Genetic selfishness in an all-male club.

Genetic selfishness in an all-male club

Frogs and snails and puppy-dogs' tails may make little boys, but in the wasp Nasonia vitripennis, a selfish chromosome apparently can dictate which offspring will be male. Scientists report that an extra chromosome carried by males of the species somehow destroys the other paternal chromosomes -- thus enhancing its own transmission but resulting in all males.

A tiny, parasitic wasp that lays its eggs in the pupae of flies, N. vitripennis normally produces only females from fertilized eggs and only males from unfertilized eggs. If an extra chromosome called psr (paternal sex ratio) is present in the sperm, however, all five of the normal chromosomes contributed by sperm disappear after fertilization. With its "fertilized" status canceled, the egg then becomes a male capable of making more psr-containing sperm.

Uzi Nur and other researchers at the University of Rochester in New York report in the April 22 SCIENCE that, "because the psr chromosome enhances its transmission by eliminating the rest of the genome, it can be considered the most 'selfish' genetic element yet described."

Because it is an extra chromosome not found in normal wasps, psr is classified as a supernumerary chromosome, or "B" chromosome. Scientists have found B chromosomes in more than 800 species of plants and animals thus far, Nur told SCIENCE NEW. "More often than not, they have some trick up their sleeve to enhance their transmission [to the next generation]," he says. In the mealy bug, for example, the B chromosome hitches a ride on those chromosomes that remain active whether offspring become male or female. But psr takes this trickery to the extreme by simply eliminating the posibility that offspring will be female.

Coauthor John H. Werren discovered the psr trait several years ago (SN: 3/1/86, p. 134). But prior to the current study, the scientists thought a parasitic virus was causing the peculiar all-male phenomenon. Normally, female N. vitripennis leave about 5 to 15 percent of their eggs unfertilized to guarantee an adequate number of males. If there are several females laying eggs on the same fly, however, they adjust the sex ratio of their offspring to 50 percent males. Females can readily mandate the sex of young wasps by rationing sperm held in a special organ found in their bodies. "All these elements fighting each other for the sex ratio is a fascinating story," Nur says.

Although it is still unclear exactly how psr functions, Nur says he and his colleagues suspect "it's like an infection in that it tends to spread [through a population of wasps]." If this is the case, he says, mutations or unknown environmental factors may periodically stop psr transmission, before the all-male results extinguish the species. The scientists have found the chromosome among wasps collected in Utah, but not in those from New York. The highest observed percentage of males carrying psr in any specific population was 20 percent, Nur says.

Whatever keeps psr from destroying the species, the chromosome apparently has an ambitious "meiotic drive" of its own, says Nur. The curious phenomenon of meiotic drive occurs when a genetic component is transmitted through generations more often than would be expected under accepted genetic laws. In other words, psr ignores the rule that genes from both parents have an equal chance of being transmitted to the off-spring.

With psr now characterized as a genetic element, scientists may also find the selfish DNA in other species, Nur says. Werren currently is looking at a related wasp capable of mating with N. vitripennis, to see whether psr can spread from one species to another. Nur says those experiments may show whether psr was derived from the normal chromosomes of N. vitripennis, or whether it came from cross-breeding between two species.