One single hormone determines whether a threadworm larva turns into an animal parasite or whether it develops into a free-living soil-dwelling worm. This fact has recently been discovered by researchers from the Max Planck Institute for Developmental Biology in Tübingen. As the hormone is a small molecule that can easily be synthesized chemically, it stands a good chance of being used in the combat of parasites (Current Biology, 13 January 2009).
In contrast to free-living organisms, parasites gain sustenance from their host and also reproduce in the host. This has proven a very successful strategy for life: more than fifty percent of all animal species are believed to be parasites. Some animal groups have repeatedly evolved from free-living to parasitism. For example, in nematodes, animal parasitism has developed at least four times independently; plant parasitism has developed at least three times independently in a variety of species.
Adult, parasitic nematodes usually produce a high number of progeny which are released by the host - which may be a mammal, fish, beetle or plant - into the environment. They then develop in the soil or water into a stage that is able to find a new host and infect it (infective larvae). Once inside the host, the infective larvae develop into adult organisms. "The existence of these infective larvae offers a unique starting point for the specific combat of parasites," said Akira Ogawa, lead author of the paper.
Until recently, little was known about how parasitism developed, because the differences between free-living and parasitic forms were far too big to enable conclusions to be drawn regarding possible intermediate forms. Researchers of Ralf Sommer's team at the Max Planck Institute for Developmental Biology in Tübingen have, for the first time ever, discovered molecular and pharmacological similarities between larvae of the free-living nematode species Caenorhabditis elegans and Pristionchus pacificus and larvae of the Strongyloides papillosus nematode species parasitizing on sheep, hares and other mammals. "Similarities between the three species allowed us to elucidate information on how parasitism developed during evolution," said Ralf Sommer.
Scientists have known for a long time that the infective larvae of the nematode parasites were morphologically quite similar to the dauer larvae of free-living nematodes. However, a common molecular basis connecting the dauer and infective larvae stages had been missing. Akira Ogawa has been able to show that the development of the specialised larvae stages in the free-living nematode species C. elegans and P. pacificus and in the mammalian parasite S. papillosus is controlled by the same hormone. In C. elegans and P. pacificus, this hormone prevents the induction of dauer larvae stages and in S. papillosus the induction of infective larvae stages. The nematodes develop directly into free-living adult animals. “Since the hormone is a small molecule that can easily be synthesized chemically, our findings might have an impact on the pharmacological control of parasites,” said Akira Ogawa. “We will now use other parasitic nematodes in order to find out whether they too react to this hormone.”The nematodes, which are only a few millimetres long, are one of the most diverse phyla of all animals. It is estimated that there might be more than a million nematode species. Nematodes occur on all continents and are found in almost all ecosystems on earth. Although the majority of nematodes are harmless and live in the soil of the earth surface or seabed, some of them are highly infectious animal, plant and human pathogens. For example, pinworms (Enterobius vermicularis) and the roundworm Ascaris lumbricoides are also members of the nematode phylum. While roundworms are usually harmless for humans, pigs, dogs and cats, hookworms of the species Ancylostoma duodenale and Necator americanus infect one billion people worldwide of whom about 50,000 die per year from the consequences of infection. River blindness, a widespread disease in Africa, is also caused by nematodes. In addition, nematodes also cause severe plant damage on plantations, thus leading to harvest losses.
Original publication: Akira Ogawa, Adrian Streit, Adam Antebi, Ralf J. Sommer; "A Conserved Endocrine Mechanism Controls the Formation of Dauer and Infective Larvae in Nematodes"; Current Biology, 13 Januar 2009
Source: Max Planck Institute for Developmental Biology