How All-Female Lizards Keep Their Genes Fresh Without Sex
Sure, creatures that reproduce asexually get to avoid some of the hangups that come with sex, but the strategy brings its own problems. First and foremost, how do you prevent genetic deterioration without the fresh infusion of new genes that results from the mixing of male and female DNA? For the all-female whiptail lizard, the solution is to hedge its bets.
In a study forthcoming in Nature, researcher Peter Baumann found that each whiptail lizard egg cells contains twice the number of chromosomes you’d expect. In the fertilized egg cell of a sexually reproducing lizard species, you’d expect to see much what you see in humans—23 chromosomes from the father and 23 from the mother combining into 46. (Most human cells contain 46 chromosomes, but egg and sperm cells contain only 23, so that they can combine to give an offspring a compete, but genetically new, set of chromosomes.)
But the whiptail eggs instead begin with two identical copies of each of their mother’s chromosomes, for a total of 92. Those chromosomes then pair with their identical duplicates, and after two cell divisions, a mature egg with 46 chromosomes is produced. Since crossing-over during the cell divisions occurs only between pairs of identical chromosomes, the lizard that develops from the unfertilized egg is identical to its mother [The New York Times].
Curiously, the whiptail lizards came to be through the fusion of two other lizard species, Baumann says. That gave it a rich genetic diversity, but without sex the lizards needed a new way to maintain that diversity. “There’s an absence of sperm, and genetic information is never provided by another source. Anything that’s lost is lost for good” [Wired.com], Baumann says. This trick he found provides a tidy explanation for how these all-female lizards maintain those rich genetics.
Still, while the whiptail’s trick allows for generation after generation of identical lizards, that’s not necessarily advantageous for long term survival. Unless an animal can recombine the DNA they already have, they will produce an offspring with an identical set of chromosomes, in which any genetic weakness, such as disease susceptibility or physical mutation, would have no chance to be overridden by outside genetic material from a mate [Scientific American]. Asexual reproduction is beneficial in the short run, and species like Komodo dragons will do it if they have to. But relying on it exclusively might cost the whiptails in the long run, especially if they should need to adapt to a changing habitat.
There are other creative solutions invented by other asexual species, like the bdelloid rotifers. Unfortunately, though, lizards probably can’t steal the rotifers’ trick of ripping apart their genome and stealing foreign DNA from the surrounding environment.
Image: Peter Baumann