Waspish son-killers and sex-switchers.

Waspish son-killers and sex-switchers

Birds do it and bees do it. But bees do it differently, as do wasps and other members of the order Hymenoptera. In general, while females of these species develop from the union of sperm and egg, males--in the process called parthenogenesis--develop from unfertilized eggs. Now researchers report in the Feb. 28 SCIENCE that a bacterium newly discovered in a parasitic wasp may eventually help explain this mechanism.

The wasp, Nasonia vitripennis, lays her eggs--fertilized and unfertilized--on the pupa of a fly. If the mother wasp is infected with the yet-unnamed bacterium, development of most of the unfertilized eggs will be prevented. The fertilized eggs hatch unharmed, resulting in a mostly female brood. The researchers, at the University of Maryland in College Park, the University of Wisconsin in Madison and the West German Institute for Biological Pest Control in Darmstadt, speculate that the bacterium produces a chemical in the female that can distinguish fertilized from unfertilized eggs, killing the male eggs.

"People gave up on [trying to find the mechanism for parthenogenesis] a long time ago," says John Werren of the University at Maryland. "The bacterium interferes with something that plays a key role in the development of unfertilized eggs. It gives you a tool."

Every female of this wasp species can determine the sex of her offspring, since she stores sperm in a sac in her reproductive tract. By relaxing or contracting a muscle to release the sperm, she controls the proportion of fertilized eggs and hence the proportion of daughters. But about 35 percent of the females also carry one of three factors that will distort the sex ratio of offspring; of the three factors, the son-killing bacterium is perhaps the least bizarre.

The "maternal sex ratio" factor--perhaps another microorganisn -- induces females to contract the sperm-releasing muscle more frequently, boosting the proportion of daughters.

And the strangest of the sex-ratio distorters is a "paternal sex ratio" factor that causes the wasp to produce only sons, the researchers say. "It shows paternal inheritance, meaning it's transmitted through the male line," says Sam Skinner of the University of Wisconsin. "That's a fairly extraordinary phenonmenon because there is no male line [since males are produced from unfertilized eggs]." Since the factor is transmitted with sperm into an egg during fertilization, it affects only eggs that would normally become female. "It comes in with the sperm and then seems to turn around and destroy the male-derived chromosome," he says. "The egg would have been diploid and female; it's now haploid and male. But it's a male that has inherited this [paternal sex ratio] trait."

The maternal and paternal factors allow the females to make gender adjustments in offspring to maximize the number of grandchildren. The son-killing bacterium is different: The death of male eggs leaves more food for female offspring, which grow bigger and have more female, infected offspring. But the loss of eggs also means the wasp pays a price in absolute numbers of offspring and in opportunities to transmit her genes. In the end, only the bacterium collects the benefit.