DYNAMICS OF MHC EVOLUTION IN BIRDS AND CROCODILIANS - AMPLIFICATION OF CLASS-II GENES WITH DEGENERATE PRIMERS

Genes of the major histocompatibility complex (Mhc) are the most polym orphic functional loci in mammalian populations, but little is known o f Mhc variability in natural populations of nonmammalian vertebrates. To help extend such studies to birds and relatives, we present a pair of degenerate primers that amplify polymorphic segments of one chain ( the beta chain) of the class Ii genes from the major histocompatibilit y complex (Mhc) of archosaurs (birds + crocodilians). The primers targ et two conserved regions lying within portions of the antigen-binding site (ABS) encoded by the second exon and amplify multiple genes from both genomic DNA and cDNA. The pattern of nucleotide substitution in A BS codons of 51 sequences amplified and cloned from five species of pa sserine birds and an alligator (Alligator mississippiensis) indicates that archosaurian class II beta genes are subject to selective forces similar to those operating in mammalian populations. Hybridization of a genomic clone generated by the primers revealed highly polymorphic b ands in a sample of Florida scrub jays (Aphelocoma coerulescens coerul escens). Because the primers amplify only part of the ABS from multipl e class II genes, they will be useful primarily for generating species specific clones, thereby providing a critical inroad to more detailed structural and evolutionary studies.