Sexual reproduction requires significantly higher energy costs than asexual reproduction. In addition, populations that reproduce asexually increase their numbers faster: they do not need to look for partners, and coadapted gene complexes are not destroyed during recombination. Despite this, sexual reproduction is the most common in eukaryotes.
The reasons for this are unknown. According to one hypothesis, living organisms must evolve not only to gain advantages, but also to survive in general, for example, in the fight against parasites. This hypothesis is associated with the phrase of the Black Queen from the fairy tale "Alice Through the Looking Glass" by Lewis Carroll: "Well, here, you know, you have to run as fast just to stay in the same place."
To test the Black Queen's hypothesis, scientists from the University of Stirling used Daphnia, a crustacean with a body length of 0.2–6 millimeters. The fact is that Daphnia can reproduce both sexually and asexually. Most of the year, reproduction occurs by parthenogenesis, but in the fall, under the influence of external factors, males develop from their eggs.
In the experiment, the authors collected daphnia in a pond and after a while received two groups of offspring: those that appeared as a result of parthenogenesis and that appeared as a result of sexual reproduction. Both groups were infected with parasites - bacteria of the Pasteuria ramosa species, which reduce the reproductive function of daphnia. The collection of bacteria was carried out simultaneously with crustaceans and a year later.
The analysis showed that the parasites of the same year spread faster among the offspring that appeared as a result of parthenogenesis. In addition, animals obtained by sexual reproduction were twice as resistant to the next year's pathogens (which co-evolved at the same time as their parents) compared to offspring obtained asexually.
According to scientists, this is the first short-term animal experiment that has confirmed the Black Queen's hypothesis. The data obtained also indicate the special role of parasites in the emergence and maintenance of sexual reproduction. Moreover, the advantage of this method of reproduction is observed already in the first generation of offspring - they demonstrate increased resistance to the pathogen.