Assessment of allelic Mi-1.2 dosage effects on levels of resistance to virulent and avirulent Meloidogyne spp. populations in some tomato rootstocks

The Mi-1.2 gene confers broad-spectrum resistance to populations of at least 13 Meloidogyne species, being present in a wide range of commercial tomato (Solanum lycopersicum) cultivars. A more refined assessment of the allelic dosage effects of the Mi-1.2 gene to virulent and avirulent Meloidogyne populations may provide relevant information for the management of the durability of the resistance under field conditions. Herein, the allelic dosage effects (homozygous vs heterozygous at the Mi-1.2 locus) on the phenotypic expression of resistance were evaluated against one Mi-1.2-virulent and six avirulent Meloidogyne spp. populations. Seven tomato rootstocks with distinct allelic conditions at the Mi-1.2 locus and one susceptible control ('Santa Clara') were used in the present study. Significant dosage effects of Mi-1.2 alleles in reducing the reproduction factors (RF) were observed, but restricted to the avirulent Meloidogyne populations. Homozygous resistant (Mi-1.2/Mi-1.2) cultivars, except TD1, prevented reproduction of almost avirulent Meloidogyne spp. populations in contrast with the heterozygous allele combination (Mi-1.2/mi-1.2), which allowed a certain level of nematode reproduction. The efficiency and durability of the Mi-1.2 gene depend on its proper management under commercial field conditions. Heterozygous (Mi-1.2/mi-1.2) plants will increase the population of Meloidogyne spp., with no drastic reduction as occurring in the case of homozygous resistant, and they allow a small reproduction of more adapted sub -populations of the nematode, which might increase over time. Therefore, our results strongly indicated that the employment (whenever possible) of hybrids carrying the double dosage of the Mi-1.2 gene would reduce the potential of high nematode populations in plants with this allelic configuration.