Sex costs amazing amounts of time and energy. Just take birds of paradise touting their tails, stags jousting with their antlers or singles spending their weekends in loud and sweaty bars. Is sex really worth all the effort that we, sexual species, collectively put into it?
Despite many teenage frustrations and MTV, most biologists think that sex is totally worth it. With sex, every new generation receives a fresh combination of genes from its parents. This makes it easier to adapt to changing environments, as genes can spread quickly through a population.
In asexual species every child will be genetically identical to its parents, making it hard to compensate for disadvantageous mutations. Biologists expect that deleterious mutations will pile up in asexual species in a process known as Muller's ratchet. With every mutation in an asexual lineage, Muller's ratchet clicks one step closer to extinction.
While sexuality might save a lot of time and energy on the short term, asexual species are expected to lose to their sexual cousins in the long run. Sex is the driver of genetic diversity on which we depend .
Yet, asexual species keep popping up all over the tree of life, with over 90 vertebrates and many more insects currently committed to an asexual lifestyle . Did all these species buy a one way ticket to extinction? Or have they found ways to make asexuality work in the long run?
In some populations of mole salamanders (or Ambystoma), all the individuals are female. They live alongside four sexually reproducing salamander species in northeastern North America. Weirdly enough, when you look at the genes of unisexual salamanders, you’ll find that they’re a mixed bag of genes that can be found in the sexual salamanders.
A single female can carry up to five copies of entire genomes from these different salamanders in her cells. For example, a unisexual salamander with the biotype ‘LLJ’ combines two copies of the Blue-spotted salamander (L) genome with one copy from the Jefferson salamander (J). (Image Left: The males of the four salamander species whose genomes can be found in unisexual species. From left to right: A. jeffersonianum, A. tigrinum, A. laterale and A. texanum.)
Since every unisexual salamander is a genetic mashup of multiple different salamander species, it’s difficult to track down when and where they evolved. Luckily, mitochondrial DNA (which is transmitted from mother to daughter largely unaltered) provides a much clearer picture. This mtDNA indicates that all unisexual Ambystoma evolved from a single salamander closely related to the Streamside Salamander an impressive 5 million years ago ! The unisexual salamanders were the hybrid outcome of a romantic encounter between a Streamside Salamander and another unidentified species.
At 5 million years old, they are the oldest unisexual vertebrates ever recorded. But can a species really survive 5 million years without sex? Shouldn’t Muller's ratchet have driven them extinct a long time ago?
The secret of these salamanders is that while they might be unisexual, they’re not asexual. In other words: they’re having sex. They still require a sperm cell to trigger their egg cells to develop into embryos. Since there are no males in their own populations, they mate with their closely related sexual neighbors .
Normally the sperm isn’t allowed to contribute its DNA to the embryo. But sometimes the security measures fail, and one of the maternal genome copies gets exchanged for a fresh paternal one. This is also the reason why they carry so many different genome copy combinations. (Image Right: When a unisexual salamander and sexual male mate, a maternal genome copy (B) of an egg cell sometimes gets exchanged for the paternal copy present in sperm (M).)
Despite having all-female populations, these salamanders can still avoid genetic stasis in this way. Their sexual strategy gives them access to the sexual efforts from four different species. All they have to do is tempt a male into having sex, and steal the results of their hard work every now and then!
Bdelloid Rotifers have taken asexuality one step further. They have ditched sex at least 35-40 million years ago ! Bdelloid Rotifers are microscopic animals that live in fresh water environments, where they feed on detritus and algae. Despite their small size they are proper animals, complete with jaws, throat, stomach and a gut . They are well known for their ability to dry themselves out when the water in which they live dries up. When things turn more favourable (read: wetter) they reboot by rehydrating themselves and resume living as if nothing ever happened.
