Sex protects against infections
About the advantages of sexual reproduction
Sofia Kovaleva, XX2 century, based on the materials of the University of Stirling: Scientists show sex evolved to help future generations fight infection
Why is sexual reproduction so common if it is much easier to produce offspring by division or budding? One of the answers to this question is the "Black Queen hypothesis". A team of biologists from the UK, which has been studying large Daphnia (Daphnia magna) – planktonic crustaceans for many years, was able to test this hypothesis "in numbers". The results of the work are published in the journal Proceedings of the Royal Society B (Auld et al., Sex as a strategy against rapidly evolving parasites).
Freshwater large daphnia are interesting because they can reproduce both sexually and asexually (parthenogenetically). During the study, biologists decided to find out which offspring of crustaceans is more vulnerable – which carries the genes of two parental individuals, or only one.
To do this, scientists have been observing the life of more than 6 thousand Daphnia magna for several years, resulting from both sexual and asexual reproduction. In the course of observations, the researchers subjected the young crustaceans to a serious test: they were planted with parasites – Pasteuria ramosa bacteria (which, by the way, reduce the fertility of infected daphnia).
The experiment was conducted as follows: the bacteria were divided into 2 colonies. Bacteria from one of them parasitized daphnia for a year. And the second group of bacteria was isolated from the "hosts" during the same year. The researchers then tested the resistance of daphnia to bacteria from both groups. In terms of resistance to bacteria from an isolated colony, daphnia obtained by asexual reproduction did not differ much from sexually born daphnia. But when bacteria that had previously evolved with them were "let loose" on daphnia, the "sexual" daphnia turned out to be more than twice as resistant as the "asexual" ones: only 15% of daphnia were infected in the former, while all 46% in the latter. When parasites developed already inside the body of daphnia, the "sexual" daphnia also resisted more strongly: inside them, researchers found half as many spores of parasitic bacteria as inside the "asexual" ones.
So, it turned out that daphnia, born as a result of sexual reproduction, developed resistance to rapidly evolving parasites – bacteria much faster. According to the researchers, the benefits of sexual reproduction, as a result of mixing the genes of the parents, turned out to be strong and fast enough to compensate for the costs of finding a sexual partner (a problem that does not concern organisms that reproduce asexually). Thus, the so-called "hypothesis of the Black Queen" (Red Queen hypothesis) is confirmed, justifying the existence of sexual reproduction by faster adaptation to changing environmental conditions. As you might guess, the name of the term originates in Lewis Carroll's book about Alice's adventures. In the book "Alice through the Looking Glass" there is such a dialogue:
– Here, – said Alice, catching her breath with difficulty, – when you run as fast as you can for a long time, you will certainly get to another place.
– What a slow country! The Queen cried. "Well, here, you know, you have to run as fast as you can just to stay in the same place. "We proved the Black Queen hypothesis for the first time during a short–term animal experiment," the authors of the work summarize.
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21.12.2016