RFLP mapping of chromosome 6S in a backcross 1 population --Douglas Mead, Christine Bredenkamp, Michael Kiefer, Ann Majerus, Hope Sunderland, Carol Wangen, and Edward Weck In order to establish the relative order of loci on chromosome 6S, a backcross 1 population of 95 plants from the cross (NK1 x NK2wx) x NK1 was utilized. The advantages of using a backcross population for mapping are: 1) the number of individuals displaying a recessive trait is greater than in an F2 population; 2) there are only two RFLP patterns to score; and 3) there is the possibility of gaining information about genomic constitution in a different set of genetic materials. The disadvantages of using a backcross population (compared with recombinant inbred lines) are: 1) each backcross 1 individual is different and only a limited amount of tissue can be obtained from each plant; 2) data can not be correlated across laboratories. The first disadvantage can be overcome by the utilization of wide crosses of heterotic elite materials to generate large amounts of tissue (we had great difficulty during the drought summer of 1988 in obtaining sufficient tissue samples from the recombinant inbred lines to allow the routine isolation of quality DNA). The second disadvantage is only significant if a universal mapping strategy is required.

In order to analyze the mapping data for loci on chromosome 6S in the cross (NK1 x NK2wx) x NK1, a SAS program was written which makes pairwise probe pattern comparisons. The number of differences between pairs of loci (including double crossovers) are shown in a "relatedness" triangle (Figure 1). The distance between loci was calculated (to the nearest 0.1 cM) using the number of probe differences. A map of chromosome 6S is shown in Figure 2. Comparison of this map with the 6S map from the 1988 MNL suggests that: 1) different mapping methods result in different relative amounts of recombination or 2) there is a parental line effect in the various mapping populations.

To examine these possibilities further, a number of probes on chromosome 3 and chromosome 9 were analyzed in the (NK1 x NK2wx) x NK1 BC 1 population and compared with the 1988 MNL maps (Figure 3a and b). The mapped distances differ significantly in the (NK1 x NK2wx) x NK1 population from those observed in the 1988 MNL maps. In this BC1 population the markers M81 and 5.10 are not linked to wx or 8.17. The presence of other numerous mapping differences makes it difficult to ascertain whether parental or test population differences are responsible for the discrepancies.

The reason for examining the BC1 population was to map chromosome 6S markers as a prelude to incorporating the morphological markers into the RFLP map. A number of crosses will be made in an attempt to localize various morphological markers on the RFLP map.

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