Inheritance of the resistant reaction of the lettuce cultivar `Grand Rapids' to the southern root-knot nematode Meloidogyne incognita (Kofoid & White) Chitwood

Abstract

Resistance to the southern root-knot nematode Meloidogyne incognita Chitwood would be an important attribute of lettuce Lactuca sativa L. cultivars adapted to both protected and field cultivation in tropical regions. `Grand Rapids' and a few other cultivars are reported to be resistant to this nematode. In this paper, we studied the inheritance of the resistant reaction of `Grand Rapids' (P2) in a cross with a standard nematode-susceptible cultivar Regma-71 (P1). F1(Regina-71 × Grand Rapids) and F2 seed were obtained, and inoculated along with the parental cultivars with different races of M. incognita to evaluate nematode resistance. Broad sense heritability estimates for the number of galls and of egg masses per root system, gall size and gall index were generally in the order of 0.5 or higher. Class distributions of these variables over generations P1, P2, F1 and F2 were in agreement with simulated theoretical distributions based on monogenic inheritance models. F3 families were obtained from randomly sampled F2 plants and tested for reaction to the nematode. The frequency ratio of homozygous resistant, segregating and homozygous susceptible F3 families did not differ from the 1:2:1 ratio expected from monogenic inheritance. M. incognita resistance appears to be under control of a single gene locus. The Grand Rapids allele (for which the symbol Me is proposed) is responsible for the resistant reaction, and shows high (though incomplete) penetrance, variable expressivity and predominantly additive gene action.