Staying celibate can be a difficult task, but bdelloid rotifers have managed to survive without sex for nearly 50 million years.
Scientists now think they have cracked the secret to these microscopic animals’ success: recombining their own genes in new ways and stealing genes from other organisms living nearby, thus keeping genetic diversity alive and well—even without the DNA from a mate.Bdelloid rotifers are supertough little creatures. Photograph by M. Shribak and I. Arkhipova, Marine Biological Laboratory.
“This animal has lost its sexuality,” said study co-author Olivier Jaillon of Genoscope, part of the Institut de Génomique du CEA in France.
Jaillon said the results of the study gave him one of the very rare moments in a career when you feel you’ve really “found something.”
Bdelloid rotifers are microscopic, multicellular animals that look and move a lot like leeches (“bdelloid” is from the Greek for “leech”). They generally live in freshwater, moist soil, and other damp environments. And these unassuming animals have some pretty cool superpowers: They can withstand long periods of being dried out, as well as massive doses of radiation that would kill pretty much every other living thing.
Despite these traits, bdelloid rotifers are mainly known in the research world for their 40-million-year-long dry spell in the sack. Although biologists long suspected that these microscopic animals never reproduced sexually—they generally reproduce via an asexual method known as parthenogenesis, in which the offspring is the clone of the parent—the assertion remained controversial for several reasons.
One of the key purposes of sexual reproduction is to provide an ongoing source of genetic diversity and not let harmful mutations accumulate. Since these rotifers were such an evolutionary success, scientists found it difficult to believe the animals weren’t reproducing sexually. (Get a genetics overview.)
Although sexual reproduction is extremely beneficial, it’s not without costs: Time spent finding a mate is time not spent finding food or hiding from predators. There’s also no guarantee that the offspring will be as well adapted to the environment as the parents.
Yet just because sexual reproduction in bdelloid rotifers had never been observed didn’t mean it never happened.
Dr. Ruth of Rotifers
To settle this issue, Jaillon, along with Jean-François Flot and Karine Van Doninck, at the University of Namur in Belgium, and colleagues focused their efforts on one particular species of bdelloid rotifer, Adineta vaga. This species is easy to raise in the lab, and previous work had indicated that it had one of the smallest genomes of any of the bdelloid rotifers, which would make it easier for the scientists to sequence.
The sequencing results weren’t quite what the researchers had anticipated. “We were all surprised by the genome structure, as nothing like this had been observed before,” said Flot, whose study appeared this week in the journal Nature.
The genome of A. vaga had an unusual array of characteristics that, together, made it basically impossible for the rotifer to reproduce sexually. (Also see “Wild Romance: Weird Animal Courtship and Mating Rituals.”)
A. vaga has modifications that made its chromosomes—or DNA molecules—nonidentical, which is also unusual in the animal kingdom. This meant that A. vaga‘s sex cells couldn’t complete a key phase of meiosis—or cell division—known as crossing over, in which each chromosome lines up next to its partner and they swap portions of DNA. The genes present on each chromosome have been shuffled across the genome, which means the rotifers’ chromosomes aren’t alike enough to line up for the crossing over.
In fact, Van Donink pointed out, the chromosomes of A. vaga had many of the same genetic characteristics of the human Y chromosome, which also does not undergo crossing over. This similarity helps confirm that this step does not happen and these bdelloid rotifers are not capable of sexual reproduction.
Mission to Mars?
Although A. vaga can’t use crossing over to get rid of harmful mutations, it does use a similar method to shuffle its genes. And, like all bdelloids, A. vaga is a notorious gene thief. Its genome sequence revealed that 8 percent of its genes had come from non-animals like prokaryotes and fungi. This, along with its ability to shuffle genes, will likely keep the rotifer alive and well for at least another 40 million years.
“Because of these incredible survival abilities and the fact that, being asexual, a single individual can start a whole population, I wouldn’t be surprised if bdelloid rotifers were able to survive space travel and colonize other planets such as Mars,” Flot said.
We will leave it up to NASA to find humans with these same capabilities to travel alongside rotifers to Mars.