IN an obscure field of study, a few biologists have slowly been piecing together the bizarre story of a group of microorganisms that live as stowaways inside insect cells and specialize in turning the lives of their hosts upside down.
Scientists say these peculiar microorganisms have become expert in disrupting sexual reproduction, an intriguing ability that may have implications for pest control. The effects range from turning sexually reproducing species into asexual, clonally reproducing species, to preventing the production of any offspring between an insect male and female. Until recently, however, researchers had viewed these bacteria and protozoa as quaint biological curiosities, a handful of unrelated creatures that performed their strange tricks in a few insect species.
Now new studies, including a recent report in The Proceedings of the National Academy of Sciences as well as a study in the current issue of the journal Nature, indicate that these microorganisms are extremely widespread among insects and many appear to have sprung from one original insect-manipulating stock. Possible Use in Pest Control
Found in everything from crop-eating moths and beetles to tsetse flies, these microorganisms can halt or alter insect reproduction so effectively that they have become a new focus for scientists seeking ways to destroy insect pest species.
"It's mind boggling," Dr. Jack Werren, a biologist at the University of Rochester, said of the microorganisms' abilities. Dr. Werren, a leader in the small but growing field of study, said: "It's a very exciting new area. They're no longer just interesting novelties."
Researchers say that the new studies suggest there may be more oddities of insect reproduction now attributed to genetic mutations that will turn out to have been caused by microorganisms.
Until recently, asexually reproducing species like the wasps studied by Dr. Richard Stouthamer, a geneticist at the Agricultural University in Wageningen, the Netherlands, were assumed to be asexual as a result of a genetic mutation. Like other asexual species, those Dr. Stouthamer studies consist only of females that reproduce nothing but clonal copies of themselves, without the help of sperm or males. He said he had searched long and hard for a genetic basis for his wasps' asexual reproduction before considering the possibility that it might be caused by bacteria living inside them. Discovering the 'Impossible'
"We thought that it wasn't possible," said Dr. Stouthamer, "but we eventually found that the microorganisms were involved."
Scientists have found that these microorganisms, tucked away inside the insect's cells, are quite well entrenched, passing through one cell division after another as regularly as strands of an insect's own DNA.
But only a female's egg, with its large stores of cytoplasm in which the microorganisms can live, can carry the microorganisms into the next generation. Sperm are too tiny to hold the hitchhikers and bacteria found in males are never passed on.
"No bacteria wants to end up in a male," said Dr. Werren, who studies tiny wasps called Nasonia that are infected by male-killing bacteria. "So they have come up with a lot of ways to insure that they end up in females." While some bacteria make females reproduce more females asexually, others like those Dr. Werren studies simply kill off male embryos before they develop.
Scientists still do not know the exact details of how the bacteria do their work, but they say at least some of these microorganisms achieve their effects by manipulating the chromosomes of cells they infect.
Under normal circumstances, a cell destined to become an egg goes through a cell division in which the normal number of chromosomes is split between the two new cells, leaving the egg with only half the number of chromosomes typically found in a cell. In wasps, such eggs when unfertilized develop into males and when fertilized develop into females.
Dr. Stouthamer said the bacteria in infected eggs appeared able to prevent the eggs from giving away half of their chromosomes as they normally would. By maintaining all the female's chromosomes in the egg, the bacteria insure that the egg will develop into a female, a clone of its mother. This simple chromosomal manipulation keeps the bacteria in a female that can pass them on to the next generation through her eggs. Manipulation of Chromosomes
Bacteria also seem able to manipulate chromosomes in those situations where they cause mating incompatibility, stopping sexually reproducing pairs of insects from producing offspring. Scientists say the bacteria from the egg appear to destroy the chromosomes donated by the father, killing the embryo.
Why the bacteria do this, as it insures they will not be passed on to the next generation at all, is still unclear to scientists. But because the microorganisms seem able to selectively destroy or manipulate the insects' chromosomes, they have captured the attention of researchers who now study the bacteria for clues to how molecular biologists might perform similar tricks.