Question

Why are parasitic nematodes generally ubiquitous and difficult to control?

Answer 1

Parasitic nematodes generally ubiquitous and difficult to control because they can survive adverse conditions, thus assisting their chances of establishment and persistence in new areas.

Some of them are

Cryptobiosis

Cryptobiotic mechanisms are physiological changes which enable parasitic nematodes to survive for long periods under unfavourable conditions (e.g. in the absence of a host, unfavourable temperatures, and during storage or transportation by entering a quiescent or dormant state)

Eggs, cysts and egg mass

The chances of plant parasites surviving transit may be dependent also on the life-stage associated with a particular pathway because adaptations to survive adverse conditions in new areas may be specific to particular life stages. Live, non-quiescent adult nematodes can survive in plant materials and soil contaminants for relatively short periods, but eggs can survive longer. The eggs of parasitic nemotodes are enclosed in a multilayered eggshell typically with three layers:

an outer vitelline layer; a middle chitinous layer; and

an inner lipid layer

The eggshell protects the contents from desiccation, extreme temperatures and chemicals such as nematicides.

The eggshell in parasitic nematodes enables enclosed juveniles to supercool to temperatures below −38◦C enabling them to survive extreme winter temperatures. In addition to the protection of the eggshell,many hundreds of eggs may be contained within another protectivepackage formed by the tanning of the cuticle of the dead fertilised female. This is known as the ‘cyst’

Cysts have been implicated in the widespread dispersal of these PN as they can survive harsh conditions and be spread by many different pathways including being ingestedby birds and released with their droppings, as well as being blown in dust storms.

Host location and invasion by juveniles

For a successful establishment, PPN need to find a susceptible host. Adaptations for host finding are welldeveloped in the best-known PN.

For example, in globodera, Heterodera and Meloidogyne, root exudates from hostplants stimulate egg hatching thereby synchronising thepresence of juveniles and hosts, which increases juvenile survival. Second stage juveniles are also specifically adapted for host location and are the infective stage in the lifecycle.

In Meloidogyne it has been shown that secretions from juveniles can influence root development and responsiveness to nematodes, similar to mechanisms used by Rhizobia bacteria while forming root nodules

Specially adapted juveniles

Other strategies frequently found in PN are lipid reserves in the intestine of juveniles and specially adapted stages for dispersal. Lipid reserves act as a food source while the nematodes are trying to locate host plants. These juveniles are adapted for dispersal by beetle vectors.

Changing sex ratios

Adult PN are adapted for feeding and reproduction rather than dispersal and finding hosts like the eggs and juveniles.

Female nematodes require greater amounts of nutrients than males. As a survival mechanism, the sex of developing juveniles in species of Globodera, Heterodera and Meloidogyne can change with food availability

Adult females are produced than males when food is abundant whereas more adult males develop when food is scarce. Such a strategy is thought to reduce competition for food because males can survive longer without a host