Insights from the timber rattlesnake (Crotalus horridus) genome for MHC gene architecture and evolution in threatened rattlesnakes

Conservation of threatened species can benefit from an evaluation of genes in the Major Histocompatibility Complex (MHC), whose loci encode proteins that bind pathogens and are often under strong selection to maintain diversity in immune response to diseases. Despite this gene family's importance to disease resistance, little is known about these genes in reptiles including snakes. To address this issue, we assembled and annotated a highly-contiguous genome assembly for the timber rattlesnake (Crotalus horridus), a pit viper which is threatened or endangered in parts of its range, and analyzed this new genome along with three other rattlesnake genomes to characterize snake MHC loci. We identified highly-duplicated MHC class I and class IIβ genes in all species typified by a genomic architecture of discrete gene clusters localized on chromosome 2. Number of loci varied between species from 14 to 23 for MHC I and from 8 to 32 for MHC IIβ and was greater than previously identified in the few non-genome-based studies of reptile MHC to date. We present evidence of the gene family's complex evolutionary history, with extensive duplication and loss concurrent with speciation resulting in incomplete lineage sorting. The differences in gene number between species combined with a dynamic evolutionary history suggests that gene family expansion/contraction via rapid duplication/gene loss may represent an important mechanism for generating genetic diversity in rattlesnake MHC. Our work demonstrates the utility of whole genome sequences for identifying functional genetic variation in the form of MHC genes relevant for conservation genomic studies in threatened snakes.