Evolution of mouse major histocompatibility complex genes borne by t chromosomes.

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Abstract
Virtually all wild mouse populations carry t haploytypes that cause embryonic lethality or semilethality, distortion of segregation ratios, suppression of crossing-over and male sterility. The t complex of genes is located on chromosome 17, closely linked to the H-2, the major histocompatibility complex of the mouse. The t haplotypes differ from each other not only in lethal genes they carry but also in their linked H-2 haplotypes. The class II H-2 genes present on 31 t chromosomes extracted from wild populations in different parts of the world were compared. The comparison was based on the analysis of DNA fragment obtained after digestion with restriction endonucleases. The results reveal the existence of 3 major groups of class II alleles representing main branches on the evolutionary tree of the t chromosomes. Alleles within each group are similar if not identical, although they are born by chromosomes that were separated in time and space. They presence of similar alleles in Mus musculus and M. domesticus suggests that some of them may have been separated for more than 1 million yr. This must also be the minimal age of the t chromosomes but, because at least 2 of the 3 main branches appear to be related in their origin, the actual age of t chromosomes could be much greater. The observations support the proposal that H-2 genes evolve slowly.