Scientists have long wondered as to how bdelloid rotifers are able to escape extinction as they do not have sexual intercourse to reproduce. Now a group of researchers has theorized that these animals have a mechanism that repairs harmful DNA mutations effectively and this may be the cause for the microscopic animals surviving extinction.
"We think, in the bdelloid rotifer, genomic changes together with environmental changes have conspired to create something that is able to exist in the absence of sex," says Mark Welch, an assistant scientist in the MBL's Josephine Bay Paul Center.
The suggestions appearing in two related papers, published this week in Proceedings of the National Academy of Sciences (PNAS), have medical implications because DNA repair capacity is an important factor in cancer, inflammation, aging, and other human conditions.
In animals that do have sex, DNA repair is accomplished during meiosis, when chromosomes pair up, one from the father and one from the mother, and "fit" genes on one chromosome can serve as templates to repair damaged genes on the other chromosome. The bdelloid, though, always seems to reproduce asexually, by making a clone of itself.
In the first study, MBL adjunct scientist Matthew Meselson and Eugene Gladyshev, both of Harvard University, demonstrate the enormous DNA repair capacity of bdelloid rotifers by zapping them with ionising radiation (gamma rays), which has the effect of shattering its DNA into many pieces.
"We kept exposing them to more and more radiation, and they didn't die and they didn't die and they didn't die," says Mark Welch.
The researchers observed that even at five times the levels of radiation that all other animals are known to endure, the bdelloids were able to continue reproducing.
"Because there is no source of such intense ionizing radiation on Earth, except if we make it, there is no way these organisms could have evolved to be radiation resistant," says Mark Welch.
The researchers, instead, propose that bdelloids' DNA repair capacity evolved due to a different environmental adaptation, tolerance of extreme dryness. They showed that desiccation (drying out), like ionising radiation, breaks up the rotifers' DNA into many pieces.
"That's the next thing we are looking at. How are the bdelloids able to repair this many double-stranded breaks in their DNA? Do they have better enzymes, more enzymes?" Mark Welch said.
In their second paper, the researchers give evidence that the bdelloid rotifer originally had two copies of each chromosome just like most animals, but at some point in its evolution, it underwent a "whole-genome duplication", giving it four copies of each chromosome and hence of each gene.
While lineages that undergo whole-genome duplication generally lose the duplicate genes over time, the bdelloid has kept most of its duplicate genes throughout its evolutionary history, say the researchers.
"We believe they have kept most of their duplicate genes because they are serving as templates for DNA repair," says Mark Welch.
One possible result of DNA repair is gene conversion, in which the gene being repaired ends up having an identical DNA sequence to the gene repairing it. This can introduce the kinds of changes into the gene pool that sex usually does.
"We think that gene conversion resulting from DNA repair resulting from adaptation to (desiccation in) its environment may provide enough of the advantages of sex that bdelloids can survive," Mark Welch says.