Are there animals that are asexual?

Asexuals crowd out the sexual

The Limnology Institute in Mondsee is investigating how the switch to asexual reproduction happens.

It is called the paradox of sexual reproduction, ”says Claus-Peter Stelzer from the OeAW Institute for Limnology in Mondsee, when one asks why most multicellular organisms reproduce sexually, even though they are expensive. When asked what sex costs, biologists have no slippery thoughts, but rather count the energy expenditure associated with sexual reproduction. First: the production of males. If the mother invested only in females, who in turn produce females through “virgin generation” (parthenogenesis), the number of offspring would double generation after generation. So why produce males who, in most animal species, do not even help raise the young? Other examples of the cost of sexual reproduction are the production of germ cells and the time and energy invested in finding and choosing a sexual partner.

Nevertheless, most animal species do not let it go: only 0.1 percent of vertebrates reproduce asexually. Evolutionary biologists seek the explanation for the paradox in the renewal of genetic material during sexual reproduction. The genes are mixed twice during the formation of egg and sperm cells (meiosis) and when they are combined. This increases the variation, an important advantage for responding to changing environmental conditions and pathogens. In addition, by recombining the genetic material, harmful genes can be replaced by “good” material from the other parent.

To solve the paradox, experiments on the evolution of sexual behavior are necessary. But these fail in many animal species because a researcher's life is not enough to observe numerous generations. "This is why the rotifers are a great model organism: they only take two days to generate," says Stelzer. The rotifers are smaller than some unicellular organisms (only 0.2 to 0.3 millimeters) and yet they are real multicellular organisms with a specialized organ system that are common in freshwater. You can find them in almost every garden pond, where they serve as food for insects and fish larvae living in the water. “Animals that regularly produce asexual offspring are suitable for clarifying the paradox of sexual reproduction. The Brachionus calyciflorus used here can switch to purely asexual reproduction within a few weeks, ”says Stelzer. In addition, rotifers can easily be kept by the thousands in the laboratory, so evolutionary changes in populations are easy to understand. “Normally the animals have a mixed reproduction and can reproduce asexually and sexually. Sexual reproduction only asserts itself in high population densities. ”This works via a chemical messenger substance that stimulates the animals in the vicinity to produce sexual offspring. “The denser the population, the easier it is for the males to find a female,” explains Stelzer.

Dwarfish little men

The rotifers produce asexual offspring in such a way that the dams form diploid egg cells without meiosis (with a double set of chromosomes), from which genetic clones of themselves grow. It is only through the chemical messenger substance that meiosis takes place during the formation of the germ cells. This creates haploid egg cells, and if these are not fertilized, a haploid male grows out of them. The same system is known from bees and other insects that produce their males via parthenogenesis. In rotifers, the males are dwarfish, even shorter-lived than the females and consist almost entirely of sexual organs. If the haploid egg cell is fertilized by a male, however, a diploid egg grows from it, which, as a thick-walled permanent stage, can survive for months.

“In the laboratory it happens again and again that the rotifers no longer react to the chemical messenger and only reproduce asexually. In a short time, all of the offspring are females, ”reports Stelzer. The laboratory results of the research group from Mondsee show what has not yet been observed in the field: The fast-growing asexual animals displace the sexual ones. So the paradox of sexual reproduction is confirmed. It is now possible to research which factors are responsible for the fact that asexuals do not get out of hand in nature: Do asexuals accumulate harmful mutations in their genetic material? Is the complexity of the outdoor habitat crucial for sexual generations? Which genetic mechanisms cause switching between the types of reproduction? This is to be researched in Stelzer's recently started FWF project.

("Die Presse", print edition, 02/20/2008)