Genetic analysis of Meloidogyne hapla and functional characterization of a putative avirulence gene Cg-1 in Meloidogyne javanica corresponding to Mi-1 in tomato

Six inbred strains of the root-knot nematode M. hapla were analyzed for karyotypes, DNA polymorphisms, and pathogenecity on five plant hosts. All strains have N = 16 chromosomes and reproduce by facultative meiotic parthenogenesis. AFLP-DNA fingerprinting revealed that DNA polymorphisms are common among strains. However, no correlation was found between geographic origins and DNA polymorphisms. Strain specific markers were developed according to sequences from AFLP polymorphisms. M. hapla strains showed significant difference in ability to detect the host plants and pathogenicity on resistance hosts. Four strains from the Netherlands are avirulent on common bean cv. NemaSnap and SB-22, an accession of the wild potato Solanum bulbocastanum. Two strains from California, USA, are virulent on both hosts. The incompatible interaction of avirulent nematodes in resistant hosts is manifested by arrested development of nematodes. Strain VW9 has the strongest capacity to detect roots, while VW8, VW10, VW11 and VW13 are weak in perception and attraction to roots in a pluronic gel assay.; M. hapla strains VW8 and VW9 were used to conduct genetic crosses, with VW8 as the female parent and VW9 as the male parent. In the controlled crosses, strain-specific DNA markers were used to identify F1 hybrids. The F1 hybrids were allowed to reproduce parthenogenically to generate 182 F2 lines. Segregation patterns of 17 DNA markers including three pairs of allelic markers were examined. In contrast to typical Mendelian genetics, markers segregated 1:1. A preliminary genetic linkage map was constructed using JoinMapRTM in which two linkage groups contained 3 or 4 markers.; A nematode gene (Cg-1) identified in M. javanica (VW4) that correlates with avirulence was characterized using RNA interference. The correlation between the loss of Cg-1 transcript in an avirulent strain VW4 after treatment with dsRNA corresponding to Cg-1 and the gain of virulence in VW4 supports the hypothesis that Cg-1 is the avirulence gene in M. javanica corresponding to the tomato resistance gene Mi-1. Virulence acquisition through RNAi has become transmissible over generations. It appeared that selection pressure by Mi-1 tomato helps maintain virulence in VW4 silenced for Cg-1 because virulence decreased in the progeny cultured on susceptible tomato.