GENE-CENTROMERE MAPPING OF BOVINE DYA, DRB3, AND PRL USING SECONDARY OOCYTES AND FIRST POLAR BODIES - EVIDENCE FOR 4-STRAND DOUBLE CROSSOVERS BETWEEN DYA AND DRB3

A genetic map consisting of three loci anchored at the centromere of b ovine chromosome 23 was constructed by genotyping secondary oocytes (S O) and first polar bodies (PB1) using a polymerase chain reaction (PCR )-based approach. Primary oocytes arrested in prophase of meiosis I we re stimulated in vitro to resume division and extrude PB1. Sixty SO an d their matched PB1 were collected from 14 cows by micro-manipulation, subjected to amplication of the whole genome by primer extension prea mplification, and genotyped independently for the linked genes PRL, DR B3, and DYA by PCR-RFLP analysis. Single-locus analysis of gene-centro mere recombination rates were theta(cen-DYA) = 0.042, theta(cen-DRBS) = 0.113, and theta(cen-PRL) = 0.166. The most likely order is cen-DYA- DRB3-PRL. Analysis of typing data from 3 cows revealed three meiotic d ivisions consistent with ''linkage phase exchange'' between DYA and DR B3 or PRL. One of the three linkage phase exchanges was confirmed by c omplementary genotypes in a matched secondary oocyte-first polar body pair. Such linkage phase exchanges could result from four-strand doubl e crossovers between homologous chromosomes. Because all four gametes produced by four-strand double crossovers will be recombinant, more fr equent occurrence of such events in females may explain the sexual dim orphism in genetic maps. Alternatively, four-strand crossovers could r epresent a type of recombination hotspot between DYA and DRB3, suggest ing a mechanism for the high recombination frequency (15%) between the se two class II genes of the bovine major histocompatibility complex. (C) 1995 Academic Press, Inc.