Whiptail lizards are a fairly ordinary-looking bunch, but some species are among the strangest animals around. You might not be able to work out why at first glance, but looking at their genes soon reveals their secret – they’re all female, every single one. A third of whiptails have done away with males completely, a trick that only a small minority of animals have accomplished without going extinct.
Some readers might rejoice at the prospect of a world without males but in general, this isn’t good news for a species. Sex has tremendous benefits. Every fling shuffles the genes of the two partners and deals them out to the next generation in new combinations. Sex creates genetic diversity and in doing so, it arms a population with new weapons against parasites and predators. These benefits are so big that sex is nigh universal among complex life. Only a few groups, like the incredible bdelloid rotifers, have found ways of becoming permanently asexual.
Doing away with sex is even rarer for vertebrates (back-boned animals). The whiptails of the genus Aspidocelis are a flagrant exception. Their forays into asexuality started when two closely related species mated. For some reason, these encounters produced asexual hybrids. For example, the New Mexico whiptail (Aspidocelis neomexicana) is a hybrid of the Western whiptail (A. Inornatus) and the little striped whiptail (A. tigris). In the hybrid species, the females (and there are only females) reproduce by laying eggs that have never encountered any sperm.
The problem is that this really shouldn’t work. Sperm and egg cells are created through a process called meiosis, where a cell’s chromosomes are duplicated before the cell divides twice. This produces four daughter cells, each with half the DNA of the original. This means that egg cells only contain half the total number of chromosomes that most other cells in the body do. It’s their union with sperm, which are also genetically half-cocked, that restores the full balance of chromosomes, ready for the next generation.
So how do the lizards get their full set? The answer is deceptively simple. They start off with twice as many.