Three genes control resistance to wheat streak mosaic virus in the maize inbred Pa405
--M. D. McMullen, M. W. Jones, K. D. Simcox and R. Louie

Wheat streak mosaic virus (WSMV), a mite-transmitted potyvirus, infects certain maize inbreds. We (McMullen and Louie, Mol. Plant-Microbe Interact. 2:309-314) have previously reported on a gene (Wsm1) for resistance to WSMV on chromosome 6S of B73 and Pa405. However, segregation ratios of resistant to susceptible plants from a cross involving Pa405 and Oh28 indicated that, in addition to Wsm1, other genetic factors controlling symptom resistance to WSMV are present in Pa405. To identify these other genes in Pa405 controlling resistance to WSMV, (Pa405 x Oh28) F2 plants were inoculated with WSMV and symptom responses observed. In addition to resistant (symptomless) plants, two types of symptomatic plants were noted: plants with generalized mosaic (GM) similar to the symptoms observed on the susceptible inbred Oh28, and plants with dispersed, chlorotic spots and rings (DSR). DNAs pooled from 25 plants with GM symptoms and from 25 plants with DSR symptoms were used to detect linkage to RFLP loci by a "bulked segregant" approach. Southern hybridization analysis was performed with DNAs of Pa405, Oh28, F1, GM pool, and DSR pool, all cleaved with three to six restriction enzymes, and hybridized with RFLP probes. This analysis identified two additional genes in Pa405 for resistance to WSMV, designated Wsm2 (chromosome 3 near umc102) and Wsm3 (chromosome 10 near umc163), and reconfirmed the presence of Wsm1 on chromosome 6S. RFLP analyses of DNA from individual plants revealed that the plants that exhibited GM symptoms were homozygous for Oh28 alleles at the wsm1, wsm2 and wsm3 loci. Plants that exhibited DSR symptoms were homozygous for Oh28 alleles at wsm1 and wsm2, but had one or two alleles from Pa405 at the wsm3 locus. 

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