It's hard to say what is more curious about the bdelloid rotifer: the ability of these microscopic invertebrates--which look like tiny, transparent leeches--to survive for years without water, or the fact that the 450 species that make up this class of freshwater animals have survived for tens of millions of years without sex. A new study links the two curiosities by suggesting that rotifers developed a talent for living without water in order to avoid lethal parasites. And this defense in turn may have allowed the organisms to dodge an evolutionary bullet called the Red Queen Hypothesis and remain asexual for eons.
The romantic poets believed that sex and death were inextricably intertwined; but, from an evolutionary perspective, a species is far more likely to be driven extinct by asexuality. Numerous explanations for this have been proposed, but one of the most popular is the Red Queen Hypothesis, named for a character in Alice's Adventures in Wonderland who has to permanently run just to stay in place. The hypothesis holds that asexual species "run slowly"--that is, they evolve less rapidly because favorable genes do not spread throughout the population. That makes these species more vulnerable to sexually reproducing pathogens, which can evolve offensive weapons faster than asexual creatures can evolve an effective defense. Indeed, individual rotifer colonies are often exterminated by a vicious fungus, called Rotiferophthora angustispora, that eats them from the inside out.
But rotifers seem to have found a way to beat the system. Curious whether the organism's desiccation strategy helps them avoid annihilation, evolutionary biologists Christopher Wilson and Paul Sherman of Cornell University created seven groups of genetically identical rotifers, totalling 95 colonies, in separate petri dishes. They then inoculated all but one group with R. angustispora. The duo allowed five of the six inoculated groups to dry out and maintained them in this condition for between 7 and 35 days before rehydrating them.
The longer the rotifers stayed dry, the better they did. The fungus wiped out all colonies dehydrated for 14 days or less, but 60% of those dehydrated for 21 days remained completely free from infection. The percentage was higher still when the rotifers were dehydrated for longer.
Wind also helped. Wilson and Sherman created a heavily infected population of rotifers, dried it out, and placed it in a wind tunnel. They then placed 24 clean dishes downwind of the infected population. Rotifer colonies grew in 17 of these dishes, ostensibly blown by the wind from the infected dish, and 10 of these remained fungus free. In the wild, wind may carry uninfected rotifers from infected populations to new habitats that their parasites have not yet colonized, the researchers speculate in tomorrow's issue of Science.
Evolutionary biologist Mark Pagel of the University of Reading in the United Kingdom is impressed. "Rotifers are a real enigma, and ... these researchers are offering a fascinating suggestion as to why they've been able to evade one of the things that should have knocked them off," he says. Wilson says researchers should next explore whether other asexual species have evolved creative ways to avoid an evolutionary death spiral.