Revenge of the placid protozoan.

Revenge of the placid protozoan 

A mild-mannered, single-celled organism that lives in water-filled tree holes changes into a ruthless parasite when it detects the presence of predatory mosquito larvae, providing a new and unusual model for studying cell differentiation and regulation, new research shows.

According to a paper in the May 27 SCIENCE, mosquito larvae--Which feed on the tiny organisms, called protozoan ciliates--unwittingly secrete a water-soluble factor that stimulates the normally free-living ciliates to undergo a rapid transformation into a parasitic form. As parasites, the ciliates infect the mosquito larvae and ultimately kill their would-be predators.

Such "inducible parasitism" is an extreme example of an antipredator defense in which prey becomes parasites and would-be predator becomes prey, the researchers report. More commonly, water-borne prey develop spines or other protuberances as defenses in response of predatory threats.

Entomologists from the University of California at Berkeley performed a series of experiments showing that the single-celled ciliates, which normally feed on bacteria and other microorganisms, undergo rapid cell division and morphological changes in the presence of predatory mosquito larvae. Moreover, they found the same changes could be stimulated by water previously inhabited by the larvae but from which all larvae had been removed -- indicating the transformation is triggered by a secreted factor, as yet unidentified.

In their parasitic form, the ciliates may prove useful as biological controls against mosquitoes, the researchers suggest. And the identification of the chemical, or "morphogen," responsible for the parasitic transofrmation could add to an understanding of cell regulation.