- Not Exactly Rocket Science
Extra chromosomes allow all-female lizards to reproduce without males
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.
Aracely Lutes from the Stowers Institute for Medical Research showed that all-female whiptails have a subtly different style of meiosis. They double their chromosomes twice before everything kicks off, creating eight copies of each. During the normal two rounds of cell division, these copies are partitioned two apiece among the four daughter cells.
Lutes measured the amount of DNA in the egg cells (oocytes) of two closely related whiptails, just before they went through the first round of meiosis. She found that, at this stage, the chromosomes of the asexual checkered whiptail (A. tesselatus) take up twice as much room as those of the sexual Texan spotted whiptail (A. gularis), even though both species have similarly sized genomes. Under a microscope, Lutes even managed to count twice the normal number of chromosomes in the oocytes of checkered whiptails.
Having eight sets of chromosomes rather than four might seem like a big deal, but it doesn’t actually take very much to make this happen. There are two possible routes. The cell could duplicate its DNA but fail to actually split into two, or two cells could fuse together. This seems to have happened in other asexual animals too, including some salamanders and one species of grasshopper. Having extra chromosomes may be a common solution to the problems of ditching males.
Reference: Lutes, A., Neaves, W., Baumann, D., Wiegraebe, W., & Baumann, P. (2010). Sister chromosome pairing maintains heterozygosity in parthenogenetic lizards Nature DOI: 10.1038/nature08818