Stressed Worms Make Sexier Babies

(Inside Science) -- We’d probably all be supermodels if stress could make us more attractive. But for a type of worm called nematodes, stress actually can influence whether they’re hot or not. According to a new study, nematodes (Caenorhabditis elegans) grown under environmental stress will amp up their offspring’s sex appeal over several generations.

Some nematodes are male, but most are hermaphroditic and make both sperm and eggs. The hermaphroditic worms usually reproduce via self-fertilization. But as they age, the quantity and quality of their sperm tanks. Old hermaphrodites will shift to sexual reproduction instead, relying on higher quality sperm from a male worm to reproduce. To announce their mating intentions, hermaphroditic worms secrete a tempting pheromone that attracts a male. Researchers from Tel Aviv University in Israel and The Rockefeller University in New York reported recently in the journal Cell that stressed worms start producing this pheromone prematurely and pass this trait down to their babies.

“We asked the simple question whether parental responses to environmental triggers could affect these decisions about mating or not mating,” said Itai Toker, a biologist now at Columbia University and one of the study’s lead authors.

Toker and his team stressed the worms by nudging up the heat to 25 C (77 F) from the usual 20 C (68 F). Nematodes can survive and reproduce at this temperature, but it still evokes a stress response. After 10 generations -- around a month -- the stress triggered premature sperm defects. Although the worms were still fertile, this decline caused them to begin producing pheromones and attracting mates at a younger age. Baby nematodes inherited this tendency toward earlier-than-normal sexiness from their parents through small RNA molecules.

RNA, the single-helix counterpart to DNA, is a bridge between DNA and protein production. Unlike mutations, it doesn’t directly change the genes that are coded into the DNA. But RNA can still meddle in cellular blueprints: It turns genes on or off. In the case of the C. elegans, environmental stress causes RNA to alter sperm quality and trigger mating pheromones in offspring. Environmental influences that affect gene expression are called epigenetics, an emerging field of genetics that is helping scientists to understand evolution and inherited traits.

The evolutionary advantage of extra-sexy worms is a diverse gene pool, said the study’s senior author Oded Rechavi, a biologist at Tel Aviv University. While mating can be a dangerous activity -- exposing worms to predators and injury -- diversified offspring have greater chances of adapting. With more combinations of genes, natural selection has more options to play with, he said. Heat-stressed worm parents are giving the next generations more opportunities to evolve in the face of environmental changes like higher temperatures.

“What’s super cool here is essentially that you have this epigenetic effect that is induced by stress, but it can have these incredibly, potentially long-term evolutionary ramifications,” said Levi Morran, an evolutionary biologist who also studies C. elegans but was not involved in the study.

Even after the temperatures returned to normal, nematodes continued to make prematurely attractive offspring. It took around three generations for the normal life cycle of asexual reproduction to resume. By that time, thousands of nematodes had mixed their genes.

“This was very exciting,” said Toker. “This is a transgenerational effect on mating behavior that’s derived from ancestral stress that is not there.”

Toker and his colleagues confirmed the attractiveness trait was passed on through small RNA molecules through an experiment in which they removed the mechanism that transfers RNA between generations. A nifty protein called HRDE-1 acts as a shuttle, “physically carrying small RNAs across generations,” said Rechavi. But without HRDE-1, there can be no small RNA movement: The team blocked the function of HRDE-1, essentially halting small RNAs in their tracks. For stressed worms with disabled RNA shuttles, life continued as normal. No early sperm declines, or sexy smells -- each successive generation remained unaffected.

However, the team couldn’t confirm what gene, or genes, could be responsible for the changes in sperm or pheromones. It could be a combination of genes being turned on and off that elicit the strategic response.

“None of this says anything about humans, or mammals in general,” Toker added. “We hope it could give new ideas and new avenues for research in other organisms.”