Sex has long been an intriguing topic for biologists. Traditional sexual reproduction requires a large amount of energy and even more energy when we consider the time to find a right partner. That being said, despite all the energy investment only 50% of your genetic material goes to the next generation. Not only that, chromosome recombination makes sexual reproduction not ideal in terms of conserving your genetic information. Furthermore, asexual reproduction is faster, capable of generating more offspring and does not require shared paternity for the next generation.
So with all these disadvantages, why would sexual reproduction evolve? The short answer is it provides diversity and a way to purge disadvantageous mutations from the genome, making offspring more likely to thrive in a changing environment.
Scientists have long believed that lack of sexual reproduction was an evolutionary dead- end, due to the fact asexual organisms would have no way of getting rid of these mutations. However, a group of microscopic animals known as bdelloid rotifers, which comprise of several different species, don’t have any records of males, hermaphrodites and not even meiosis; the process which forms sperm and egg cells.
Microscopic photograph of bdelloid rotifer Adineta vaga. Credit: Michael Plewka
This discovery has led scientists Jean-François Flot et al., in a recent publication in Nature, to analyse the genome of such animals in order to find out if they were indeed a long lineage of asexuals.
The answer is yes.
For meiosis to occur, chromosomes have to have pairs. Humans, for instance, have 23 double-copy chromosomes, a condition we call diploidy. On the other hand, the bdelloid rotifers genome does not present chromosome pairs. In fact, analyses have shown that their ancestral genome was tetraploid (x four-copy chromosomes) and their chromosomes have merged into fewer, but larger, chromosomes with several similarity regions across them called collinear regions. Therefore, their genome does not comport meiosis.
This is strong evidence that they have evolved asexually for thousands of years.
And how have they not become extinct, you may ask? Well, there are two important processes which they have evolved bypass the problems of being asexual. The first is Gene Conversion, where a homologous DNA sequence replaces the other, resulting in two identical sequences; this allows them to deal with mutations that may arise. The other process is called Horizontal Gene Transfer (HGT), where a gene is transferred between organisms without traditional reproduction (vertical transfer, from generation to generation) occurring; this gives them genetic diversity. Though not abundant in animals, HGT is responsible for about 8% of their genome, a percentage similar to that in bacteria.
All in all, this research has shown once again that evolution does not follow traditional paths. As humans, we have a tendency to think sexual reproduction is better. This is not true as these rotifers have shown us that under the right conditions asexual reproduction can be just as advantageous as its counterpart.
Flot, J. F. et al. Genomic evidence for ameiotic evolution in the bdelloid rotifer Adineta vaga. Nature 500 (2013): 453-457, doi:10.1038/nature12326.
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