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Sex in the soil

Nematode worm can change its sex based on mating chances

University of Alberta

Even in the animal world, mating is so desirable that the nematode worm will change its sex to increase the chances of partners--a groundbreaking discovery of nurture changing nature, says a University of Alberta scientist, part of a team which conducted the research.

"We all know that we can alter our behaviour, depending on the environment in which we are raised," said Dr. David Pilgrim, from the U of A's Faculty of Science. "But it was thought that our basic genetic makeup is unaltered by these effects. What we have now shown is that our nature--our genes--may be altered by our nurture, the environment."

Pilgrim, Dr. Veena Prahlad and Dr. Elizabeth Goodwin have published their results in the prestigious international journal, Science. Prahlad was a post-doctoral student in Pilgrim's lab before moving to the University of Wisconsin, where she completed the work in Goodwin's lab.

Like humans, the female nematode worm bears XX chromosomes but the male nematode has only a single X. The team showed that the sex ratio--percentage of males and females--could be altered depending on the amount of food the animal senses. While the young female is still too young to display any sexual characteristics, it judges how much food will be available once it grows up.

If it thinks there will be a lot of food available once it is sexually mature, a significant number of XX animals will lose one of their X chromosomes--making them genetically male. If they think that food will be scarce, they will keep both their XX chromosomes and grow up to be female.

Also, the female--actually a hermaphrodite since it can produce sperm as well as egg--can self-fertilize if she doesn't find a male but the offspring then are only female (XX).

If the population density is high, as it would be near food, then there is a benefit to being a male since the male nematode is rarer and the chances of finding a potential female partner are higher. If it is low, then the worm is safer being a female since she can still have offspring even if she never meets another animal.

"The trick comes in being able to estimate whether the food will be plentiful or not when it is ready to reproduce, because it needs to make the decision to be male or female well before that," said Pilgrim. "This research helps understand how animals adapt to a variable environment, and to a certain extent, why sex exists."


The U of A in Edmonton, Alberta is one of Canada's premier teaching and research universities serving more than 33,000 students with 6,000 faculty and staff. It continues to lead the country with the most 3M Teaching Fellows, Canada's only national award recognizing teaching excellence.

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