Co-segregation of the maize dwarf mosaic virus resistance gene, Mdm1, with the nucleolus organizer region in maize.

The mdm1 locus on the short arm of chromosome six confers resistance in maize to five strains of the maize dwarf mosaic virus (MDMV), an aphid transmitted potyvirus. The location of mdm1 in relation to RFLP and morphological loci on the short arm of chromosome six was determined using BC1 and F2 mapping populations. The following map order and distance in cM was obtained from the F2 population; jc1270-2.5-npi245-1.6-umc85/po1-0.5-mdm1/nor-0.5-bnl6.29A-0.5-npi235-0.8-npi101A-4.3-numc59. No recombination between mdm1 and the nucleolus organizer region (nor) was detected, as determined using a probe from the intergenic spacer region of the rDNA repeat. In order to resolve the relationship between mdm1 and the nor, and to recover recombinants around mdm1, a highresolution map within the polymitotic1 (po1) yellow kernel1 (y1) interval was generated using [po1 y1 tester (po1 mdm1 y1) x Pa405 (Po1 Mdm1 Y1)] F2 plants. The recessive po1 allele imparts a male-sterile phenotype when homozygous and since po1 and y1 are closely linked, the majority of fertile plants from white endosperm (y1/y1) F2 kernels will arise though a recombination event between the Pa405 Po1 allele and the y1 allele of the po1 y1 tester. Plants from 7,650 white (y1/y1) F2 kernels were examined (15,300 chromosomes) and a total of 626 F2∶3 recombinant families was recovered. Analysis of these recombinants revealed that mdm1 cosegregates with the nor. This lack of recombination between mdm1 and the nor suggests that: either (1) mdm1 is located in the region flanking the nor and recombination is suppressed within that region, or (2) mdm1 is located within the nor.