Major histocompatibility complex (MHC) class II genes are usually among the most polymorphic in vertebrate genomes because of their critical role (antigen presentation) in immune response. Prior to this study, the MHC was poorly characterized in tiger salamanders (Ambystoma tigrinum), but the congeneric axolotl (Ambystoma mexicanum) is thought to have an unusual MHC. Most notably, axolotl class II genes lack allelic variation and possess a splice variant without a full peptide binding region (PBR). The axolotl is considered immunodeficient, but it is unclear how or to what extent MHC genetics and immunodeficiency are interrelated. To study the evolution of MHC genes in urodele amphibians, we describe for the first time an expressed polymorphic class II gene in wild tiger salamanders. We sequenced the PBR of a class II gene from wild A. tigrinum (n=33) and identified nine distinct alleles. Observed heterozygosity was 73%, and there were a total of 46 polymorphic sites, most of which correspond to amino acid positions that bind peptides. Patterns of nucleotide substitutions exhibit the signature of diversifying selection, but no recombination was detected. Not surprisingly, transspecies evolution of tiger salamander and axolotl class II alleles was apparent. We have no direct data on the immunodeficiency of tiger salamanders, but the levels of polymorphism in our study population should suffice to bind a variety of foreign peptides (unlike axolotls). Our tiger salamander data suggest that the monomorphism and immunodeficiencies associated with axolotl class II genes is a relict of their unique historical demography, not their phylogenetic legacy.
MHC Natural selection Axolotl Evolution Alternative splicing
Fig. S1Alignment of salamander class II DAB nucleotide sequences. The 5 longer sequences are derived from cDNA purified from fresh adult and larval spleen and larval gill tissue. Complete tiger salamander (Ambystoma tigrinum) sequences include partial 5′ and 3′ untranslated regions, and the entire coding region of the gene; these represent full-length mRNA transcripts of the gene locus. One sequence from the tiger salamander (Amti-DAB.019s) represents the alternately spliced transcript of the same locus that lacks the polymorphic β1 domain. The 264 nt sequence are derived from genomic DNA using primers designed to capture variation at the β1 domain of the DAB locus. These represent allelic polymorphism of samples from a single location. For comparative purposes,sequence from the orthologous DAB locus of the axolotl (A. mexicanum) is included. Dots represent identical nucleotides to the top reference sequence, dashes represent gapsor missing data (PDF 63 kb)