Phenotypic and molecular-genetic analysis of resistance to Aphis gossypii (cotton-melon aphid) in Cucumis melo (melon)

Abstract

Aphis gossypii Glover (cotton-melon aphid) is a major pest of agriculture worldwide. Cucumis melo L. (melon) possesses monogenic resistance to this aphid, and is a good model for the study of aphid resistance mechanisms in plants. This dissertation presents analyses of the effects of the resistance gene on A. gossypii, and of the gene's effects on biochemical and molecular-genetic properties of melon plants. Nearly isogenic lines (NILs) of melon, either resistant or susceptible to A. gossypii, were compared for their influence on aphid life history traits and feeding behavior. The resistance trait delayed development, increased mortality, and markedly decreased reproduction of aphids confined to leaves of resistant plants. Aphids on resistant plants salivated into phloem sieve elements significantly longer, and were less likely to begin sap ingestion after salivation, suggesting that the resistance factor acts within phloem sieve elements. Biochemical properties of callose synthase were compared between NILs to test the hypothesis that callose deposition plays a role in the resistance mechanism. No differences were detected between resistant and susceptible melon genotypes with respect to callose synthase subunit abundance or in vitro enzyme activity. Sixty-four F₃ families from a melon mapping population were tested for aphid resistance to place the resistance locus on a genetic map of the melon genome. Four molecular markers were found to be linked to the aphid resistance phenotype. The name Agr ( Aphis gossypii resistance) is proposed for this locus. The closest flanking markers were positioned at 4.3 and 7.0 cM from Agr. Evidence suggests Agr might be a member of the nucleotide binding site-leucine-rich repeat (NBS-LRR) family of plant resistance genes, which are known to cluster in plant genomes. Melon genomic DNA sequences homologous to this gene family were isolated to test the hypothesis that Agr is an NBS-LRR homolog. Two of these sequences were tested for genetic linkage to Agr in a population of F₂ plants segregating for the resistance trait. DNA gel blot analysis determined that one sequence, NBS-2, is approximately 2.7 cM distant from Agr, which suggests Agr resides in a cluster of NBS-LRR homologs and could be a member of this gene family.