Creature that has not had sex for 100m years

A tiny creature that has not had sex for 100 million years has overturned the theory that animals need to mate to create variety.

Analysis of the jaw shapes of bdelloid rotifers, combined with genetic data, revealed that the animals have diversified under pressure of natural selection.

Researchers say that their study “refutes the idea that sex is necessary for diversification into evolutionary species”.

The microscopic animals, less than four times the length of a human sperm, are all female, yet have evolved into different species that fill different ecological niches. Two sister species were found to be living together on the body of a water louse. One of them specialised in living around the louse’s legs and the other stayed close to the chest.

Genetic analysis showed that the two creatures were distinct, a fact backed up by observations that each type had differently shaped jaws.

Asexual animals and plants usually die out quickly in evolutionary terms but the ability of bdelloid rotifers to diversify may explain why they have survived so long.

A specimen trapped in amber has shown that the animals were living at least 40 million years ago and DNA studies have suggested they have been around for 100 million years. Modern Man has notched up about 160,000 years.

It had previously been recognised that asexual animals and plants can evolve through mutations into another species, but only into one species and at the cost of its original form. Bdelloid rotifers have displayed the ability to evolve into many different forms.

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Introduction to the Rotifera
Rotifers : the "wheel animalcules"

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Rotifers are microscopic aquatic animals of the phylum Rotifera. Rotifers can be found in many freshwater environments and in moist soil, where they inhabit the thin films of water that are formed around soil particles. The habitat of rotifers may include still water environments, such as lake bottoms, as well as flowing water environments, such as rivers or streams. Rotifers are also commonly found on mosses and lichens growing on tree trunks and rocks, in rain gutters and puddles, in soil or leaf litter, on mushrooms growing near dead trees, in tanks of sewage treatment plants, and even on freshwater crustaceans and aquatic insect larvae. (Örstan, 1999)

Because of their very small size and mostly soft bodies, rotifers are not commonly favored for fossilization. Their only hard parts, their jaws, might be preserved in the fossil record, but their tiny size makes detection a serious challenge (Örstan, 1999). However, fossils of the species Habrotrocha angusticollis have been found in 6000 year old Pleistocene peat deposits of Ontario, Canada (Warner et al., 1988). The oldest reported fossil rotifers have been found in Dominican amber dating to the Eocene (Waggoner & Poinar, 1993).

Rotifers : The rotifers are microscopic animals, and under high magnification will look something like the picture at upper left, for most perople using a light microscope. Those with more sophisticated microscopes and lighting techniques can give rotifers such as Philodina, grazing at lower left, a beautiful glow. But not all rotifers swim freely; some like the Flosculariacean rotifer above at center, will cement themselves by their foot to a handy alga or bit of dirt and sift the water for food. At right, Collotheca is another monogonont rotifer, shown here bearing an egg on its stalk end. Notice the extemely long coronal cilia this rotifer uses to catch food. (Click on any of the pictures above for a larger image).

Rotifers are multicellular animals with body cavities that are partially lined by mesoderm. These organisms have specialized organ systems and a complete digestive tract that includes both a mouth and anus. Since these characteristics are all uniquely animal characteristics, rotifers are recognized as animals, even though they are microscopic. Most species of rotifers are about 200 to 500 micrometers long. However a few species, such as Rotaria neptunia may be longer than a millimeter (Orstan 1999). Rotifers are thus multicellular creatures who make make their living at the scale of unicellular protists.

The Rotifer Jaw

The basic constituents of the jaw are one fulcrum, and a pair each of unci, rami and manubria. There are several jaw types: malleate, incudate, virgate, uncinate, ramate and malleoramate. The type of jaw observed forms the basis for division into families.

Key:
F fulcrum; M manubrium; R ramus; U uncus.

The easiest jaw type to recognize, and shown here, is the malleate type, which is characteristic of the Brachionidae. All parts of the jaw are well developed and strong. The fulcrum is short and the rami broad. The unci have ridges across the surface terminating in teeth at the inner edge. The action of the jaw is to cut and chew, between the teeth, particles collected.

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