Birds do it, a few bees get to do it 鈥� but how did sexual reproduction evolve in the first place? An evolutionary model suggests that it could have started as a way for two cells to pool their resources when favourable environments turn harsh.
鈥淭he idea is when times get hard, you fuse with another cell,鈥� says at the University of York, UK. 鈥淭hen you鈥檝e got this big cell which has more chance of surviving.鈥�
The evolution of sexual reproduction is a long-standing mystery. Simple cells like bacteria swap bits of DNA in a number of ways, but they don鈥檛 engage in sexual reproduction in the same way as complex cells, where two cells fuse with each other and intermingle their entire genomes.
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While sexual reproduction has a number of disadvantages compared with asexual reproduction 鈥� such as having to find a mate 鈥� in the long run, species that can mix their genomes have a major advantage: some offspring end up with more beneficial mutations than either parent alone, while others get landed with more harmful mutations and are eliminated. As a result, very few organisms have given up on sexual reproduction altogether.
However, the advantages of sexual reproduction don鈥檛 explain how it came to evolve in the first place, says Constable. A sticking point is that sexual reproduction requires cell fusion, but why would cells fuse before they evolved the ability to combine their genomes?
To get some insight, his team looked at single-celled complex cells, the first organisms to evolve sexual reproduction. In fact, it鈥檚 clear that sexual reproduction evolved very soon after the first complex cells appeared, and perhaps even at the same time.
For these cells,聽sexual reproduction is usually optional and happens when the environment is no longer favourable, says Constable. Cells then often form a dormant stage, such as a spore.
What鈥檚 more, while sexual reproduction in animals involves minuscule sperm fusing with much larger eggs, in single-celled organisms it involves two cells of similar sizes. This means the fusion of the cells creates a larger cell.
This led evolutionary biologist to suggest that these larger cells created by fusion would be more likely to survive during tough times because of their larger food reserves. 鈥淭he mathematical model supports this,鈥� says Constable.
In his team鈥檚 model, small cells thriving in a benign environment are switched to a harsh environment, where larger cells have a survival advantage. The cells can evolve fusion, but it is assumed that they aren鈥檛 very good at it at first. Modern cells have lots of highly evolved machinery for fusion, but this wouldn鈥檛 have existed early on.
According to the study, which has yet to be peer-reviewed, the evolution of cell fusion is favoured even if up to 86 per cent of the cells that try to fuse die. 鈥淐ell fusion can still evolve even with a remarkably high cost,鈥� says Constable.
鈥淢aybe he鈥檚 correct,鈥� says at University College London. 鈥淏ut I鈥檓 pretty sceptical that this is the fundamental reason behind cell fusion.鈥�
If fusion is such an advantage, asks Pomiankowski, why don鈥檛 simple cells do it? Constable, however, points out that simple cells have thick cell walls, so for them, the costs of fusion may outweigh the benefits.
There are also reasons to think that there are limits to the size of simple cells that only complex cells have managed to overcome.
We may never get definitive answers on the origin of sexual reproduction as it is very difficult to study. 鈥淚 think we just don鈥檛 know,鈥� says Pomiankowski.
bioRxiv
