Some Snakes Can Reproduce Without Sperm, Researchers Say

On a Sunday morning two years ago, David Chiszar walked into his office at the University of Colorado and noticed a bigger-than-usual mess surrounding his 2 1/2-foot timber rattlesnake - one of several reptiles the researcher keeps for animal-behavior experiments.

He couldn't believe his eyes when he saw the source of the mess: The 14-year-old rattler had given birth to her first litter.

"I said, `Now wait a minute,' " Chiszar recalled. " `I've had this female since she was 2 days old and she's never been with a male. Something funny is going on here.' "

Thus began a series of phone calls, e-mail discussions and a battery of genetic tests on three different species of snakes, culminating this month in the publication of a groundbreaking pair of scientific papers that document the existence of fatherless reproduction in snakes.

Virgin birth may seem an unlikely talent for the animal that closed the gates on the Garden of Eden. Moreover, of the smattering of other species known to reproduce without males - a process called parthenogenesis - most produce female offspring exclusively. These young snakes were all males.

Conclusive evidence

The new research offers conclusive evidence that several species of snakes can indeed produce offspring with nary a sperm - at least while they're in captivity. Further studies are needed to see if the process occurs in nature.

But already the work is forcing scientists to reconsider the

current explanation for births that occasionally occur among captive snakes with no documented contacts with males: the long-held notion that females can store sperm for many years after a single, clandestine mating in their youth.

Parthenogenesis has been documented in some lizards, insects and other species, including domestic turkeys. In all of herpetological history, however, virgin births have been reported in but one species of snake - the blind Asian Brahminy - and DNA testing has not been available to provide molecular confirmation that no males have made contributions.

University of Arizona herpetologist Gordon W. Schuett, senior researcher on one of the two new papers, has long suspected that parthenogenesis is more widespread among snakes, and at a scientific meeting a few years ago, he described an apparent case in a garter snake. The snake had been caught as a young adult in Arizona in 1983. She lived only with other females for the next year and was kept completely isolated from other snakes after that. Yet she produced four litters between 1988 and 1994.

Having heard that snake tale from colleagues, Chiszar called Schuett the day after his rattler gave birth. The two agreed to collaborate in a study to determine whether the offspring of their snakes harbored genetic material from any snake other than the mothers. They and several colleagues report in the just-released summer issue of Herpetological Natural History that only the mother's genes were present in the young.

How it's done

Scientists don't know how snakes are produced without sperm, but they have two leading theories.

They know that egg cells are created through a process called meiosis, in which a precursor cell loses half its chromosomes so it can accommodate a similar number of chromosomes from a sperm during fertilization. In snake parthenogenesis, they suspect, the ejected female chromosomes - which together are known as the polar body - get reabsorbed by the egg cell in an act of "self-fertilization," triggering embryo development.

Alternatively, the polar body may never leave the egg; instead, an unknown trigger - perhaps a hormonal signal - might make an egg start dividing as though it had been fertilized.

In either case, only males can be made. That's because in snakes and birds, where there are two kinds of sex chromosomes called Z and W, individuals with a pair of identical sex chromosomes are male (ZZ), while those with one of each (WZ) are female. Since parthenogenesis requires that an egg cell containing either a Z or a W essentially fertilize itself, and since WW snakes can't survive, only ZZ individuals can be made.

However it happens, scientists said, the evolutionary value of the feat is undeniable.

"Wild snakes in some parts of their range are not very common and a female might live her entire life with a low probability of encountering a male," said Jeffry Mitton, a University of Colorado biologist who contributed to the Schuett paper.