Evolution of Plant Parasitism

The conserved morphology of nematodes and the absence of extensive fossil records make discussion of the evolution of parasitism in nematodes problematic (Poinar 2011). Several hypotheses about the origins of plant parasitism have been put forward by nematode taxonomists (for example, Maggenti 1971; Poinar 1983; Siddiqi 1983) with little agreement. However, more recent studies using molecular phylogenies based on the small subunit ribosomal RNA (SSU RNA) demonstrate that parasitism of plants by nematodes has arisen independently on at least three separate occasions (reviewed by Baldwin et al. 2004). The rRNA phylogenies also support convergent evolution of sedentary endoparasitism and feeding site establishment by root-knot and cyst nematodes, rather than the theory that the two groups shared a common ancestor.

The co-evolution of plants and plant-parasitic nematodes has resulted in remarkable synchrony of the host and parasite life cycles that enhances the chances of the nematode infection and, thus, survival and reproduction. This integration between host and nematode has progressed furthest in cyst nematodes and the dependency of some species on stimulation from host plants to cause hatch is one aspect of this integration (see Sect. 1.3).

Comparative genomics have been used to provide insights into the evolution of parasitism in the phylum Nematoda, especially the acquisition of novel genes associated with parasitic lifestyles (Rosso et al. 2009). Several genes have been identified in the transcriptomes of plant-parasitic nematodes that are most similar to mi-crobial genes, and these may have been acquired by horizontal gene transfer (HGT) from microbes associated with ancestral nematodes. Jones et al. (2005) argued that acquisition of such genes via HGT has played a critical role in the evolution of plant parasitism. For example, several genes coding for enzymes such as cellulase, pec-tate lyase and chorismate mutase have been identified in cyst and root-knot nema-

todes with bacteria as the likely origin (Jones et al. 2005). Fungi are the probable origin for the gene coding for GHF45 cellulase, which is vital for the parasitic phase of the life cycle of Bursaphelenchus xylophilus (Kikuchi et al. 2004).

The use of RNAi enables loss-of-function phenotypes to be analysed and will provide information on the evolution of nematode parasitism. Rosso et al. (2009) consider that RNAi will facilitate the elucidation of the molecular determinants of parasitism. Information from this approach, together with functional and behavioural data, may provide pointers to new control targets centred on perturbing aspects of the parasitic life cycle such as hatching, host location and survival.

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