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 i n maize to five strains of the maize dwarf mosaic virus (MDMV), an aph id 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 F-2 mapping populations. The following map order and dis tance in cM was obtained from the F-2 population; or-0.5-bnl6.29A-0.5- npi235-0.8-npi101A-4.3-numc59. No recombination between mdm1 and the n ucleolus organizer region (nor) was detected, as determined using a pr obe from the intergenic spacer region of the rDNA repeat. In order to resolve the relationship between mdm1 and the nor, and to recover reco mbinants around mdm1, a high-resolution map within the polymitotic1 (p o1) yellow kernel1 (y1) interval was generated using [po1 y1 tester (p o1 mdm1 y1) x Pa405(Po1 Mdm1 Y1)] F-2 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 endospe rm (y1/y1) F-2 kernels will arise though a recombination event between the Pa405 Po1 allele and the y1 allele of the po1 y1 tester. Plants f rom 7, 650 white (y1/y1) F-2 kernels were examined (15,300 chromosomes ) and a total of 626 F-2.3 recombinant families was recovered. Analysi s of these recombinants revealed that mdm1 cosegregates with the nor. This lack of recombination between mdm1 and the nor suggests that: eit her(1) mdm1 is located in the region flanking the nor and recombinatio n is suppressed within that region, or (2) mdm1 is located within the nor.