Evolution of MHC class I in the Order Crocodylia
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The major histocompatibility complex (MHC) is a dynamic genomic region with an essential role in the adaptive immunity of jawed vertebrates. The evolution of the MHC has been dominated by gene duplication and gene loss, commonly known as the birth-and-death process. Evolutionary studies of the MHC have mostly focused on model species. However, the investigation of this region in non-avian reptiles is still in its infancy. To provide insights into the evolutionary mechanisms that have shaped the diversity of this region in the Order Crocodylia, we investigated MHC class I exon 3, intron 3, and exon 4 across 20 species of the families Alligatoridae and Crocodilidae. We generated 124 DNA sequences and identified 31 putative functional variants as well as 14 null variants. Phylogenetic analyses revealed three gene groups, all of which were present in Crocodilidae but only one in Alligatoridae. Within these groups, variants generally appear to cluster at the genus or family level rather than in species-specific groups. In addition, we found variation in gene copy number and some indication of interlocus recombination. These results suggest that MHC class I in Crocodylia underwent independent events of gene duplication, particularly in Crocodilidae. These findings enhance our understanding of MHC class I evolution and provide a preliminary framework for comparative studies of other non-avian reptiles as well as diversity assessment within Crocodylia.
KeywordsMHC class I Crocodilidae Alligatoridae MHC evolution Balancing selection Recombination
We thank Dr Travis Glenn, Dr Kent Vliet, Robert Godshalk, Mitch Eaton and Dr Matthew Shirley, who kindly provided us with many of the DNA samples included in this investigation. We are grateful to Porosus Pty. Ltd. for providing the Australian saltwater and Johnston's crocodile samples. We also thank Dr Camilla Whittington for copy editing an early draft of the manuscript.
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