By David N. Leff

Of the Ten Commandments in the Bible, No. 3 prohibits adultery. And farther down the list is written: "Whosoever lieth with a beast, shall surely be put to death." But - curiously - in this Judeo-Christian legal code one finds no injunction against sons lying with their mothers, daughters with their fathers, or brothers and sisters having intercourse with each other.

It's curious because incest is, and always has been, taboo by all societies throughout human history. Not that this outlawing of inbreeding was motivated by any knowledge of genetics. "All religions ban marriage between close relatives," observed Swedish animal ecologist Thomas Madsen, a research associate at the University of Sydney, Australia.

"And of course," he continued, "there is a reason for this taboo. The reason - although the religious people didn't think about it - comes from when we humans were ape men, or whatever we were. Those ancestors realized that to marry your sister, or to have children with your mother, or a daughter with her father - that's the worst inbreeding you can get - causes severe problems."

Today we know this fact of life at the genetic level. Everyone born carries within his or her chromosomes two or three harmful genes programmed to cause inherited disease. Fortunately, most of us escape that fate because we carry two sets of chromosomes, one from each parent. (That makes us heterozygotes.) Only when those black-hatted variant genes come down to us from both father and mother (i.e., homozygotes) do we run the risk of familial disorders.

If non-incestuous couples marry, their risk of having a homozygous child is slim. But if brother and sister have a baby together, its probability of acquiring that disease rises to 25 percent - one in four - because siblings share all of their grandparents' identical alleles. For first cousins, who share only half of those aberrant genes, it's one in eight.

Pioneer geneticist Gregor Mendel worked out those odds in the mid-19th century by crossbreeding garden peas. Statistically, highly inbred populations have reduced reproductive success. This applies not only to people and peas, but to all forms of life - including snakes.

A Tribe Of Adders In Need Of Help

Thus, in 1981, Madsen, a doctoral candidate at the University of Sydney, happened upon a colony of common European adders (Vipera berus) in Sweden, on the brink of extinction from extreme inbreeding.

"Adders are one of the most widespread vertebrates on the face of the planet," pointed out Australian evolutionary biologist Richard Shine. "They range from England all the way across Northern Europe to Sakhalin Island off the coast of Siberia. These snakes have heavy bodies, broad heads, and look rather like one of the smaller species of American rattlesnakes, but without the rattle. Although venomous, adders are not particularly dangerous to human beings. They're quiet little creatures, fairly reluctant to bite."

But less docile in the springtime mating season.

"The males fight each other over the females," Shine recounted. "They engage in ritual wrestling matches, which the larger males win. They then court the female by lying beside her and rubbing their chin along her back. After about half an hour of intense foreplay, the female will accept a copulation by raising her tail. The male then inserts his forked penis - he's got two penises, but uses only one at a time - and transfers sperm, while they copulate for 45 minutes or so." He added, "The female may then mate with other males, or that first one again, in the course of the mating season."

Madsen is senior author, and Shine corresponding author, of a brief paper in this week's Nature, dated Nov. 4, 1999. Its title: "Restoration of an inbred adder population."

What drives these reptiles to inbreed is the extremely restricted area they inhabit on the southern tip of Sweden. "It's bordered to the south by the Baltic Sea," the co-authors report "to the north by arable land, to the west by a village and to the east by a harbor. The adders are confined to a coastal strip 1 kilometer long by 50 to 200 meters wide, totally isolated from other adder communities in Sweden."

"What I was really interested in 19 years ago," Madsen told BioWorld Today, "was to look at the adders' sexual dimorphism. In most snakes, the males that fight for females are larger than females. But male adders, though they fight for females, are smaller - about 55 centimeters long to the females' 65. So that's why I wanted to study how come this exception to a very general rule.

"That was the work I did from '81 to '87," he went on. "Then I became interested in the genetics of it all, because I brought back pregnant females every year to give birth in laboratories at the University of Lund, which I visited from Sydney every few months. Then I found out that these mother snakes produced a very high proportion of malformed and stillborn babies, and had very low genetic variability, compared to other adder populations. So I realized that they were all suffering from some kind of inbreeding depression, with their population starting to decline rapidly."

Live, Vigorous Gene Delivery Vectors

So Madsen decided in 1992 to apply an ecological version of gene therapy. Instead of injecting DNA sequences into individual snakes, he inserted fresh genes into the entire population, by importing and releasing 20 live and vigorous male adders he collected at distant sites.

"I saw that these new guys settled in fairly well," he recalled. "All females in '92 mated with the new males. And when I brought these females back to the lab, they had no stillborns, no malformed babies, so then I realized: 'Wow, it seems to be working!' "

This year Madsen caught more snakes than he ever captured before in that area. "Virtually all of them still carry new genes," he pointed out, "and the population is the most genetically variable adder community in all Sweden. I'm certain they will survive. So I'm definitely not going to stop the work now," he concluded. "It will be interesting to see what will eventually regulate this population's number. What will be the carrying capacity of the area."