Sexual reproduction is usually touted as the best way to ensure a species’ survival. Not for geckos. Some of them thrive without sex.
Geckos have unusual sex lives. First of all, different species of these small lizards can interbreed. Moreover, although the offspring of such interspecies unions are often sterile, some of the females are quite the opposite: they can reproduce without even having sex with a male. For some reason their eggs contain the same 44 chromosomes as their body cells—22 from the mother and 22 from the father—instead of half, and so the eggs can grow into gecko hatchlings without first being fertilized by sperm. The hatchlings in turn are all females that give birth in the same way. Thus two species of sexual geckos can produce a new species of asexual hybrid. This has happened more than once on islands in the Pacific.
The mother and father species of some asexual geckos are known. But the origin of Lepidodactylus lugubris, an asexual gecko found on many central Pacific islands, had remained a genealogical whodunit until Ray Radtkey, an evolutionary ecologist at the University of California at San Diego, and his colleagues turned their attention to the problem. They have not only figured out L. lugubris’s pedigree but pinpointed its likely birthplace—Arno Atoll in the Marshall Islands. They have also found reason to believe that—on islands at least—virgin births can create sturdier geckos.
The researchers began tracking down L. lugubris’s ancestry by trying to match the lizard’s genes to a paternal and maternal forebear. Finding the maternal ancestor was easy, says Radtkey, because the DNA contained in mitochondria—the cell’s batteries—is inherited only from the mother. So he and his colleagues simply sequenced L. lugubris mitochondrial DNA and found that it matched that of L. moestus, a sexual species of gecko common on a few central Pacific islands.
But Radtkey’s group lacked an equivalent marker for establishing paternity. So they started searching for other species that shared a set of proteins found in L. lugubris’s cells but not in L. moestus’s, which therefore must have come from the paternal ancestor. When they rounded up all the likely candidates, the father proved to be a gecko species so little known that it doesn’t even have a name. What is known is that its distribution overlaps with that of L. moestus only on Arno Atoll. This tiny island, which surfaced only about 4,000 years ago, may well be L. lugubris’s birthplace.
The new species has prospered to the extent that it is now more widespread in the Pacific than either of its parents. University of Hawaii researchers have recently discovered one possible reason: L. lugubris, they say, is more resistant to mites than are sexually reproducing geckos.
Doesn’t that fly in the face of the theory that sexual reproduction, through the mingling of genes, helps create hardier offspring? Not necessarily, says Radtkey. The benefits of sexual recombination depend on the availability of lots of different genes. On islands, sexually reproducing species have a smaller gene pool to draw on than their mainland counterparts. Instead of hardier offspring, they may be just as apt to produce feeble inbreds.
L. lugubris also has a small gene pool: its population consists of clones descended from four or five interspecies couplings. Each individual L. lugubris, on the other hand, has highly heterogeneous DNA because it is a hybrid of two species. Interspecies mating, says Radtkey, probably produces a lot of feeble offspring that don’t survive. But it can also produce organisms with new combinations of genes that are better adapted to their environment—which is apparently what happened in the case of L. lugubris. And because it reproduces asexually, its genetic edge over its parent species is frozen in; it can’t be eroded by inbreeding.
But that edge won’t last forever, says Radtkey. On these islands, it’s better to be asexual in the short-term ecological sense, he says, but any time the environment changes, you’re going to get shut out. If your parasites are changing, for instance, and you don’t have any way to change, then you’ll go extinct rapidly. One bit of circumstantial evidence that asexuality is not that great is that you don’t find any old asexual species—meaning evolutionarily old.