Published online 12 November 2009 | Nature | doi:10.1038/news.2009.1082

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Biologists turn against worm

Researchers seek out alternative model organisms to C. elegans.

C. elegansResearchers are exploring alternative experimental systems to C. elegans.James King-Holmes / Science Photo Library

The nematode worm Caenorhabditis elegans has been one of the most important biological model systems for more than 30 years. But now growing numbers of researchers are abandoning the stalwart species to investigate closely related worms that offer better insights into the origins of complex biological traits.

" C. elegans is still the best experimental system, but if you want to know something about evolution then you need to wade out into these other species," says Eric Haag, an evolutionary biologist at the University of Maryland, College Park.

For instance, C. elegans reproduces via self-fertile hermaphrodites, but is descended from a species with separate male and female sexes. In fact, self-fertilization has evolved at least three times independently among the 20-odd species in the worm's genus that have been identified.

To tease apart this reproductive transition, Ronald Ellis, a developmental geneticist at the University of Medicine and Dentistry of New Jersey in Stratford, and his colleagues turned their attention to C. remanei, a species that has only male and female sexes. Publishing today in Science1, Ellis's team showed that by lowering the expression of just two genes — one involved in the sex-determination pathway, the other in sperm activation — they could transform C. remanei females into sperm-producing, self-fertile hermaphrodites.

"That's a big, dramatic evolutionary change," says Ellis. He notes that the worms are sickly and would still need other genetic changes to become fully fledged, healthy hermaphrodites. But with further experiments, he believes, a plausible model for this major evolutionary shift will emerge. "We think we can really piece this out," he says. An equivalent approach for elucidating the origins of other complex features, such as insect wings or the vertebrate eye, would be near impossible, Ellis adds.

Sister act

Patrick Phillips, an evolutionary geneticist at the University of Oregon in Eugene, is also focusing on C. remanei. But rather than trying to modify the mating system, Phillips is using this species precisely because of its more typical male-female reproductive mode. " Remanei is much more like a 'normal' organism in terms of its levels of genetic variation," says Phillips, who is inspecting how the genetic pathways influencing the ageing process vary within natural populations of the worm.

“Elegans is still alone on its branch and the prize is still to be won.”

Marie-Anne Félix
Institut Jacques Monod

Other researchers continue to direct their efforts towards hermaphroditic species. But instead of studying C. elegans, they are turning to the related hermaphrodite C. briggsae.

C. briggsae displays higher levels of natural variation than C. elegans, particularly between worms from different parts of the world. This allows researchers to better detect the signatures of local adaptation, notes Asher Cutter, an evolutionary geneticist at the University of Toronto in Ontario, Canada. "Understanding the genetic basis to ecologically important phenotypic differences will be easier in C. briggsae than in C. elegans," he says.

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Another shortcoming of C. elegans is that it lacks a closely related 'sister' species on which comparative genomics studies could be performed or speciation studied. Despite exhaustive international searches and a prize of US$5,000 offered by a consortium of worm biologists to the first researcher to find a closer relative, " elegans is still alone on its branch and the prize is still to be won", says Marie-Anne Félix, a developmental biologist at the Institut Jacques Monod in Paris who has spearheaded global efforts to find new Caenorhabditis species.

By contrast, C. briggsae can cross-fertilize with a closely related, as-yet unnamed male-female species that Félix found two years ago in southern India. Haag, who has been trying to breed the new Indian species with C. briggsae, has managed to make viable hybrids, although the males are all sterile. He is now trying to find the genes responsible for keeping the two species apart and for driving self-fertility in C. briggsae. "This briggsae system might just be flat out better than elegans," he says. 

  • References

    1. Baldi, C., Cho, S. & Ellis, R. E. Science 326, 1002-1005 (2009).
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