The origin of vertebrates and their symmetry, segmentation, chord and tubular nervous system

Development of vertebrata begins with formation of a multicellular organism by ordered repeated division of a reproductive cell and nondisjunction of the new formed cells, which have kept connection by means of the extracellular matrix. Further there is a consecutive formation of organisms due to aggregation of similar structures: blastaea; segmentella, supersegmentella. Supersegmentella gave development to tunicates, hemichordates, chordates like lancelet and to predecessors of vertebrata. Segmentation of organisms is determined by aggregation of supergastraeas into one integrated. Symmetry is determined by structure-forming quality of extracellular matrix. Symmetry of primary organisms was radial; then radial-bilateral, and the first plane of symmetry divided the organism into dorsal and ventral sides. With the arrangement of supergastraeas in a line radial-two-plane symmetry consistently formed. Radial-three-plane symmetry formed by association of two segmentellas by posterior edges. The third plane of symmetry divided the organism into anterior and posterior antimeres. From extracellular matrix originated mesogloea, and then a chord; endodermic embolies gave development to the primary gut; ectodermic embolies after the concentration there earlier diffusely located nervous cells transformed first into a trench, and then into a tubular nervous system; the condensed nervous fabric of aboral poles gave development to the central nervous system. The glandulocytes of supergastraeas became starting material for all glands of the organism. Introduction The origin of vertebrates remains an unresolved question in biology. Based on a comparison of the structure and development of animals and on their own conjectures, anatomists have suggested a large number of concepts of the origin of Bilateria, including vertebrates. These hypotheses and their analysis are presented in monographs (Bresslau and Reisinger 1933; Hyman 1951; Willmer 1994; Iordansky 2001; Saveliev 2005), theses (Gorodilov 2002), and reviews (Arendt and NüblerJung 1999; Erwin, 1999; Malakhov 2004; Gerhart et al 2005; Gerhart 2006). One of the earliest concepts was proposed by Geoffroy Saint-Hilaire (1970) in the first half of the 19 century. SaintHilaire argued for the radical idea that vertebrates were inverted copies of arthropods. A similar idea that ancestors of chordates were annelid-like worms was put forward by Dohrn (1937). This N at ur e P re ce di ng s : h dl :1 01 01 /n pr e. 20 10 .4 16 4. 1 : P os te d 19 J an 2 01 0 2 idea is still alive in the 21 century and has many advocates (Bonik et al. 1976; Foreman 1985; Jefferies 1986; Nielsen 1999; Malakhov 2004). It arises from the fact that the position of the heart and the direction of the blood flow in chordates are virtually the same as in arthropods but turned upside down. It is equally true of the coelome position. This gives reason to claims by supporters of this concept that ancestors of chordate animals underwent inversion of body parts in the course of evolution; in other words, they flipped themselves upside down and began to move on the morphologically reverse side. In the end, it became to function as the physiologically ventral surface, while the morphological downside turned into a physiologically dorsal one. The direction of blood flow in annelides, arthropods and inverted ancestral chordates also coincides. At the same time, the different position of the heart (on the underside in chordates and high on the dorsal side in invertebrates) may be a result of the independent origins of their blood circulatory systems. Adepts of the hypothesis of dorsoventral inversion in chordates argue that certain extant species still live upside down (belly upwards), although both ventral and dorsal sides continue to function as such. As is known, “wrong” jelly-fish (Ctenophora) have displaced oral and/or anal openings, although their entoand ectodermal origin is not in doubt. Adherents of this hypothesis adduce data from molecular biology in support (Arendt and Nubler-Jung 1994; Ferguson 1996; Voronov 2000.). Gastrula-stage embryos of certain vertebrates have been shown to synthesize bone morphogenic protein-4 (BMP-4) and chordin (CHD) on their ventral and dorsal sides respectively. Normally, CHD is synthesized on the dorsal side, but its experimental injection into the ventral side to increase protein concentration triggers the development of intrinsic dorsal structures. Similarly, the administration of BMP-4 into the embryo’s dorsal side induces the formation of ventral morphological structures. Virtually identical results were obtained in Drosophila experiments and in a study by Slack et al. (1993) on the role of group Hox genes in the development of different invertebrate and vertebrate animals. The authors showed that the Hox gene expression marks the ventral side in invertebrates and the dorsal one in vertebrates. However, localization of these proteins at a given site neither confirms nor disproves the inversion hypothesis; it only suggests that they are involved in the formation of certain morphological structures or somehow influence it. They are supposed to be derivatives of blastopore inducer (organizer of development at early gastrulation stages). Supporters of this idea seem unwilling to notice a significant difference in the development and organization of the above animal groups or the absence of homologous organs in them. The primary difference between protostomes and deuterostomes is in cleavage patterns of the fertilized ovum, spiral in the former and radial in the latter. Another difference consists in the mode of coelome initiation. In protostomes the walls of the coelome develop from two teloblasts, while in deuterostomes they originate from an outpouching of the embryonic intestine. The critical N at ur e P re ce di ng s : h dl :1 01 01 /n pr e. 20 10 .4 16 4. 1 : P os te d 19 J an 2 01 0