Centromere mapping in triploid families of the Pacific oyster Crassostrea gigas (Thunberg)

Gene-centromere mapping, a tool for improving linkage maps, can be accomplished in families of triploid bivalve molluscs induced by inhibiting the second meiotic division (MII) of fertilized oocytes. Here, we report half-tetrad analyses of seven triploid families of the Pacific oyster Crassostrea gigas, using 56 microsatellite DNA markers and the amylase gene. Most MII segregations (88/93) show 1:1 ratios of homozygotes, as expected, and marker-centromere distance are calculated for these cases as one-half the proportion of heterozygotes, y. Heterogeneity of y is observed in nine of 25 cases in which markers were typed in more than one family, suggesting variation in gene-centromere recombination among families. Altogether, 64 statistically independent marker-centromere distances are obtained, 61 of which can be arranged onto previously published linkage groups (LG) and three of which are linked to a centromere but unlinked to any other marker. Differences in marker orders between linkage and centromere maps imply polymorphisms for chromosomal rearrangements. Six of the 10 centromeres of the Pacific oyster genome are closely linked to nine microsatellite DNA markers, facilitating future linkage assignments. Distances among markers on the centromere map correlate closely with distances among markers separated by 30 cM or less on the linkage map, supporting the assumption in centromere mapping of complete interference (i.e. one and only one cross-over between a marker and the centromere). High interference, reported in previous studies, is not supported here by a uniform distribution of y-values from zero to one. Detailed analysis of segregation patterns reveals more cross-over events than counted under the assumption of complete interference, with no or even partial negative interference calculated for pairs of markers on LG4.