Phylogenetic position of Meloidogyne within the phylum Nematoda
The phylum Nematoda comprises over 25,000 species, including free-living species, animal and plant parasitic species.
Meloidogyne belongs to the order Tylenchida, a very large and diverse group of nematodes, which contains a majority of the known plant parasitic species. Representatives of this order have a worldwide distribution and are encountered more frequently than any other group of nematodes. Within Tylenchida, members of the family Heteroderidae are by far the most damaging to world agriculture, among which the RKN (Meloidogyne spp.) and the cyst nematodes (Globodera spp. and Heterodera spp.). To date, more than 80 RKN species are described. Although representative of the genus, M. incognita M.arenari and M. Javanica, as the other mitotic RKN species, holds a phylogenetic position quite distant from RKN meiotic species, e.g. M. hapla (Castagnone-Sereno et al., 1993 and 1994).
Biological and phytopathological traits
The nematodes Meloidogyne interacts with their hosts in a remarkable manner. This obligate pathogens have evolved strategies for infesting numerous plant species in a similar manner, probably by manipulating fundamental elements of plant cell development. This biotrophic parasite do not kill the host cells from which they feed. Instead, it induces a redifferentiation redifferentiation of roots cells into specialized feeding cells essential for nematode growth and reproduction. Like other plant parasitic nematodes, RKN have a stylet, a hollow retractable needle connected to the esophagus and three unicellular esophageal glands This structure is used to pierce and penetrate plant cell walls, to release esophageal secretions into the host tissue and to take up nutrients from the plant cells. Plant nutrient and water uptake are substantially reduced by the resulting damage to the root system, and infested plants are therefore weak and give low yields.
Plant-parasitic nematodes are annually responsible for an estimated 100 billion euros in crop damage worldwide. Among them, root-knot nematodes (RKN), Meloidogyne spp., are the most important of the plant parasitic nematodes, infecting almost all cultivated plants, being responsible for billions of euros in crop losses annually (rice, potato, soybean, and legumes). The potential host range of these obligate, sedentary endoparasites encompasses more than 3 000 plant species. M. incognita, representing the most widespread species, is found in every country in which the lowest temperature is more than 3°C and is therefore possibly the most damaging crop pathogen in the world (Trudgill and Block, 2001).