More bathroom biodiversity: the Celibate Rotifer

A few days ago I posted some pictures of tiny animals called tardigrades that I found living in the wet mould growing on our bathroom window. 

They weren't the only wildlife that I found there; when I looked at two more pinhead-sized samples of the mould under the microscope theycontained several of these bdelloid rotifers. These are about a quarter of a millimeter long and are truly extraordinary animals.

Bdelloid rotifers consist of an elogated body containing complex internal organs. Normally they are anchored by their tail and when at rest they extend a pair of organs on their head that form the corona, each with a circle of cilia. The  cilia are anchored but because they beat rhythmically it appears as though they rotate - which is why the paired structures are sometimes known as wheel organs. 

The beating cilia create a vortex in the water that draws small food particles into the rotifer's mouth - click here to view a couple of movies which illustrate this far better than words can.

One peculiarity of bdelloid rotifers that intrigues biologists is that they never undergo sexual reproduction; they reproduce clonally and have done so for around 80 million years. 

It's generally assumed that sexual reproduction evolved because it generates offspring that are genetically different from their parents, which is beneficial in a constantly changing environment; somewhere amongst the genetically variable population there are always likely to be some that are better equipped than others to deal with new challenges, like changing climate or new diseases. 

Clonally reproducing organisms, like these rotifers, are all genetically identical so they should all be equally susceptible to change or disease and would be prone to extinction. So how have these rotifer clones managed to survive after 80 million years of celibacy?

The answer seems to be that they can incorporate DNA from their food - which consists of fungi, algae and bacteria - into their own DNA, so generating genetic variation in their population. Like tardigrades they are very drought-resistant and when their surroundings dry out they form a tough, durable cyst. 

When the cysts hatch again and rehydrate breaks develop in their DNA molecules that would normally be fatal, but DNA from their partially-digested food is used to repair these, so about eight per cent of rotifer DNA actually comes from fungi, bacteria and algae. It gives new meaning to the old  saying "you are what you eat".

You can see partially-digested food (most likely mould from our bathroom window) inside this rotifer's gut.

Rotifers like this are ubiquitous. Their durable eggs are transported by water and wind and they hatch anywhere that is moist. Mosses are habitats where you can find them in very  large numbers. If you keep cut flowers in a vase you can almost guarantee that there will be a large population of rotifers living on the submerged stems within a week. 

This is a higher magnification image that shows the corona, where you can just make out those beating hairs ....

 ..... and these are the jaws (the mastax) in the neck of the animal. They chew constantly. Here they are open ....

.... and here they are closed.

There's one other amazing property of rotifers that sets them apart - they are more resistant to radiation than any other animal. Tests have shown that they can survive doses of radiation that are 100 times greater than would kill a human, which means that when there are nuclear reactor accidents these will be last animals to succumb. This again is probably due to the unusual ease with which they can repair breaks in their DNA.

It's amazing what you can find in your bathroom. I wonder what's lurking behind the sink................


For pictures and information about some more rotifers, click here.


Sources:

Gladyshev, E.A., Meselson, M., Arkhipova, I.R. (2008). Massive Horizontal Gene Transfer in Bdelloid Rotifers. Science, 320, 5880

Gladyshev, E., Meselson, M. (2008). Extreme resistance of bdelloid rotifers to ionizing radiation. Proceedings of the National Academy of Sciences 105 (13), 5139-5144

Jean-Francois Flot et al (2013) Genomic evidence for ameiotic evolution in the bdelloid rotifer Adineta vaga. Nature 500, 453–457