How did a group of tiny organisms develop into hundreds of subspecies without sexual reproduction?

A study published this week in the journal Nature may demystify the proliferative and enduring nature of the bdelloid rotifer — a group of water dwelling animals, whose members appear to get by perfectly well without ever having sex. ScienceDaily reports that by sequencing the rotifer genome, the research team may have taken a step towards a more sophisticated understanding of the animal's curious evolutionary strategies.

"Rather than the standard way of using sexual reproduction to weed out harmful mutations to its DNA, this tiny aquatic animal appears to have adopted other strategies to maintain lineages over millennia that aren't burdened by genetic damage or killed off altogether," David Mark Welch, scientist at the Marine Biological Laboratory, told reporters.

While a wide variety of organisms are capable of spawning offspring by themselves, most of them still rely on sexual reproduction to some extent. The land snail, for example, is hermaphroditic, meaning that the animal possesses reproductive organs of both sexes. In that sense, the snail plays by the same evolutionary rules as mammals, as the biological minutia of their reproductive act is essentially the same. Whether it's with themselves or someone else, these animals still have sex.

From an evolutionary perspective, sexual reproduction is extremely beneficial, as it allows an organism to circumvent genetic dead ends, avoid pernicious mutations, and ultimately tailor their genome to thrive over millennia.

The rotifer, lacking the reproductive features generally associated with gender, is able to maintain its lineage by dividing unfertilized eggs. Where the DNA of other animals is informed by two gametes — known more commonly as sperm and egg cells — that combine during meiosis, rotifer's is largely the product of a single "parent."

"[The genome] is completely consistent with what you would expect to see with a long-term absence of meiosis," Mark Welch says. "It's hard to prove a negative, and we can never say there is no chance the rotifer is ever having sex. But it would have to be some kind of crazy meiosis."

Welch and his team found that besides a series of genetic "repair mechanisms" working to prevent deleterious mutations, the rotifer has the ability to "dry up" for prolonged periods of time until water once again becomes available.

"When they rehydrate, this might be an opportunity for foreign DNA fragments from ingested bacteria, fungi, or microalgae to transfer into the rotifer genome," says Irina Arkhipova, a researcher who worked on the project.

The rehydration process may also allow the rotifer to acquire genetic material from its own peers. "In this way, the processes of mutation and DNA repair mimic some aspects of sex," Welch says.

In light of the new findings, the researchers are eager to continue their exploration of the processes whereby the rotifer protects and improves its genome without sexual reproduction — the strategy long thought to be the most viable.

Source: Flot JF, et al. Genomic evidence for ameiotic evolution in the bdelloid rotifer Adineta vaga. Nature. 2013.