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Powered by Website address: https://www.gesundheitsindustriebw.de/en/article/news/the-inversion-of-the-dorsoventralaxis-in-the-separation-of-bilataria/ The inversion of the dorsoventral axis in the separation of Bilataria The basic classification of bilateral animals is based on the comparison of their embryonic development and was confirmed in modern molecular phylogeny by the analysis of the relationship of the genes involved. Conserved groups of genes are responsible for the formation of the body axes. The separation of protostomes and deuterostomes is based on the inversion of the dorsoventral axis. The basic division of the majority of the animal kingdom into protostomes and deuterostomes is one of the most important findings from comparative embryology of the late 19th century. These bilaterian phyla differ mainly in embryonic development. During gastrulation, the cells continue to be rearranged to form the archenteron (primitive gut), the open end of which is called the blastopore. Gastrulation is a process in multicellular animals (with the exception of sponges) in which mesenchyme cells enter the blastocoel. The cells at the vegetal pole buckle inwards towards the blastocoel, a narrow pouch is formed by the gastrula's endoderm. The endoderm of the archenteron fuses with the ectoderm of the blastocoel wall. Schematic of gastrulation. The primary body opening (vertical arrow) is the blastopore. (Figure: University of Heidelberg) The open end of the archenteron is connected with the outside by way of the blastopore. In protostomes, the blastopore becomes a mouth, and the anus develops at the other end of the archenteron. [However, this is more complicated in many protosomes: the blastopore is prolonged to form a shallow dip along the longitudinal axis of the embryo and closes up from 1 the middle like a zipper; the mouth at one end and the anus at the other end remain open.] In deuterostomes, which include humans and our relatives among the chordates, the echinoderms and other less known phyla, the blastopore becomes the anus (or develops at least in close vicinity to it) and the mouth develops at the other end of the archenteron. “Protostome” means “mouth first” while deuterostome means “mouth comes second”. Dichotomy of bilateria Since the majority of phyla – with the exception of Porifera (sponges), Cnidaria and Ctenophora (comb jellies) – can be grouped into protostomes or deuterostomes, it can be assumed that this is a fundamental, very old distinctive feature. It must have formed before the Cambrian Era (more than 545 million years ago) when the early representatives of phyla appeared and which can now be clearly classified as either protostomes or deuterostomes. Unfortunately, no fossils are available that could provide us with information as to how these two lineages developed. Protostomes: Longitudinal cut through the front end of an earthworm (Figure: fsbio-hannover.de) It was possible to confirm this basic division of the animal kingdom into protostomes and deuterostomes by modern analyses used in molecular phylogeny. Of course, there were also some new views: A few invisible candidates – such as Brachiopoda and Bryozoa – which were often regarded as deuterostomes, now count as protostomes. In terms of the division of major phyla, the molecular data suggest some radical changes. Previously, scientists thought that annelids (bristle worms) were related to arthropods due to their segmentation. Nowadays, they are grouped as Lophotrochozoa, together with molluscs. This classification can be seen as a late triumph for the classical developmental biologists or embryologists, who suggested a big similarity between Trochophora larvae and marine annelids and snails. The arthropods (insects, crayfish, spiders, centipedes, etc.) and other relatives are nowadays grouped as Ecdysozoa together with the unsegmented nematodes (ring worms like for example the favourite experimental animal of molecular biologists , Caenorhabditis elegans). They differ from other phyla because they shed their skin (Greek: Ecdysis) during individual development. Conserved genes during embryonic development 2 Protostomes and deuterostomes together (plus a small group of primitive worms, known as Acoela or acoelomorphic platyhelminthes, which are nowadays no longer in the group of protosomic flatworms) are grouped as Bilateria (animals with two sides), which are more or less symmetric along their longitudinal axis. Their ancestor is a small worm with an end-to-end digestive tract, with a front and a back end. It is likely that our Precambrian ancestor was such an inconspicuous worm. Deuterostomes: Longitudinal cut through amphioxus (Photo: Int. J. Biol. Sci. 2006; 2: 149-160) One of the most