Phoresy In Entomopathogenic
Summary: Phoresy is a process whereby a hitchhiker organism attaches to a transporter organism and becomes dormant until the transporter enters a habitat conducive to rapid reproduction of the hitchhiker. Within such a habitat, the hitchhiker breaks dormancy, detaches from the transporter, and begins to multiply. Scroll down to read full text of of article. Next... Home...
In the case of entomopathogenic nematodes (EPN), the hitchhiker is a bacterium that is carried by the EPN in a special intestinal vesicle or in its anterior gut. The nematode Family Steinernematidae, for example, carries a bacterium in the genus Xenorhabdus while the nematode Family Heterorhabditidae carries a bacterium in the genus Photorhabdus.
EPN, on entering a suitable insect's body, proceed directly to its blood supply (the insect's hemocoel). Here their phoretic bacteria pass into the insect's blood, where they feed and reproduce. As the bacteria multiply, several important things happen. First, they become food for the nematodes--in fact, they and the insect's partially metabolized tissues will be the primary food for the nematodes over the next five to seven days.
Next, the bacteria secrete toxins which, while harmless to nematodes, are lethal to the insect. Studies have shown that the toxin complex produced by various species of bacteria in these genera is unique to the bacteria species involved, and that nematodes evolving with such bacteria were required to make significant adjustments, along the way, to enable the phoretic relationship to work. Thus, similar nematodes, with different phoretic bacteria, are typically unable to survive when reared in the presence of toxins produced by bacteria from foreign nematode species.
This suggests that each EPN/bacteria symbiotic complex has a long history that is not susceptible to sudden. significant, and sustainable mutations. Such mutations would likely disrupt the phoretic relationship to the point that neither organism can survive. On the other hand, a slow series of mutations, taking place over sufficient time to allow the symbiotic partner to adjust, would be less likely to be fatally disruptive. This underscores the safety of EPN for bio-control work.
Along with a unique toxin complex, the phoretic bacteria also secrete special antibiotics that keep foreign bacteria from putrefying the insect's dead body--this allows the nematodes and their phoretic bacteria to thrive in the host insect's cadaver until fresh infective juvenile nematodes are ready to emerge. The entire process can take as many as eighteen or more days to complete, though the first batch of infective juveniles can begin emerging as early as the third day, post-infection.
As the nematodes feed, they proceed through several molts and lay eggs. After the eggs hatch, nematode larvae develop to the third juvenile molt, known as the J3 or dauer stage. At this point they cease feeding, conserve their store of nutrient, and stow a quantity of phoretic bacteria internally. Thus equipped, the infective juveniles (referred to as IJ's) begin to emerge from the insect cadaver where they were reared.
What happens next varies between nematode species and a number of environmental and temporal conditions. In general, a certain number of IJ's will, on emerging from the insect cadaver, immediately seek out another insect to attack. Some of the IJ's go dormant, ceasing activity for days, weeks, or months, before actively searching for insect prey. Others parasitize the bodies of non-insect organisms, such as earthworms, apparently causing little or no injury to their hosts. These latter IJ's may later emerge from their non-insect hosts to search for insect prey.
When dauer-stage nematodes (IJ's) find and infect new insects, the life cycle, for the nematode species involved, starts afresh.
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