Selection and parasite evolution: a reproductive fitness cost associated with virulence in the parthenogenetic nematode Meloidogyne incognita

Selection in plant parasites for virulence on resistant hosts and the resulting effects on parasite fitness may be considered as a driving force in host-parasite coevolution. In the present study, we tested the hypothesis that a fitness cost may be associated with nematode virulence, using the interaction between the parthenogenetic species Meloidogyne incognita and tomato as a model system. The reproductive parameters of near-isogenic lines of the nematode, selected for avirulence or virulence against the tomato Mi resistance gene, were analysed and combined into a reproductive index that was taken as a measure of fitness. The lower fitness of the virulent lines on the susceptible tomato cultivar showed for the first time that a measurable fitness cost is associated with unnecessary virulence in the nematode. Although parthenogenesis should theoretically lead to little genetic variability, such cost may impose a direct constraint on the coevolution between the plant and the nematode populations, and suggests an adaptive significance of trade-offs between selected characters and fitness-related traits. These results indicate that, although plant resistance can be broken, it might prove durable in some conditions if the virulent nematodes are counterselected in susceptible plants, which could have important consequences for the management of resistant cultivars in the field.