Evolution of Plant Parasitism in the Phylum Nematoda

Nematodes constitute a fairly speciose (˜ 27,000 documented species) and trophically diverse group of metazoans positioned close to the base of the superphylum Ecdysozoa. Bacterial feeding is thought to be the ancestral feeding type of nematodes from which a multitude of lineages with other food preferences arose. At least four independent major lineages of plant parasites have evolved, and in at least one of these major lineages plant parasitism arose independently multiple times. Ribosomal DNA data, sequence information from nematode-produced, plant cell wall–modifying enzymes, and the morphology and origin of the style(t), a protrusible piercing device used to penetrate the plant cell wall, all suggest that facultative and obligate plant parasites originate from fungivorous ancestors. Data on the nature and diversification of plant cell wall–modifying enzymes point at multiple horizontal gene transfer events from soil bacteria to bacterivorous nematodes resulting in several distinct lineages of fungal or oomycete-feeding nematodes. Phylogenetic patterns arising from a ribosomal DNA framework with sequence data from over 2,700 nematode taxa will be compared with diversification patterns of nematode effectors such as cellulases, venom allergen-like proteins, and members of the SPRYSEC family.