And maybe they are not all female.
A new study has cast doubt on leading theory for how tiny creatures have evolved for tens of millions of years — without ever having sex.
they are all female, and their offspring are clones of their mothers. Bdelloids are microscopic animals that live in freshwater and damp habitats across the world. Despite their apparent lack of sex, we know they have evolved for tens of millions of years into more than 500 species.
By studying their genomes — the set of all the genes that define an animal’s characteristics — researchers thought they had identified an explanation for how bdelloids had ‘gotten away’ with no sex for millions of years.
However, a new study, published today in PLOS Biology and led by Imperial College London researchers, reveals this mechanism may not be the main explanation for the bdelloids’ success.
Many species of bdelloid endure periods of drying out, called desiccation. Although they survive desiccation, the process damages their DNA, which they need to repair when dehydrated.
Based on a previous study of the genome of a species that survives desiccation, researchers had proposed that the repair of DNA might remove some of the problems of being asexual, for example by removing harmful mutations and possibly allowing occasional recombination of genes to occur.
This theory made key predictions about what the genomes of the small number of bdelloid species that cannot survive desiccation should look like. The new study looked at the genomes of three further species, including some that do not undergo desiccation.
The researchers found that the predicted differences between species that can and cannot survive desiccation were not observed. This suggests that DNA repair following desiccation may not be as important as previously thought, and that other factors may need to be considered to explain bdelloid evolution.
Finally, although no males have ever been found, this new study of rotifers’ genomes suggests scientists can’t rule out sex as firmly as previously thought. Previous evidence had suggested that the structure of the bdelloid genome made conventional sex impossible.
Sexual animals have two copies of each gene arranged along matching chromosomes, one from the father and one from the mother. The first bdelloid genome sequenced revealed that the two copies of genes were often on the same chromosome, which is incompatible with their inheritance on chromosomes from a father and a mother. The new study, however, did not find these same patterns in the three new genomes.
Lead author Dr Reuben Nowell said: “We haven’t proved they are having sex, it’s just that we can’t prove that they aren’t based on the current genome results. Other species of rotifers have identifiable males, but no bdelloid rotifer males have ever been found. Paper. open access – Reuben W. Nowell, Pedro Almeida, Christopher G. Wilson, Thomas P. Smith, Diego Fontaneto, Alastair Crisp, Gos Micklem, Alan Tunnacliffe, Chiara Boschetti, Timothy G. Barraclough. Comparative genomics of bdelloid rotifers: Insights from desiccating and nondesiccating species. PLOS Biology, 2018; 16 (4): e2004830 DOI: 10.1371/journal.pbio.2004830
The bad news is that the simple explanation probably isn’t the answer. The good news is that we found out that it probably isn’t the answer. We could have been content to make up a Darwinian just-so story and insert it into the textbooks for memorization.
See also: Bdelloid rotifer uses horizontal gene transfer (HGT), dispenses with sex
Top life sciences micro photo: The rotifer, filtering water for food, like a vacuum cleaner