important discoveries of modern evo-devo research (evolution plus development) is that the embryonic longitudinal axis in animals develops according to the same principle, i.e. controlled by the Hox genes. The Hox genes were initially discovered in Drosophila and have a typical sequence of approximately 180 base pairs (homeobox), which encode a particular DNA-binding region. The proteins encoded by the Hox genes either directly or indirectly control the transcription of numerous genes, including those that are responsible for the formation of typical features in the individual embryo segments. Drosophila has eight Hox genes that are arranged in linear order on the chromosome. Each of the genes is expressed in particular body segments. It is slightly confusing that these segments do not correspond with the developing segments of the fly. Scientists found that the same homologous gene cassette also occurs in other animals, although the number of Hox genes varies. In the amphioxus lancelet (Branchiostoma lanceolatum), a primitive deuterostome regarded as a model of the ancestors of vertebrates, there are 10 Hox genes, mice and humans have 38 arranged in four clusters that developed through duplication. Inversion of the body axis The researchers found that a conserved gene cassette controls the development of the dorsoventral axis in Drosophila. The key gene in this process, dpp, codes for the synthesis of a protein that activates other genes during early embryogenesis. These genes are responsible for the development of dorsal structures and which suppress the formation of ventral structures, including neurogenesis. Although a homologue of dpp, bmp-4, was found and cloned in frog embryos, scientists did not find a parallel between insect morphogenesis and vertebrate embryos. Apparently, a clear and undisguised view was required. In 1994, Detlev Arendt, then a PhD student at the Institute of Zoology in Freiburg (now head of a group of researchers at EMBL in Heidelberg), and Dr. 3 Dr. Detlev Arendt, European Molecular Biology Laboratory in Heidelberg. (Photo: EMBL) Katharina Nübler-Jung (from Freiburg), published a short paper in which they showed that the two genes had similar functions, albeit in opposing body regions. They concluded that the body axis must have been inversed not long after the evolutionary lineages separated from a common ancestor (“Nature“ 371: 26, 1994; “Development“ 126: 2309-2325, 1999). In protostomes (for example in Drosophila or earthworms) the central nervous system is located on the ventral side of the intestine. The blood is pumped from the heart, which is located on the dorsal side of the intestine, through a main artery, which is also located on the dorsal side. In deuterostomes (for example amphioxus or vertebrates) it is exactly the opposite: the central nerve cord is located on the dorsal side of the intestine, while the heart and the major blood vessels are located on the ventral side of the intestine. Many people regarded the idea that deuterostomes developed from ancient Bilateria through the conversion of the axis as extremely heretical. Nowadays, the idea is accepted by the majority of scientists as it has been confirmed by molecular data. Artemia salina, a prostome swimming on its back (Photo: Tierreich.de) © Tierreich.de It is a lot easier to imagine such a conversion rather than imagining that the entire internal design of the animals had to be changed. In addition, there are living animals that have become specialised in swimming on their backs and that can be used as models, including the Notonectidae (backswimmers), a family of water bugs, Artemia salina which is a popular aquarium fish food, and some fish such as Synodontis nigriventris (upside down catfish). Assuming that Artemia, which have most likely acquired this behaviour in the recent geological past, would have millions of years to evolve and adapt to this new way of movement, then what was formerly the belly would become the back and the ventral nervous system would become a dorsal one. 4 Nothing new under the sun Back in 1822, decades before Darwin wrote his “Origin of Species”, the French zoologist Geoffroy St.-Hilaire proposed the controversial hypothesis that vertebrate were upside down arthropods. He was heavily attacked and mocked by the dominant scientist of the time, George Cuvier. Now, Geoffroy St.-Hilaire has been rediscovered as the father of modern evolutionary research (Panchen, A.L. 2001: Ètienne Geoffroy St.-Hilaire: Father of ’evo-devo’?, in: Evolution and Development 3, 41-46). 'And there is nothing new under the sun. Is there anything of which one can say, “Look! This is something new”? It was here already, long ago; it was here before our time.’ (Ecclesiastes 1, 9-10 NIV). Article 21-Feb-2008 EJ BioRN 5