Their shapes are so different that they look like five different species.

But genetic studies have shown single species of nematode worm, newly discovered inside figs, can develop into five distinct forms. It is a striking example of physical divergence without genetic divergence.

“We were shocked,” says team member Erik Ragsdale of the University of Indiana. “It is remarkable and unusual.”

Young Pristionchus nematodes hitch a ride to new figs on the wasps that fertilise the figs. If you look inside the fig soon after the wasps arrive, only a small form of the nematode can be found. It has a simple tube-like mouth for feeding on microbes.

Three forms have teeth

But the offspring of these colonists can develop into five distinct forms: two that feed on microbes and three larger ones with bigger mouths equipped with rows of teeth, for catching other species of nematodes. “They have different ecological roles,” Ragsdale says.

Many animals, particularly insects, can develop into different forms depending on the environment, season or social cues – a phenomenon known as polyphenism. The white winter coats of many animals are a familiar example.

More dramatic examples include the moth Nemoria arizonaria, whose caterpillars grow to resemble oak catkins if they hatch in spring but oak twigs if they hatch later on.

Cannibal forms

In other animals including a few fish, birds, frogs and salamanders, there are distinct cannibalistic phenotypes with features such as larger mouths for eating their peers. And some arachnids have distinct forms armed with different weapons.

Some social insects do have even more than five forms – with two or more types of the soldier, worker and reproductive castes. “There is an enormous diversity of forms and tasks,” says Fred Nijhout of Duke University in Durham, North Carolina, who studies insect polyphenisms and was not involved in the nematode research.

Most polyphenisms associated with different ecological roles, though, involve just two different forms. For a single species to have five different forms each with different ecological roles may be unique. “It is totally unexpected,” Nijhout says.

Why did it evolve? Perhaps because figs are like islands, offering lots of opportunities to colonists that can take various forms to exploit them.

But figs are ephemeral ecosystems, and only a few nematodes ever manage to colonise new fruits. So a single species that can diverge soon after reaching a fig will have a big advantage over others.

So far the team has identified three related species of Pristionchus nematodes – collected in South Africa, Réunion Island and Vietnam – that each have five phenotypes. There may be species with even higher numbers, says team member Ralf Sommer of the Max Planck Institute for Developmental Biology in Germany.

“That is why we continue to collect,” he says. “I have a hard time, though, imagining that there could be even more microhabitats in fig fruits.”

Journal reference: Science Advances , DOI