| The root-knot nematode, Meloidogyne incognita, reproduces by obligate mitotic parthenogenesis and is polyploid. Virulence, the ability of a nematode to reproduce on plants that carry resistance gene(s), is variable among M. incognita populations. The Rk and Rk2 genes in cowpea, Vigna unguiculata, confer resistance to M. incognita although Rk-virulent isolates occur. The effects of host selection on (a) virulent phenotypes and their associated fitness were examined for up to 27 generations in M. incognita field populations, greenhouse isolates, isofemale lines (IFLs) (an isolate started from a single parthenogenetic female), and single descent lines (SDLs) (lineages started from the progeny of a single parthenogenetic female). Hatch, root penetration, egg mass production, and egg production were assayed under controlled-environment conditions and percent virulence and fecundity were calculated. Five different profiles for (a) virulence and genetic variation between M. incognita populations, isolates, IFLs, and SDLs were found. Four profiles were represented by a stable avirulent condition, a stable virulent condition, stabilizing selection from the virulent to the avirulent condition, and nondirectional selection of virulence to the Rk gene when continuously cultured in the absence of the Rk gene. A fifth profile represented directional selection from the avirulent to the virulent condition when selected on the Rk gene. Some IFLs became extinct during attempts to select for virulence to Rk or when continuously cultured on susceptible cowpea. All SDLs derived from the female of an avirulent IFL became extinct within six generations. SDLs derived from a virulent female were represented by three distinct profiles: virulent individuals that produced virulent progeny, avirulent individuals that produced avirulent progeny that became extinct, and an avirulent individual that produced virulent and avirulent progeny. Isolates and IFLs showed variability in level of hatch, rate of penetration into susceptible and resistant roots, egg mass production, and fecundity. Egg mass production and fecundity levels on susceptible or resistant plants but not egg hatch and penetration of roots were associated with virulence in some lineages. The association of fitness and virulence, influence of host selection on the occurrence of the virulent phenotype, and insights into the genetics of M. incognita are discussed. |
