Genetic variation in Meloidogyne hapla and inheritance of virulence to resistance in common bean (Phaseolus vulgaris)

Root-knot nematodes (Meloidogyne spp.) are the most important plant parasitic nematodes in agriculture as they cause large economic losses to food and fiber crops. Host plant resistance is being used increasingly as a nematode control tactic to prevent such losses. However, genetic variation in nematode species for virulence to host resistance genes occurs in the field, and this host-parasite interaction is poorly understood. The present study represents the first formal analysis of virulence factors in Meloidogyne using a Mendelian genetic approach. Seven M. hapla race A isolates were used to challenge common bean (Phaseolus vulgaris ) genotypes that were potential resistance sources. Cultivar NemaSnap was found to have a single dominant gene for resistance, effective against two of the M. hapla isolates (avirulent) but not against the five other isolates (virulent). A transagonal relationship was found among M. hapla isolates, in which differential interactions of isolates with NemaSnap and Yolano genotypes indicated a gene-for-gene relationship. The rDNA D3 regions of the M. hapla isolates were sequenced to determine if polymorphism at this locus might produce possible genetic markers. All seven M. hapla isolates were found to have the same D3 sequence profiles, and some individuals carried two sequence haplotypes. Crosses between virulent and avirulent M. hapla isolates were conducted in seedling growth pouches. Putative F₁ lines were analyzed at the F₃ stage and three lines had significant virulence levels introduced into the population from the male parent through cross-fertilization. The percentage of virulence in F₃ progenies indicated a single dominant avirulence gene present in the nematode that matched the resistance gene in NemaSnap. F₄ families from those three lines were established, selfed, and analyzed at the F₅ stage. A 3 avirulence: 1 virulence segregation pattern was found in the F₅ families. This is the first evidence in Meloidogyne that avirulence is controlled by a single gene, with virulence recessive to avirulence. These findings indicate that the virulence factor can be transferred within and between M. hapla populations that have the ability to reproduce sexually, thereby maintaining genetic variation for virulence. The inheritance of virulence also indicates that other important biological traits can be transferred through this process.