But it is their ongoing celibacy that has made Bdelloids truly famous. Not one male Bdelloid Rotifer has ever been observed. Unlike the unisexual salamanders, the female Bdelloids have done away with sex and males entirely. From every unfertilized egg, a new rotifer emerges that is an exact clone of its mother. The fact that they have been doing this for millions of years and managed to evolve into different Bdelloid species, led John Maynard Smith to describe them as ‘an evolutionary scandal’ in 1986 . No animal lineage should be able to survive without sex for so long!
But Bdelloids have found their own a unique way of maintaining genetic diversity. Instead of having sex, they import foreign genes on a massive scale . They aren’t exactly picky in which genes they take up either. As far as Bdelloids are concerned, the entire tree of life is fair game. They have picked up genes from bacteria, plants and fungi. In just 1 percent of its genome researchers have already found dozens of genes of foreign origin. While other animals occasionally import a foreign gene, none can rival the genetic flexibility of Bdelloid Rotifers. Genes that normally maintain the bacterial cell wall or produce antibiotics and toxins in bacteria have all found a new home in the Rotifer genome. Whereas some older genes have already been retrofitted to be more like original Rotifer genes, newer ones still look like the bacterial and fungal originals. This means that Bdelloids have been stealing genes for a long time and show no sign of stopping any time soon.
The Bdelloid’s kleptomania can give them a big edge over other species. Bacteria are biochemically way more diverse than animals could ever hope to be, so stealing their genes sounds like a great strategy to increase your own metabolic capabilities. Why have sex when the combined evolutionary potential of billions of organisms lies at your feet?
The Bdelloid’s resistance to drought is linked to their exceptional skill in stealing genes. When they dry themselves out, they inevitably damage their own membranes and DNA. When they rehydrate, they have to repair their DNA to undo the damage. This period of repairs is the window of opportunity where foreign genes can get integrated in the genome. It is possible that the Bdelloids even exchange genetic material between themselves in this way, in a strange kind of pseudo-sex.
Becoming asexual is not easy. Genetic and developmental constraints prevent easy transitions to asexuality when sexuality has become established . Those species that do manage to bail out need other processes that generate enough genetic diversity to avoid extinction by genetic stasis. From salamanders, to rotifers, to humans…We all need a little bit of sex.
1. Engelstädter J (2008) Constraints on the evolution of asexual reproduction. BioEssays 30: 1138-50.
2. Lampert KP, Schartl M (2010) A little bit is better than nothing: the incomplete parthenogenesis of salamanders, frogs and fish. BMC biology 8: 78.
3. Bi K, Bogart JP (2010) Time and time again: unisexual salamanders (genus Ambystoma) are the oldest unisexual vertebrates. BMC evolutionary biology 10: 238.
4. Bogart JP, Bi K, Fu J, Noble DW, Niedzwiecki J (2007) Unisexual salamanders (genus Ambystoma) present a new reproductive mode for eukaryotes. Genome 50: 119-36.
5. Waggoner B, Poinar G (1993) Fossil habrotrochid rotifers in Dominican amber. Experientia 49: 354-357.
6. Baqai A, Guruswamy V, Liu J, Rizki G, Speer BR (2000) Introduction to the Rotifera. Available: http://www.ucmp.berkeley.edu/phyla/rotifera/rotifera.html.
7. Maynard Smith J (1986) Contemplating life without sex. Nature 324: 300-301.
8. Gladyshev Ea, Meselson M, Arkhipova IR (2008) Massive Horizontal Gene Transfer in Bdelloid Rotifers. Science 320: 1210-1213.
Photo Credits: The salamander picture comes from the third reference (Bi and Bogart 2010), kleptogenesis is from the second reference (Lampert and Schartl 2010). Bdelloid Rotifer picture courtesy of Umberto Salvagnin.
The views expressed are those of the author and are not necessarily those of Scientific American.
About the author
Lucas Brouwers is a recent college graduate who obtained his MSc degree in Molecular Mechanisms of Disease from Radboud University in Nijmegen, the Netherlands. Lucas blogs on evolution at thoughtomics and tweets as @lucasbrouwers. Besides writing about science, you’re likely to find Lucas listening to electronic music with his headphones on, or cycling through the Low Countries.