MULTIPLE ALLELES FOR ZUCCHINI YELLOW MOSAIC-VIRUS RESISTANCE AT THE ZYM LOCUS IN CUCUMBER

Sources of resistance to several potyviruses have been identified and characterized within the cucumber (Cucumis sativus L.) germplasm. Resi stance to zucchini yellow mosaic virus (ZYMV) is present in inbred lin es derived from the Dutch hybrid Dina (Dina-1) and from the Chinese cu ltivar 'Taichung Mou Gua' (TMG-1). Tests of allelism indicated that th e genes for resistance to ZYMV in TMG-1 and Dina-1 are at the same loc us; however, the two genotypes exhibited different phenotypes in respo nse to cotyledon inoculation with ZYMV. Dina-1 exhibited a distinct ve inal chlorosis and accumulation of virus limited to the first and/or s econd true leaves, while TMG-1 remained symptom-free and did not accum ulate virus. The distinct veinal chlorosis phenotype in Dina-1 was dom inant to the symptom-free phenotype in TMG-1 and was shown not to be d ue to a separate gene. These results indicate that a series of alleles differing in effectiveness and dominance relationships occurs at the zym locus such that Zym > zym(Dina) > zym(TMG-1). I, addition to ZYMV resistance, TMG-1 is also resistant to watermelon mosaic virus (WMV), the watermelon strain of papaya ringspot virus (PRSV-W) and the Morocc an watermelon mosaic virus (MWMV); the WMV and MWMV resistances are at the same locus, or tightly linked to the zym locus. Dina-1 also was-f ound to be resistant to PRSV-W and MWMV. The gene for MWMV resistance in Dina-1 appeared to be at the same locus or tightly linked (<1% reco mbination) to the gene for ZYMV resistance. In contrast to the respons e to ZYMV inoculation, Dina-1 does not exhibit distinct veinal chloros is when inoculated with PRSV-W or MWMV. Collectively, these observatio ns suggest that the gene(s) conferring resistance to ZYMV, WMV, and MW MV may be part of a gene cluster for potyvirus resistance in cucumber.