Biologists have found that two species of crustaceans called water fleas — Daphnia atkinsoni and Daphnia triquetra — independently acquired a crown-of-thorns-like structure on their heads during the course of evolution. This crown helps them protect themselves from predators, and therefore increases the crustaceans’ chances of survival. Genome analysis has shown that the “crown of thorns” is a very ancient acquisition that arose in daphnia as early as the Mesozoic era (roughly estimated at 145–66 million years ago). The data obtained help to better understand how protective adaptations evolved in invertebrates. The results of the study, supported by a grant from the Presidential Program of the Russian Science Foundation (RSF), were published in the journal Zoologica Scripta.
Daphnia, also known as water fleas, are crustaceans that inhabit most continental bodies of water in the world. They serve as food for small fish and other vertebrates and invertebrates. As early as the 18th century, scientists discovered that some species of daphnia have special adaptations to protect themselves from predators, such as shield shrimp (Notostraca). These include a tail spine, a hard chitinous plate on the head, and surrounding spines. The spines around the head plate of daphnia resemble a crown, which is why biologists call such structures a "crown of thorns." The "crown" is not permanent - it can form in crustaceans when predators are present in a body of water, and be lost when there is no danger. Until now, it remained unknown whether the "crown of thorns" in daphnia arose once in the process of evolution, or whether different species acquired it independently of each other.
Biologists from the A.N. Severtsov Institute of Ecology and Evolution of the Russian Academy of Sciences (Moscow), in collaboration with colleagues from other institutes, studied two species of daphnia capable of forming a "crown of thorns" - Daphnia atkinsoni and Daphnia triquetra. The scientists isolated DNA from crustaceans previously caught in reservoirs in the south of Russia, Mongolia and Kazakhstan. The researchers decoded the obtained genetic sequences and compared them with the genomes of daphnia that do not have a similar protective structure on their heads.
Genomic analysis showed that Daphnia atkinsoni and Daphnia triquetra are two genetically independent lineages, each of which has close relatives among species without a "crown of thorns". Thus, Daphnia atkinsoni turned out to be related to the "crownless" species Daphnia tibetana, and Daphnia triquetra to the species Daphnia studeri.
In addition, the authors determined that the group of species that includes Daphnia atkinsoni and Daphnia triquetra arose in the late Mesozoic - approximately 145-66 million years ago. Accordingly, the appearance of the "crown of thorns" in daphnia as a means of protection from predators, which have existed for just as long, is just as ancient.
"The data we have obtained allows us to better understand the history of the emergence of protective adaptations in freshwater animals during the course of evolution. It is the cladocerans that serve as model objects for such studies. Structures similar to the "crown of thorns" increase the chances of survival in the presence of natural enemies, and are therefore important in terms of maintaining the species. In the future, we plan to continue studying the crustaceans Daphnia atkinsoni, since there is evidence that this name hides a whole group of related species. We want not only to clarify the number of species in the group, but also to understand how they differ in the regulation of the appearance and disappearance of the "crown of thorns" in the presence or absence of a predator in the reservoir," says Alexey Kotov, a participant in the project supported by a grant from the Russian Science Foundation, Corresponding Member of the Russian Academy of Sciences, Professor of the Russian Academy of Sciences, Chief Researcher of the Laboratory of Ecology of Aquatic Communities and Invasions of the A.N. Severtsov Institute of Ecology and Evolution of the Russian Academy of Sciences.
The study involved staff from the Papanin Institute of Inland Water Biology of the Russian Academy of Sciences (Borok), the Koltsov Institute of Developmental Biology of the Russian Academy of Sciences (Moscow), and Charles University (Czech Republic).
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