The major histocompatibility complex (MHC), an important genomic region for adaptive immunity, has been extensively studied in mammals and birds, but very little in squamate reptiles.
In a study in Frontiers in Genetics researchers provide the first detailed characterization of the squamate MHC using the genomes of two anole species, the green anole (Anolis carolinensis) and the brown anole (Anolis sagrei), revealing the complexity of the structure and evolution of MHC.
"Relative to mammals and birds, our understanding of this important genomic region that functions in immunity is minimal in squamate reptiles, a major radiation of vertebrates," said lead author Daren C. Card, postdoctoral researcher in the Department of Organismic and Evolutionary Biology at Harvard.
The researchers localized the MHC in the genomes of each species, detailing the composition of genes and the overall genomic structure and composition in this general region of the genome. They found that the MHC is located at the end of chromosome 2 in both species and that the composition of genes more closely resembles mammals versus birds.
The team observed differences in the compositions of repetitive elements in the MHC regions of squamates versus other vertebrates; squamates also had noticeably longer introns and intergenic regions. "Altogether, these features have contributed to a much larger MHC region in the anole genomes (~12 to 25 Mb) than what we observe in other vertebrates studied to date (< 5 Mb)," said senior author Professor Scott V. Edwards.
Card and team also observed a loss of the peptide-binding domain exon 2 in one of two mhc2β gene homologs of each anole species and signals of diversifying selection in each anole species across several codons of mhc1, many of which appear functionally relevant given known structures of this protein.
The study is the first to thoroughly describe the MHC immune region of squamates and has acquired data and knowledge that will be important for future studies of squamate and vertebrate immunogenetics and evolution.
Images: Green anole and brown anole (Credit: Nicholas Herrmann)