Diapause in parasitic nematodes: a review

This review considers in a selective way the literature on diapause in parasitic nematodes, concentrating on four species of animal parasites and three species of plant parasites. We define diapause as a developmental arrest which is temporarily irreversible, so development will not resume, even under favourable conditions, until some intrinsic changes have been completed. Our analysis recognises four stages in diapause. The first is induction, typically brought about by environmental signals (although diapause may be genetically programmed independently of the environment). These environmental signals typically do not have an immediate effect on development, but we recognise a second phase, which we call the diapause pathway, in which worms have been induced to enter diapause at a later developmental stage. Surprisingly, entry into the diapause pathway may under some circumstances be reversible. The third stage is diapause development, a period during which development is suspended, but some ill-understood process must be completed prior to the fourth stage, emergence from diapause. Although diapause development is complete, resumption of development may be further delayed because of conditions in the host or in the environment: the worm is once more capable of development, but development is prevented by unfavourable conditions extrinsic to the worm. These may include the immune state of the host or the total parasite burden in animal hosts.