Origin of the nematode plant parasitism and coevolution of plant parasitic nematodes with their hosts and vectors, with special regard to the aphelenchoidid nematodes

The chapter aims to analyze the evolution of the relations of parasitic plant nematodes with their plant hosts and insect vectors in a process of speciation. The life cycle of tylenchoid nematodes is usually homoxenic with a plant host, whereas the aphelenchoidid nematodes have two hosts in their polyxenic life cycles: a fungus and a plant. In the most advanced aphelenchoidid cycles the insect vector was acquired. The analysis of the phylogeny of tylenchoid nematodes provides an opportunity to reconstruct the hypothetical transformation series of their trophic evolution: ectotrophism – temporary endotrophism – stationary endotrophism (the feeding on the fixed specialized site with a parasitic induction of modified polynucleate host cells). A derivation of several phylogenetic lines of specialized ecto and endoparasites with similar trophic types indicates the unevenness of the evolution rates in the trophic and topological links within the host-parasite system. In nematodes with the homoxenic cycles the role of plant host has increased: the driver of speciation has changed from the soil-climatic conditions to the coevolution with plant hosts’ phylogenetic lines. The evolution of the polyxenic life cycles of aphelenchoidid nematodes started from еру ancestors combined mycotrophism and predation. Their life cycles diversified to include an adaptation either to the plant host or to the insect vector (a detritophage or pollinator). Then the latter became a true host of the parasitic nematode with a cycle involving two hosts (plant and insect) or into an obligatory entomoparasitic mode with a monoxenic cycle. The success of the aphelenchoidid colonisation of the cold Holarctic territories was ensured by the acquisition of the resistant juvenile. There were two ways of the specialization to the insect vector. First way: the resistant nematode juveniles were transformed into transmissive stages (dauer juveniles) and then into the endoparasitic juveniles. Second way: the transmissive function was taken by inseminated but immature (non-egg-laying) females.