It is the story of a tiny animal, invisible to the naked eye. It measures only about a fifth of a millimeter on average and lives in forest soil where it works to break down leaf litter. But how can we explain that zoologists, bioinformaticians and specialists in evolutionary genetics are interested in this species of oribate mite called Oppiella nova, who participates in the recycling of nutrients by breaking down organic matter in the soil into humus?
This interest is due to the fact that this small animal defies the scientific consensus according to which the survival of an animal species is conditioned on the sexual reproduction of single or multicellular organisms (eukaryotes). Indeed, sexual reproduction confers an advantage in terms of genetic diversity, so important when it comes to adapting to a new environment. In theory, therefore, it is improbable that an animal species can survive over a very long period by resorting exclusively to asexual reproduction.
Oribate mites, “evolutionary scandals”
However, it appears that rare species have managed to persist in the absence of sex for millions of years. Such organisms, which have remained asexual for very long periods of time, have been described as “ancient evolutionary scandals” (ancient asexual scandals) insofar as asexual species have a much greater probability of extinction than multicellular sexual species.
Oribate mites shave examples of such “evolutionary scandals”. They reproduce by parthenogenesis, a phenomenon characterized by the development of an egg in the absence of fertilization. Here, parthenogenesis is called thelytoque because it generates exclusively females.
It remains to be seen whether this tiny multicellular organism (150-1400 micrometers) is characterized by what biologists call “obligatory asexuality”. In other words, can this little animal completely do without sex?
In the absence of sexual reproduction, there is neither recombination nor segregation between the genes carried by two homologous chromosomes. In other words, no mixing or redistribution of maternal and paternal chromosomes occurs. Asexuality therefore results in asexual species in the absence of “mixing”.
Evolutionary genetics specialists assume that exclusively asexual reproduction should, however, leave traces in the genome of the animal organism. In fact, in such a context, genetic diversity can only come from mutations accumulated in each of the two copies of genes inherited from the parents, independently of one another. The genetic sequences then diverge more and more over time, leading to an increase in the differences between the two copies of the genes.
In the hypothesis of an exclusive asexual reproduction, we can deduce that the copies of the same group of genes (haplotype*) within individuals are more divergent than these same haplotypes normally are in closely related but sexually reproducing species. This divergence, within the same individual, between haplotypes possessing the same genes (intraindividual haplotype divergence) is known as the “Meselson effect”.
Meselson effect
The oribate mite turns out to be an excellent study model for understanding the persistence of possible exclusive asexual reproduction through the ages. A team of international researchers, made up of Swiss (University of Lausanne), German (Universities of Cologne and Göttingen) and French (University of Montpellier) biologists set out to verify whether the Meselson effect could be confirmed in this small mite.
Published September 17, 2021 in the proceedings of the American Academy of Sciences (PNAS), the results of this international team indicate that the mite oribate Oppiella nova does reproduce exclusively asexually.
These specialists collected, in 2015 and 2017, several populations ofOppiella nova in the soil of several forest areas in Germany, in this case in the Hainich National Park (Thuringia), the Kranichstein Forest (Hesse) and the Swiss Jura forest massif (Schwäbische Alb, mountain range in the southwest of country). They also collected mites Oppiella subpectinata, species very close toO. nova but whose reproduction is, on the other hand, sexual.
Exclusive asexuality
Biologists have sequenced the genomes of these two species of mites and analyzed their DNA sequences. It turns out that the genomic signatures of O. nova are specific to what is expected in the event of a Meselson effect in this small mite characterized by exclusive asexuality over the very long term. These signatures are absent from the sexual species O. subpectinata.
In particular, the results observed are not in favor of ancestral events testifying to phenomena of hybridization (cross-fertilization between different species with sexual reproduction) or of polyploidization (duplication of genetic sequences).
In summary, this international team therefore reports convincing elements in favor of the absence of sexual reproduction for millions of years in O. nova.
Dating the origin of the asexuality of this small animal is difficult, due to the fact that we do not know all the mechanisms that led to this phenomenon and that we still do not know the rate of mutations in mites. These two factors do not allow precise dating, given that it is generally estimated that the asexuality of these tiny organisms is between 6 and 16 million years old.
But one thing is certain: an exclusive asexuality did not lead these tiny animals to an evolutionary impasse, their particular mode of reproduction not having led to the extinction of the species. Far from there ! This small parthenogenetic mite now occupies a large number of habitats (forest soils, meadows, agricultural fields, canopies) and is present all over the world.
Marc Gozlan (Follow me on Twitter, Facebook, LinkedIn)
* In a diploid genome, chromosomes are present in pairs. In the case of sexual reproduction, each chromosome has genetic information in two very similar forms. Each version is called a haplotype. Two haplotypes therefore have the same genes.
To know more :
Brandt A, Tran Van P, Bluhm C, et al. Haplotype divergence supports long-term asexuality in the oribatid mite Oppiella nova. Proc Natl Acad Sci US A. 2021 Sep 21; 118 (38): e2101485118. doi: 10.1073 / pnas.2101485118
Maraun M, Caruso T, Hense J, et al. Parthenogenetic vs. sexual reproduction in oribatid mite communities. Ecol Evol. 2019 May 29; 9 (12): 7324-7332. doi: 10.1002 / ece3.5303
Schwander T. Evolution: The End of an Ancient Asexual Scandal. Curr Biol. 2016 Mar 21; 26 (6): R233-5. doi: 10.1016 / j.cub.2016.01.034
Weir W, Capewell P, Foth B, et al. Population genomics reveals the origin and asexual evolution of human infective trypanosomes. Elife. 2016 Jan 26; 5: e11473. doi: 10.7554 / eLife.11473
von Saltzwedel H, Maraun M, Scheu S, Schaefer I. Evidence for frozen-niche variation in a cosmopolitan parthenogenetic soil mite species (Acari, Oribatida). PLoS One. 2014 Nov 19; 9 (11): e113268. doi: 10.1371 / journal.pone.0113268
Birky CW Jr. Heterozygosity, heteromorphy, and phylogenetic trees in asexual eukaryotes. Genetics. 1996 Sep; 144 (1): 427-37. doi: 10.1093 / genetics / 144.1.427
www.lemonde.fr