Researchers from St. Petersburg State University and the University of Vienna have found that representatives of the ancient group of bryozoans Cyclostomata – colonial invertebrates living on the bottom of the seas and oceans, form a unique placenta. The cells surrounding the early embryo begin to grow rapidly, turning into coenocytes – large multinucleated cellular structures. Interestingly, coenocytes are widespread among plants and fungi, but rare in animals. In the placenta, they were found for the first time.
Biologists are well aware that the cells of living organisms are able to behave in the most unusual ways. It so happens that they merge with each other, forming a single structure with a large number of nuclei – syncytium, surrounded by a common membrane. This “behavior” allows them to quickly exchange nutrients without wasting time and resources to overcome cell membranes. It happens that a similar structure arises as a result of a completely different process – not fusion, but cell growth, which begins to copy nuclei and increases significantly in size. As a result, coenocyte is formed. If the first phenomenon is typical mainly for animals, for example, in the human placenta, outgrowths of the embryonic membrane are covered with syncytium, penetrating into the wall of the uterus, then the second is mainly the lot of plants and fungi.
“Starting to work with cyclostomats, together with graduate student Ulyana Neklyudova (the first author of the published article), we assumed that we would find in them exactly syncytium, known in the placentas of other organisms, and found coenocytes – multicellular structures that are not typical for animals.“— Project manager, Professor of the Department of Invertebrate Zoology, St. Petersburg State University Andrey Ostrovsky
“The embryos of this group of bryozoans do not just adhere to the placenta, as is usually the case, but are immersed in it, literally soldered. The placenta consists of several large coenocytes connected to each other by cytoplasmic bridges and two types of cell contacts. Cenocytes have a different ultrastructure, which indicates that some of them have a feeding function, and some have a conducting function. In addition, the appearance of such a complex placenta could become a necessary condition for the emergence of polyembryony in this group of animals, when an early embryo, growing up, splits into many (more than a hundred) secondary embryos, each of which develops into a larva. Such an extremely rare combination of reproductive traits (live birth, placentotrophy and polyembryony) is found in only one other group of creatures on Earth – armadillos of the genus Dasypus, ”
The researchers collected samples near the Marine Biological Station of St. Petersburg State University in the White Sea. They also found out that the placenta is formed from an organ that is initially responsible for protruding the tentacles. All bryozoans sweep sperm into the water, from where they are captured by the tentacles of zooids from other colonies. To protrude the tentacles, an organ equipped with annular muscles is used – a membrane bag. It is she who, after fertilization, becomes the basis of the placenta from coenocytes, which will nourish the embryos. This phenomenon of a change in function in evolution is called exaptation.
In general, the vast majority of bryozoans first hatch a fertilized egg from the internal cavity into the external environment, and then move it to a special brood chamber, where bearing is carried out. However, cyclostomy bryozoans act differently here too: their larvae develop directly in the cavity of a gonozooid, a member of the colony that has become a placental incubator. The process of hatching of offspring continues almost all summer – mature larvae gradually leave the gonozooid.
“Polyembryony is considered an evolutionary failure: there seem to be many children, but they are all clones – there is no genetic diversity,” explains Andrei Ostrovsky. – Usually it is the fact that the offspring are different that provides the best survival. And here the question arises – why is polyembryony, fossil evidence of which in Cyclostomata is about 200 million years old, turned out to be so stable? And with her the group itself. What’s the catch? Perhaps this is due to the fact that, producing clone larvae for several months in a row, bryozoans “confront” them with a constantly changing environment. It turns out that it is not the genomes that vary here, but the conditions in which these genomes find themselves. “
The study was supported by grants from the Russian Science Foundation No. 18-14-00086 and the Austrian Science Foundation No. P27933-B29.
Featured image: Histological section of a gonozooid filled with larvae and embryos immersed in the placenta from coenocytes (light microscopy)
Reference: Nekliudova, U.A., Schwaha, T.F., Kotenko, O.N. et al. Three in one: evolution of viviparity, coenocytic placenta and polyembryony in cyclostome bryozoans. BMC Ecol Evo 21, 54 (2021). https://doi.org/10.1186/s12862-021-01775-z
Provided by St. Petersburg State University