Evolution of Mhc-DRB introns: Implications for the origin of primates

Introns are generally believed to evolve too rapidly and too erratically to be of much use in phylogenetic reconstructions. Few phylogenetically infor mative intron sequences are available, however, to ascertain the validity o f this supposition. In the present study the supposition was tested on the example of the mammalian class II major histocompatibility complex (Mhc) ge nes of the DRB family. Since the Mhc genes evolve under balancing selection and are believed to recombine or rearrange frequently, the evolution of th eir introns could be expected to be particularly rapid and subject to scram bling. Sequences of intron 4 and 5 DRB genes were obtained from polymerase chain reaction-amplified fragments of genomic DNA from representatives of s ix eutherian orders-Primates, Scandentia, Chiroptera, Dermoptera, Lagomorph a, and Insectivora. Although short stretches of the introns have indeed pro ved to be unalignable, the bulk of the intron sequences from all six orders , spanning >85 million years (my) of evolution, could be aligned and used i n a study of the tempo and mode of intron evolution. The analysis has revea led the Mhc introns to evolve at a rate similar to that of other genes and of synonymous sites of non-Mhc genes. No evidence of homogenization or larg e-scale scrambling of the intron sequences could be found. The Mhc introns apparently evolve largely by point mutations and insertions/ deletions. The phylogenetic signals contained in the intron sequences could be used to id entify Scandentia as the sister group of Primates, to support the existence of the ARchonta superorder, and to confirm the monophyly of the Chiroptera .