family. Gene duplication time of RhR-1 and RhR-2 was estimated to be ca. 500 million years ago. It is likely that the four Rh family genes in vertebrates emerged by gene duplications in the common ancestor of vertebrates, and functional differentiation has occurred after the first gene duplication.