An international team of researchers shows that some species can survive for a long time even in the complete absence of males.

Many species – including humans – reproduce through sex. Here, the genetic material of two parents is combined and used to produce offspring. But there are also species that engage in so-called asexual reproduction. Only one parent is involved and that mother organism then produces a genetically (almost) identical copy of itself.

Unsustainable in the long run?

We regularly see the latter strategy in nature. However, it is generally accepted that it is highly unlikely, if not impossible, that a species that engages in asexual reproduction can survive for very long. In the long term, asexual reproduction does not appear to be so beneficial (see box).

Sex is better
Whereas during sexual reproduction genetic material is constantly exchanged and combined and therefore a great genetic diversity is also worked on in addition to offspring, the young that arise from asexual variation are almost all identical. And that makes them vulnerable. For example, only one disease has to appear to which the species is susceptible and this can disappear completely because all members of the species are almost identical. Species with more genetic diversity then clearly have an advantage, because there is a greater chance that there will be a few with a gene set that makes them just a bit more resilient. The idea that asexual reproduction is detrimental in the long term seems to be further supported by the fact that the vast majority of species reproduce sexually. Then that approach must be more favorable to the left or to the right, is the idea.

And yet researchers now show in a new study shows that against all odds, asexual reproduction can be very successful even in the long term. Proof? the mite Oppiella nova.

No men

The mite is less than a millimeter in size and lives mainly in forests. The species consists entirely of females and therefore does not engage in sex. Possibly not for millions of years, as it turns out.

Difficult

It is difficult to demonstrate that a species has exclusively reproduced asexually for a long time. “For example, there may have been a ‘mysterious’ sexual exchange in the past that we don’t know about,” said researcher Alexander Brandt. “Or don’t know about it yet. For example, very occasionally – perhaps accidentally – a reproductive male may have been produced.”

Meselson effect

However, it is possible to rule out such an ‘accident’. In theory, pure asexual reproduction leaves a clear trace in the genome. This is in the form of the so-called Meselson effect (see box).

Every human cell contains two copies of the genome: one comes from the father, the other from the mother. “In organisms with two sets of chromosomes — that is, that have two copies of their genome — such as humans and mites that do sexually reproduce, sex causes those two copies to be mixed,” explains researcher Tanja Schwander. This creates genetic diversity between different individuals, but the copies of the genome in the same individual usually remain almost the same. Of course, things are very different with asexual species – which produce genetic clones of themselves. It is important to know, however, that these species are also capable of introducing some genetic variation into their genome and thus adapting to their environment through evolution. But: “Without sex, or that mixing of chromosomes, the two copies in asexual species will independently collect mutations and thus slowly but surely differ more and more.” The Meselson effect describes the detection of those differences in the chromosome sets of purely asexual species. “That may sound simple,” says Schwander. “But in practice, the Meselson effect has never been convincingly demonstrated in animals. Until now.”

The researchers compared the genome of O. nova with that of the mite Oppiella subpectinata. The latter species is closely related to O. nova, but plants unlike O. nova continue sexually. The research into both mite species was not easy. “It was labor-intensive because the mites are only a fifth of a millimeter in size and difficult to identify,” says Brandt. “But our efforts have paid off: the Meselson effect has been confirmed!”

And thus it is also confirmed that O. nova really only reproduces asexually and has been doing so for a long time. “When it comes to how evolution works without sex, these mites may surprise us,” predicts researcher Jens Bast. In any case, for now they show – very surprisingly – that they are among the rare species that can survive for a long time without men and without sex. Follow-up research should show how the mites do that and what makes them so special in that area.