Cyclostomata is an ancient group of aquatic colonial suspension feeders of the Bryozoa species. The fact that they have unique placentas was discovered by researchers from the University of St. Petersburg and the University of Vienna.
Coenocytes, ie. Large multinuclear cell structures originate from nuclear proliferation and cytoplasmic growth among cells surrounding the early embryo. Interestingly, coenocytes are often found among fungi and plants, but are quite rare in animals. This is the first time that coenocytes have been discovered in the placenta.
It is well known to biologists that the cells of living organisms are incredibly different in their behavior. It can happen to them that they form a multi-structural structure that is created by cell fusion, ie. The so-called single-membrane syncytium. Such ‘behavior’ ensures the circulation of nutrients without the resources and time spent on transport between cells.
However, similar structures can be created in ways other than fusion, e.g. Coenocytes are formed by nuclear multiplication and growth of the cytoplasm. Cell fusion is generally typical of animals. For example, in the human placenta, syncytium covers the embryonic placental villi, which attack the uterine wall to establish the circulation of nutrients between the embryo and the mother. In contrast, coenocyte formation is typical of plants and fungi. However, these processes are common – cell fusion and cell growth resulting in fairly large irregular tissue-like structures.
‘Initially, my graduate student Uliana Nekliudova, who is the first author of the article, and I expected to see syncytium in Cyclostomata. However, what we came across were coenocytes, ie multi-family structures that were not previously known in animal placenta, ‘said Andrew Ostrovsky, project manager and professor at the Department of Invertebrate Zoology at St. Petersburg University. ‘Embryos in this group of bryozoans not only rely on the placenta, as is usually the case, but are embedded in it. The placenta consists of several large coenocytes, and the coenocytic “elements” are interconnected by cytoplasmic bridges and various cell contacts. Coenocytes have a different ultra-structure. This tissue also shows evidence for the synthesis and transport of nutrients. ‘
In addition, as the scientist said, the formation of such a structurally complex placenta could have contributed to the development of polygembranes in this group of animals. Polyembryos are a condition in which over a hundred embryos develop from a single fertilized egg, forming larvae. However, it is known that a unique combination of placentation, viviparity, and polyembryony occurs only in nine-striped armadillo-daypus.
The samples were collected near the Marine Biological Station of the University of St. Petersburg on the White Sea. The scientists also found that the placenta originated from the organ that was responsible for the rejection of the tentacles. All bryozoans release sperm into the water column where zooids from other colonies catch it with their tentacles. Membrane bag, ie. The organ that contains the circular muscles pushes away the tentacles. After internal fertilization, the membranous sac becomes part of the placenta, transforming into coenocytes that provide embryonic food. The evolution of the placenta cyclostomy, which involves the transformation of the hydrostatic apparatus (membrane sac) and the replacement of its function by embryonic food, is an example of exacerbation.
Most bryozoans hatch a fertilized egg and then transport it to a special incubation cavity. However, cyclostom bryozoans do it differently. The larvae develop within the gonozooid, ie. Colony members serving as a placental incubator. Larval production lasts almost all summer. Developed larvae gradually leave the gonozooids.
Polyembryos is a so-called evolutionary impasse. You have offspring, but they are clones. There is no genetic diversity. The fact that the offspring are different is key to the ability to survive. The question is why do the polygembrio – which is typical of the Cyclostomat about 200 million years ago according to fossil evidence – have resistance? What’s the catch? It is assumed that briozoans, producing clone larvae over several months, “collide” with an ever-changing environment. As a result, it is not the genomes that change, but the conditions in which those genomes can be found. “
Andrew Ostrovsky, Professor, Department of Invertebrate Zoology, University of St. Petersburg
St. Petersburg State University
Nekliudova, UA, and others. (2021) Three in one: evolution of viviparity, coenocyte placenta, and polyembryo in cyclostomy bryozoa. BMC Ecology and Evolution. doi.org/10.1186/s12862-021-01775-z.