Ways of the parasitism origin in the phytonematode orders Tylenchida and Aphelenchida

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The following principal features of the plant parasitic nematode evolution are listed and illustrated. Phytonematodes have been originated from saprotrophic nematodes (bacterio- and mycotrophic ones), firstly plant hosts and later insect vectors were consequently included in their life cycle. The speciation factors were changing depending on the stage of evolution of the host–parasite relationships. In the primitive plant parasitic nematode taxa the species divergence is associated with the soil-climatic conditions; in the most advanced taxa of the sedentary phytonema todes the co-evolution with their plant host taxa took place; in the nematode taxa associated with insect vectors the insect families served as the speciation factors for nematodes. At the key steps of the host-parasite relations evolution, similar adaptations appeared independently in different phylogenetic lines. In transition to endoparasitic feeding, the stylet enforced and the lobe of the enlarged pharyngeal glands was formed. As the adaptations to the migratory endoparasitism in plant tissues, posterior part of the female body shortened and the posterior genital system branch located there, reduced in the postembryogenesis. In the sedentary endoparasitic nematodes the topic and trophic relations with plant hosts were developing independently (incoherently) in different phylogenetic lines. But all sedentary phytonematodes have common features: the specialized infective juvenile stages were differentiated in the life cycle; the diameter of adult female body, occupied with a hypertrophied genital system with mature eggs containing infective juveniles, has been increased significantly. In insect-vectored nematode taxa the special dispersal entomophilic stages were differentiated in the life cycles by two different phylogenetic lines: in one line the transmission juvenile stages (dauerlarvae) were formed, in the second line – the inseminated but not egg-producing transmission females were arisen. The parallelisms in development of similar adaptations indicate the general principles and tendencies of the plant parasitic nematode evolution.