Researchers from the University of Oxford have investigated how so-called ‘jumping genes’ evolve in tiny animals that have apparently not had sex for millions of years. These pieces of selfish DNA normally jump around during the formation of sperm and eggs, but the new research shows that these genes are active even in asexual animals, which have evolved unusual genetic tools to keep them under control.
Bdelloid rotifers are microscopic animals that can be found living in a variety of freshwater habitats, such as ponds, puddles and the damp moss growing on trees. Bdelloids (pronounced with a silent B) have fascinated curious naturalists and scientists alike for centuries, because no one has ever found a male for any of the 500 or so species that have been described thus far. Since males are required for sexual reproduction, it means that they have apparently been evolving for millions of years as asexual females, where daughters are genetic clones of their mothers. Such ‘ancient asexuality’ is extremely unusual, because the vast majority of plants and animals need a bit of sex from time to time to ward off extinction. It is so unusual, in fact, that many researchers are convinced that bdelloids are not truly asexual – they must have some mechanism to swap their genes around.
Researchers from Oxford’s Department of Zoology, in collaboration with colleagues from the UK, Germany, Italy and the United States, have recently published 30 new genomes for bdelloid rotifers to explore one challenging puzzle around bdelloid asexuality – namely, how bdelloids cope with selfish genes known as transposable elements (TEs), or ‘jumping genes’. The study, led by Dr Reuben Nowell, Dr Chris Wilson and Professor Tim Barraclough at the Department of Zoology and recently published in the journal eLife, compared the genomes with more distantly related animals that are known to reproduce sexually.
TEs are selfish genetic elements that replicate themselves independently of the host genome, sometimes accumulating to vast numbers. For example, almost half of the human genome is made up of TEs, many of which are old and inactive. Typically, sex is important to allow TEs to maintain their selfish lifestyle, because it offers them a route out of the negative consequences of their actions.
‘In asexuals, however, the TEs are “stuck” in the same genome forever, and this can have a big effect on how they behave’, says Professor Barraclough. ‘While we already know a lot about how TEs should evolve in sexual species, there are only a few theoretical predictions about what we should find in a species that has not had sex for a very long time, and hardly any concrete data.’
By comparing and contrasting the patterns of TE evolution in bdelloids to other sexual animals, they hoped to put these predictions to the test and shine a light on the question of bdelloid asexuality.
The theory predicts one of two eventual outcomes: either the TEs are unable to change their selfish ways and they drive the asexual species to extinction, or they are able to reel in their negative effects to the point where they themselves may eventually be removed from the genome. In fact, the team found neither of these outcomes – bdelloids appeared to encode more or less as many TEs as their sexual relatives, suggesting that either the original theory was not quite correct or the assumption that they are ancient asexuals is wrong.
However, they also uncovered another genomic peculiarity that may provide a way out of the puzzle.
‘We showed that bdelloids have a really big expansion of a particular type of gene that is known to play a role in defending against the negative effects of TEs,’ says Dr Nowell. ‘It’s possible that these defence genes have them to remain asexual for so long.’
He adds: ‘We went into this project hoping that TEs might provide an answer to whether or not bdelloids are really ancient asexuals, but in the end our research may have asked more questions than it answered!’
Read the full paper here: https://doi.org/10.7554/eLife.63194