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.
When our lives are in danger, some humans go on the run, seeking refuge in other countries far away from the threats of home. Animals too migrate to escape danger but one group – the pond-living bdelloid rotifers – have taken this game of hide-and-seek to an extreme.
If they are threatened by parasitic fungi, they completely remove any trace of water in their bodies, drying themselves out to a degree that their parasites can’t stand. In this desiccated state, they ride the wind to safety, seeking fresh pastures where they can establish new populations free of any parasites.
This incredible strategy may be partially responsible for another equally remarkable one – the complete abandonment of sex. For over 80 million years, the bdelloids (pronounced with a silent ‘b’) have lived an asexual existence. Daughters are identical clones of their mothers
, budded off from her body. No males have ever been discovered. For this reason, Olivia Judson once described bdelloid rotifers as an “evolutionary scandal”. Their sexless lifestyles simply shouldn’t work in the long run.
Ditching sex allows an animal to efficiently pass all of its genes to the next generation without having to seek out a mate. This should give asexual animals a big advantage but not so. Sex provides fuel for evolution. Every time two individuals meet in flagrante, their chromosomes are joined, shuffled and re-dealt to the next generation. In this way, sex begets diversity, remixing genes into exciting new combinations.
This diversity is a vital weapon in the never-ending war against parasites. Parasites, with their large populations and short generations, are quick to evolve new ways of exploiting their hosts. They could have their run of a genetically uniform population and soon bring it to its knees. A sexually active species is a harder target. With genes that shuffle every generation, new anti-parasite adaptations are always just one bout of mating away. And so it goes, again and again, with hosts constantly having to outrun their parasites and sex acting as the getaway vehicle.
So asexual reproduction, for all its immediate gains, should be a poor long-term strategy compared to the dynamic nature of sex. Bdelloids have clearly addressed this problem and thanks to the last few years of research, we know how. They have evolved ways of achieving every single one of the many benefits of sex, without actually doing the deed. Escape parasites? They’ve got that covered. Shuffle their genes? They do that too. Generate genetic diversity? Check.
Termite colonies are families – millions of individual workers all descended from one king and one queen. But the colony itself tends to outlast this initial royal couple. When they die, new kings and queens rise to take their place. These secondary royals are a common feature of some families of termites, and they will often mate with each other for many generations. But there is more to this system than meets the eye.
Kenji Matsuura from Okayama University has found that the secondary queens are all genetically identical clones of the original. There are many copies, and they have no father – they developed from unfertilised eggs laid by the first queen through a process called parthenogenesis. These clones then mate with the king to produce the rest of the colony through normal sexual means.
It’s a fiendishly clever strategy. The original queen’s legacy to the colony is… herself. She effectively splits herself into several different bodies and in doing so, greatly increases the number of offspring she has. And because each of these descendants mates with the king, who has no genes in common with them, the colony neatly skirts around the problems of inbreeding.