Mapping of cotton-melon aphid resistance in melon

Resistance to cotton-melon aphid (Aphis gossypii Glover) segregated as a si ngle dominant gene in a melon (Cucumis melo L., mapping population derived from the cross 'Top Mark' x PI 414723. Sixty-four F-2-derived F-3 families were used to map the aphid resistance locus, Vat, with respect to randomly amplified polymorphic DNA (RAPD) and restriction fragment length polymorphi sm (RFLP) markers. RFLP markers NBS-2 and AC-39 flanked Vat at distances of 3.1 cM and 6.3 cM, respectively. NBS-2 is homologous to the nucleotide bin ding site-leucine-rich repeat (NBS-LRR) superfamily of plant resistance gen es. Another homolog of this superfamily, NBS-5, was positioned approximate to 16.8 cM from Vat, raising the possibility that Vat resides in a cluster of NBS-LRR paralogs. RFLP marker AC-8, which has similarity to plant lipoxy genases, was positioned at approximate to5.5 cM from Vat. Monogenic resista nce to A. gossypii has been identified ill two sources of melon germplasm, Indian accession PI 371795 (progenitor of PI 414723) and Korean accession P I 161375, To test for an allelic relation between the genes controlling aph id resistance in these two distinct germplasm sources, melon plants of a ba ckcross population from a cross between two resistant lines having Indian- or Korean-derived resistance were infested with aphids, At least 90 out of 92 segregating progeny were aphid resistant, suggesting that the same resis tance gene, Vat, is present in both sources of melon germplasm.