Genetic variability and adaptive evolution in parthenogenetic root-knot nematodes

Abstract

Root-knot nematodes (RKN) of the genus Meloidogyne are biotrophic plant parasites of major agricultural importance, which exhibit very variable modes of reproduction, from classical amphimixis to mitotic parthenogenesis. This review focuses on those RKN species that reproduce exclusively by mitotic parthenogenesis (apomixis), in contrast to those that have meiotic/amphimitic events in their life cycle. Although populations of clonal organisms are often represented as being ecologically isolated and evolutionary inert, a considerable volume of literature provides evidence that asexual RKN are neither: they are widely distributed, extremely polyphagous, and amenable to selection and adaptive variation. The ancestors of the genus are unknown, but it is assumed that the parthenogenetic RKN have evolved from amphimictic species through hybridization and subsequent aneuploidization and polyploidization events. Molecular studies have indeed confirmed that the phylogenetic divergence between meiotic and mitotic RKN lineages occurred early, and have revealed an unexpected level of clonal diversity among populations within apomictic species. Laboratory experiments have shown that asexual RKN can rapidly adapt to new environmental constraints (eg host resistance), although with some fitness costs. Lastly, the molecular and chromosomal mechanisms that could contribute to genome plasticity leading to persistent genetic variation and adaptive evolution in apomictic RKN are discussed. It is concluded that RKN provide an excellent model system in which to study the dynamic nature and adaptive potential of clonal genomes.

Selection of resistant rootstocks to Meloidogyne enterolobii and M. incognita for okra (Abelmoschus esculentus L. Moench)

“…The differences seen in nematode proliferation rates could be partially due to genetic factors in the host that confer susceptibility or resistance, as well as genetic differences inherent between populations of attacking nematode species (Jacquet et al, 2005;Castagnone-Sereno, 2006).…”

Development of virulence to Meloidogyne incognita on resistant pepper rootstocks

“…incognita has a great capacity to respond to environmental selection, even overcoming the known ge- netic resistance of some hosts (Castagnone-Sereno, 2006). The resistance response of pepper to M. incognita is expressed as cell necrosis and pitted tissues once juveniles have established in the roots, or as the partial or total inhibition of their multiplication once installed (Bleve-Zacheo et al, 1998;Pegard et al, 2005;Castagnone-Sereno, 2006). The selection of populations that overcome resistance under natural conditions is favoured by the repeated monoculture of cultivars carrying resistance genes.…”

Genome sequence of the metazoan plant-parasitic nematode Meloidogyne incognita

“…3). A combination of different processes may explain the observed pattern in M. incognita, including polyploidy, polysomy, aneuploidy and hybridization 10,11 ; all are frequently associated with asexual reproduction. These observations are consistent with a strictly mitotic parthenogenetic reproductive mode, which can permit homologous chromosomes to diverge considerably, as hypothesized for bdelloid rotifers 12 Supplementary Tables 2 and 3 online).…”

“…M. incognita reproduces exclusively by mitotic parthenogenesis, and males do not contribute genetically to production of offspring 11 . M. incognita also displays an environmental influence on sex determination: under less favorable environmental conditions far more males are produced.…”