By Marlene Cimons, National Science Foundation
Evolutionary biologist Ronald Fisher (1890-1962) predicted that most species would produce equal numbers of male and female offspring. He believed such reproductive behavior was a stable strategy of evolution, and his model became known as Fisher’s Principle.
But Fisher’s Principle turned out to be far too simplistic, and, in some cases, wrong.
Take the nematode, for example, more commonly known as the roundworm. Most species of nematodes do make equal numbers of male and female progeny. But certain species of roundworms defy Fisher’s Principle by producing dramatically small numbers of males, typically fewer than 5 percent. They also produce hermaphrodites, worms with female characteristics that can make both eggs and sperm and thus propagate all by themselves.
“I was curious about how this ability evolved to produce three different genders,’’ says André Pires da Silva, assistant professor of biology at the University of Texas at Arlington, of his interest in studying their complicated reproduction, specifically the reasons behind the roundworms’ mysteriously low number of male offspring. “Also, I wanted to know: what does it take for a hermaphrodite to produce sperm?"
Three years ago, da Silva, an evolutionary geneticist, contacted Diane Shakes, associate professor of biology at William & Mary and cell biology expert, and a new scientific collaboration was born.
“André and I bring very different strengths and experiences to the project,’’ Shakes says. “It’s so integrative. He asks the classic questions of evolution, for example, what kind of ecology do they live in that would foster this kind of reproductive mode? I understand how cells work, so I ask, how can you tweak the dynamics at the level of cell division? We know so little about the cellular mechanisms of how sex ratios change from 50/50 to something different. People have reported similarly unequal sex ratios in a wide variety of organisms, but haven’t gotten down to the cellular mechanisms. We’re getting pretty close."
Understanding the basic science behind their reproductive behavior, in particular how these species skew their gender ratios, could lead to new methods of roundworm population control. Ultimately, this could have an impact on pest management, as well as on new drug development for treating roundworm- related diseases, a problem in developing nations. New information about cell behavior also could provide new insights into the behavior of stem cells, and even cancer.
“This also could be applied to other species with a normal ratio population of genders," da Silva says. Shakes agrees. “Potentially, we could tinker even with the 50/50," she says.
The two scientists are studying roundworm behavior, specifically the species Rhabditis, under a three-year grant from the National Science Foundation. The combined grant totals nearly $443,000.
Another expert in nematode evolution, Marie-Anne Félix, currently at the Institut de Biologie de l’école Normale Supérieure (IBENS) in Paris, first reported the unusual gender ratios in the roundworm species Rhabditis in 2004. She discovered that mating between males and females yielded a very small number of male offspring, but couldn’t explain it. Shakes and da Silva decided to take a closer look.
In many species, males make two kinds of sperm, X and Y. An X sperm results in a female, while the Y brings a male. During the usual process of sperm production, early stage sperm cells divide twice to produce four sperm with the two X-bearing sperm contributing to the production of daughters and two Y-bearing sperm contributing to the production of sons. Nematode males represent a variation on this theme; producing X-bearing sperm that contribute to the production of daughters and non-X bearing sperm that contribute to the production of sons.
“Normally the mother cell divides twice to form four cells, all of which develop into sperm, ’’ da Silva explains. “In our worms, we found that, during the second division, the mother cell does not give the same stuff to the two daughter cells. Instead, the mother cell, just before it divides, puts all the things needed for sperm to become functional into just one cell – while the sister cell gets trash. It turns out that the X-bearing sperm gets all the good stuff while the non-X bearing sperm end up with cellular trash.’’