Ancient Asexuals

Ostracod

A second component of our research deals with the reproductive strategies of freshwater animals, especially ostracods. Sexual reproduction requires both males and females within a species and is very widespread in animals and plants. Up to 95% of all species have a sexual cycle at least every few generations. Many species, including humans, only reproduce sexually. But this constitutes an evolutionary paradox and one researcher has even called sex ‘the queen of problems in evolutionary biology’. Why?

Compare a sexual species (where mothers produce daughters and sons) to an asexual species (mothers produce only daughters). In the sexual species, only half of the offspring, the daughters, will themselves produce offspring. Males do not produce children themselves. In the asexual species, all offspring will contribute directly to the next generation. The population growth of the asexual species is thus exponential and much faster than in the sexual species. So why is sexual reproduction so successful in spite of this clear disadvantage?

Schön

Dozens of hypotheses have been put forward to explain this paradox. These either deal with the fact that asexuals have difficulty purging their genome from bad genes (this normally happens during a process of cell division called meiosis which asexuals do not possess) or with the fact that asexuals cannot form beneficial gene combinations as fast or as easily as sexuals can. In the first hypothesis, asexual clades will be short lived. In the second, asexuals are much less efficient at adapting to changing environmental conditions.

And this brings us to our second paradox: some animal groups have in fact managed to survive for millions of years, and this in spite of the absence of sexual reproduction. As these groups seemingly refuse to obey our evolutionary laws, some scientists refer to them as ‘ancient asexual scandals’. Only few animal groups claim this status: bdelloid rotifers, some groups of oribatiid mites, some groups of stick insects and darwinulid ostracods (Darwinulidae). There are about 2000 species of living, non-marine ostracods and only about 30 species belong to this family. Only the species Darwinula stevensoni is very common world-wide and can occur in a variety of habitats, most other darwinulid species are rare. And yes, the genus Darwinula is indeed named after Charles Darwin!

Our purpose is to test if these species have developed special genetic or ecological adaptations that make them less dependent on the evolutionary advantages of sexual reproduction. One of the hypotheses we are testing is the presence of more efficient DNA-repair systems in at least some darwinulid species.

But as good science requires, first we have to unequivocally demonstrate that darwinulids are indeed true ancient asexuals and do not have some kind of cryptic sexual reproduction. This has thus far been our main activity within this research programme, because our colleagues in this research field are highly sceptical. Through the analyses of fossils (ostracods have an excellent fossil record owing to their calcified valves) it has been demonstrated that the species Darwinula stevensoni has been around without sexual reproduction for at least 20 million years. We can also show that the darwinulids as a whole have survived without sexual reproduction for about 200 million years. Some darwinulids are older than that, but sexual reproduction might have occurred in some of those species.

We now think we have solved the riddle and can show how darwinulids became fully asexual a long time ago. But we’re not telling yet… we first have to publish these results!

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