Genetic analysis of (a)virulence in Meloidogyne hapla to resistance in bean (Phaseolus vulgaris)

Nematode crossing experiments were conducted to test the hypothesis that virulence in Meloidogyne hapla to a dominant major resistance gene in the bean Phaseolus vulgaris cv. NemaSnap is simply inherited by a single gene, with avirulence dominant to virulence (recessive). In a new procedure established for crossing M. hapla race A isolates, females of avirulent isolate K-6 and males of virulent isolates AN and LM were used as parental lines. Virulence frequencies and segregation patterns in the F-3 to F-5 generations indicated cross-mating with virulent males, not selection and selfing, as the source of virulence in segregating progenies. In F-3 progenies, three of 29 lines expressed virulence at levels of 10.6-36.6%. The percentage of virulence from the pooled F-3 data from these three lines indicated monogenic inheritance of virulence. F-4 families were established from single egg masses of the three virulent F-3 lines. Cohorts of F-5 juveniles derived from single egg masses of F-3 and F-4 families were tested for virulence. Due to the facultative parthenogenesis in M. hapla race A, tests were conducted of various genetic models for inheritance of (a)virulence based on different assumptions about the mode of reproduction, gamete duplication and recombination. A segregation pattern of three avirulent (AA, Aa): one virulent (aa) for the genotypes in the F-4 was determined from the F-5 virulence tests. The percentages of virulence at the F-5 stage and their patterns of segregation strongly supported the hypothesis that avirulence in M. hapla is controlled by a single gene, with virulence recessive to avirulence. The M. hapla (a)virulence matches the dominant resistance in P. vulgaris cv. NemaSnap, thus conforming to a gene-for-gene relationship. Our findings indicated that the (a)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 suggests that other important biological traits can be transferred through this process.