Characterization of MHC class II B polymorphism in bottlenecked New Zealand saddlebacks reveals low levels of genetic diversity.

The major histocompatibility complex (MHC) is integral to the vertebrate adaptive immune system. Characterizing diversity at functional MHC genes is invaluable for elucidating patterns of adaptive variation in wild populations, and is particularly interesting in species of conservation concern, which may suffer from reduced genetic diversity and compromised disease resilience. Here, we use next generation sequencing to investigate MHC class II B (MHCIIB) diversity in two sister taxa of New Zealand birds: South Island saddleback (SIS), Philesturnus carunculatus, and North Island saddleback (NIS), Philesturnus rufusater. These two species represent a passerine family outside the more extensively studied Passerida infraorder, and both have experienced historic bottlenecks. We examined exon 2 sequence data from populations that represent the majority of genetic diversity remaining in each species. A high level of locus co-amplification was detected, with from 1 to 4 and 3 to 12 putative alleles per individual for South and North Island birds, respectively. We found strong evidence for historic balancing selection in peptide-binding regions of putative alleles, and we identified a cluster combining non-classical loci and pseudogene sequences from both species, although no sequences were shared between the species. Fewer total alleles and fewer alleles per bird in SIS may be a consequence of their more severe bottleneck history; however, overall nucleotide diversity was similar between the species. Our characterization of MHCIIB diversity in two closely related species of New Zealand saddlebacks provides an important step in understanding the mechanisms shaping MHC diversity in wild, bottlenecked populations.