Long-Read Genome Assemblies Reveal Extraordinary Variation in the Number and Structure of MHC Loci in Birds.

Our knowledge of the Major Histocompatibility Complex (MHC) in birds is limited because it often consists of numerous duplicated genes within individuals that are difficult to assemble with short read sequencing technology. Long-read sequencing provides an opportunity to overcome this limitation because it allows the assembly of long regions with repetitive elements. In this study, we used genomes based on long-read sequencing to predict the number and location of MHC loci in a broad range of bird taxa. From the long-read-based genomes of 34 species, we found that there was extremely large variation in the number of MHC loci between species. Overall, there were greater numbers of both class I and II loci in passerines than nonpasserines. The highest numbers of loci (up to 193 class II loci) were found in manakins (Pipridae), which had previously not been studied at the MHC. Our results provide the first direct evidence from passerine genomes of this high level of duplication. We also found different duplication patterns between species. In some species, both MHC class I and II genes were duplicated together, whereas in most species they were duplicated independently. Our study shows that the analysis of long-read-based genomes can dramatically improve our knowledge of MHC structure, although further improvements in chromosome level assembly are needed to understand the evolutionary mechanisms producing the extraordinary interspecific variation in the architecture of the MHC region.