Theft: Better Than Sex (Bdelloid Rotifers)

Common Name: Bdelloid Rotifers

A.K.A.: Families of Order Bdelloida

Vital Stats:

• Around 360 asexual species
• All species likely descended from the same ancestor
• Common ancestor lived 50-100 million years ago

Found: Fresh water bodies of any size, on every continent, including Antarctica

It Does What?!

Here’s a creature that truly exhibits questionable evolution- as in, the kind that tends to make you go extinct in a hurry. Bdelloid rotifers (the ‘B’ is silent) are microscopic animals found in all kinds of moist, freshwater habitats- puddles, ponds, mossy areas; you name it, they’re probably there. What’s so unusual about these guys is that they’re entirely asexual, and have been for a very, very long time. In fact, bdelloid rotifers are all female, a consequence of how they reproduce.

Now, asexual reproduction isn’t so uncommon. If you look at a field of dandelions, chances are, they’re all clones derived from asexual reproduction in a single common ancestor- no second parent needed. Even such advanced creatures as komodo dragons do this periodically- a baby dragon is formed from an unfertilized egg inside the mother. What differentiates bdelloid rotifers from other asexual reproducers is that it’s all they’ve done for the last 50 million years or more. Outside of our friends the rotifers, a species must either have sex from time to time, or face extinction.

Why? Because sex solves two major problems in life (your individual results may vary..). First, it weeds out errors which tend to accumulate in DNA over time. Unlike asexuals, which pass on a copy of a copy of a copy (etc.) of their genes, sperm and egg cells contain DNA which has been mixed and matched via a process called meiosis. The gist of this is that an organism can procreate without necessarily passing on any genetic errors it may have to the next generation. Second, this same process of mixing and matching creates new combinations of DNA sequences, which in turn create the natural variation between individuals that evolution can select for or against.

For example, a genetic combination which caused a polar bear to be born with a white nose would be selected for, since it would make a more effective camouflage for hunting. On the other hand, a combination which gave polar bears big black patches on their fur would be selected against, because they’d have a harder time hunting and would therefore starve more often. Asexuals, however, can neither quickly generate useful new combinations, nor purge their populations of harmful mutations.

So on the surface, it comes as a surprise to biologists that bdelloid rotifers have been able to survive for such an epic amount of time with no sex (in addition to the absence of males, genetic tests are able to show that meiosis hasn’t occurred). However, the rotifers have two impressive ways of dealing with this. First, when times get tough, they already have a pretty good defence mechanism worked out- they just dry up. The rotifer dehydrates itself and forms a dormant cyst in which it can remain in this state until conditions improve. This is called anhydrobiosis.

Second, and more importantly, they steal genes. This is the true secret to the successful asexual lifestyle. When a rotifer emerges from dormancy and needs to patch itself up, it’s actually able to incorporate random genetic material from its environment into its own genome. A nearby bacterium, some fungus, a passing bit of rotting leaf? All fair game, apparently. Researchers have found genes from each of these three groups in the rotifer genome. Incorporating these new bits of sequence seems to give rotifers the variation they need to develop new traits and stay off the evolutionary chopping block. In fact, given the success of the bdelloid rotifers – they’ve evolved into over 300 species since giving up sex – and the ease of asexual procreation – no need to find a partner – an argument could be made that when it comes to new genes, theft really is better than sex.

Says Who?

• Gladyshev et al. (2008) Science 320(5880): 1210-1213
• Harvard Magazine, Nov.-Dec. 2000 “An Evolutionary Scandal”
• Welch & Meselson (2000) Science 288(5469): 1211-1215
• Wilson & Sherman (2010) Science 327(5965): 574-576