Bonnie Tyler didn’t realise how aptly her song ‘Holding out for a Hero’ described the lives of the Bdelloid Rotifers – in all three hundred years that we have been searching, we’ve never found a male, just millions and millions of females.
Well, there was one single report, about 100 years ago, scantily documented, that a scientist from Denmark saw one moving “round and between the females with extreme rapiditity”, but even he admitted that he wasn’t really sure. Perhaps he just didn’t like the idea that males might be a futile waste of resources.
Perhaps he just didn’t like the idea that males might be a futile waste of resources.
So Bdelloid Rotifers are all ‘female’, and therefore asexual. Why should that be surprising? We know of thousands of species of animals, fungi, plants, and bacteria that are sexual (although it can be argued that bacteria are sexual, they certainly don’t have sex in the reproductive sense that we do). The reason that the bdelloid rotifers have caused such an ‘evolutionary scandal’ by being asexual is that they don’t represent just one species, but over 450 different ones. Bdelloid rotifers all belong to the Bdelloidea class of the phylum Rotifera (hence their name, Bdelloid Rotifers) so they are all closely related on the evolutionary tree of life, but are different enough to be called separate species. Each of these is unique and identifiable – or at least they’re identifiable by the few Bdelloid rotifer taxonomy experts that exist, if they have a microscope handy.
The ‘scandal’ therefore is that at least 40,000 years ago (the age of the oldest fossilised rotifer that we’ve found so far), an ancestor of today’s rotifers gave up her need for a mate, and reproduced asexually by herself. We know that their ancestors were sexually reproducing because almost all other eukaryotes (you, me, other animals, protists, plants, and fungi) are sexual, and so it makes sense that our shared ancestors were too. If sex is evolutionary advantageous (which is surely is, given that it has been maintained in almost every other eukaryotic species), then how do the Bdelloid Rotifers manage to survive when all they have is asexual reproduction?
Asexual reproduction (cloning) seems a terrible plan for an organism trying to pass on its genes successfully. Imagine if I cloned myself, and my clone cloned herself, and so on for generations and generations. My short sightedness would be passed on unchanged in every clone, except for the tiny chance that one of the clones would have a lucky mutation that made her have perfect vision – it’s actually far more likely that one of the clones will have worse vision, or no vision at all, than one of them has improved sight. Contrast this with the real human mode of reproduction – there is a reasonable chance that the faulty gene that caused short-sightedness in the mother might not do so in the child, because the father contributes his own, non-faulty gene.
But in other ways, asexual reproduction seems to make very good sense. In sexual reproduction, a mother would only be able to pass on half of her genes to her offspring per pregnancy (effectively making ½ a clone of herself) whilst in asexual reproduction she would pass on them all (making 1 whole clone of herself). Natural selection is just organisms trying to pass on as many copies of their genes as possible to the next generation, so really asexuality makes the most sense. The only way for sex to be advantageous is if the benefits of having two parents mean that the offspring is more than twice as likely to survive as they would be with only one. And this is exactly why some species – such as ourselves – are sexually reproducing.
But what about the Rotifers?
But what about the Rotifers? We can only assume that for them, it is not worth the two-fold cost of sexually reproducing, as having two parents does not double the likelihood of survival of their offspring. So that one female Bdelloid Rotifer who had a unique genetic mutation that made her able to reproduce without needing a father for her children started a family dynasty of clones of herself, which managed to diverge into different species relying on only small genetic mutations for their evolution.
Scientists have managed to prove via genetic evidence that male Bdelloid Rotifers are not hiding, or very very small (as has turned out to be the case for several species with ‘missing’ males such as the Angler fish), but are completely unnecessary, wasteful, and gone.