Phylogenic relationships of deuterostomes and evolution of chordates. (a) Schematic representation of deuterostome groups and the evolution of chordates. Representative developmental events associated with the evolution of chordates are included. (b) A traditional and (c) the proposed view of chordate phylogeny with respect to their phylum relationship.
Download Chor Chor Super Chor 72
DOWNLOAD: https://tinourl.com/2vG1Xx
Nearly 140 years later, this notion was revitalized by the discovery of genes responsible for D-V axis formation, encoding members of TGF-β family proteins, bone-morphogenic proteins (BMPs) and their antagonists, including chordin and anti-dorsalizing morphogenetic protein (Admp) [60,61]. In Drosophila melanogaster (arthropod), Dpp (i.e. BMP) is expressed at the dorsal side of the embryo and functions in dorsalization of the embryo, while Sog (i.e. chordin) is expressed at the ventral side of the embryo and functions in ventralization [60]. By contrast, in Xenopus laevis (vertebrate), BMP is expressed at the ventral side of the embryo and chordin at the dorsal side [61].
Embryologically, the notochord and neural tube are recognized as dorsal-midline organs that are deeply involved in the formation of chordate body plans. Viewed from the vegetal pole, the early embryo of non-chordate deuterostomes is radially symmetrical, suggesting the possibility of forming the dorsal-midline structures everywhere. However, the fact is that these structures are only formed at the dorsal side, which corresponds to the aboral side of non-chordate deuterostome embryos. That is, the A-D hypothesis speculates that the oral side is spatially limited due to formation of the mouth so that the dorsal-midline organs were allowed to form in the aboral side of ancestral chordate embryos. Thus, dorsalization of the aboral side of the ancestral embryo may have been a key developmental event that led to formation of the basic chordate body plan. When compared with the inversion hypothesis, the A-D hypothesis emphasizes the role of dorsal-midline structure formation that superficially appears to be the D-V axis inversion.
The morphological similarity of extant lancelets is striking. This may not be attributable to their recent diversification, because the divergence time of the last common ancestor of the three extant genera is estimated to be 162 million years ago (Ma) [79] and that of the genus Asymmetron around 100 Ma [80]. These extant forms are reminiscent of some fossils with similar body plans, including Cathaymyrus [81] and Pikaia [82] dating back to earlier than 500 Ma. Furthermore, the similarity can be explained as morphological stasis, rather than genetic piracy [83]. The presence of extensive allelic variation (3.7% single nucleotide polymorphism, plus 6.8% polymorphic insertion/deletion) revealed by the decoded genome of an individual Branchiostoma floridae may support this notion (electronic supplementary material, table S1) [45]. The lancelet genome appears to be basic among chordates.
Features that characterize the Vertebrata as a phylum. (a) Major shared features of various vertebrate taxa. Lampreys and hagfishes (cyclostomes) lack mineralized tissues. By contrast, cartilaginous fishes produce extensive dermal bone, such as teeth, dermal denticle and fin spine. However, they lack the ability to make endochondral bone, which is unique to bony vertebrates (adapted from Venkatesh et al. [96]). (b) The neural crest GRN in vertebrates. Black arrows indicate empirically verified regulatory interactions. Shaded areas represent the conserved subcircuits of the respective GRNs between vertebrates and cephalochordates; the amphioxus genes are not used for the circuit of neural crest specifiers and the effector subcircuit controlling neural crest delamination and migration (adapted from Yu [97]). (c) Clustering of metazoan genomes in a multi-dimensional space of molecular functions. The first two principal components are displayed, accounting for 20% and 15% of variation, respectively. At least three clusters are evident, including a vertebrate cluster (red circle), a non-bilaterian metazoan, invertebrate deuterostome or spiralian cluster (green circle), and an ecdysozoan group (yellow circle). Drosophila and Tribolium (lower left) are outliers. Aqu, Amphimedon queenslandica (demosponge); Bfl, Branchiostoma floridae (amphioxus); Cel, Caenorhabditis elegans; Cin, Ciona intestinalis (sea squirt); Cte, Capitella teleta (polychaete); Dme, Drosophila melanogaster; Dpu, Daphnia pulex (water flea); Dre, Danio rerio (zebrafish); Gga, Gallus gallus (chicken); Hma, Hydra magnipapillata; Hro, Helobdella robusta (leech); Hsa, Homo sapiens (human); Isc, Ixodes scapularis (tick); Lgi, Lottia gigantea (limpet); Mmu, Mus musculus (mouse); Nve, Nematostella vectensis (sea anemone); Sma, Schistosoma mansoni; Sme, Schmidtea mediterranea (planarian); Spu, Strongylocentrotus purpuratus (sea urchin); Tad, Trichoplax adhaerens (placozoan); Tca, Tribolium castaneum (flour beetle); Xtr, Xenopus tropicalis (clawed frog) (adapted from [98]).
Neural crest. A recent view of chordate evolution, mentioned above, suggests that vertebrates evolved from a lancelet-like ancestor by developing a head and jaws, which fostered the transition from filter feeding to active predation in ancestral vertebrates. The neural crest is a key vertebrate character deeply involved in development of the head and jaws [11,101]. It is an embryonic cell population that emerges from the neural plate border. These cells migrate extensively and give rise to diverse cell lineages, including craniofacial cartilage and bone, peripheral and enteric neurons and glia, smooth muscle, and melanocytes. The gene regulatory network (GRN) underlying neural crest formation appears to be highly conserved as a vertebrate innovation (figure 2b) [97,102]. Border induction signals (BMP and Fgf) from ventral ectoderm and underlying mesendoderm pattern dorsal ectoderm, inducing expression of neural border specifiers (Zic and Dlx). These inductive signals then work with neural border specifiers to upregulate expression of neural crest specifiers (SoxE, Snail and Twist). Neural crest specifiers cross-regulate and activate various effector genes (RhoB and Cadherins), each of which mediates a different aspect of the neural crest phenotype, including cartilage (Col2a), pigment cells (Mitf) and peripheral neurons (cRet) (figure 2b).
Endoskeleton. Vertebrate cartilage and bone are used for protection, predation and endoskeletal support. As there are no similar tissues in cephalochordates and urochordates, these tissues represent a major leap in vertebrate evolution [95]. It appears that these mineralized tissues were obtained gradually during vertebrate evolution because extant jawless vertebrates (lamprey and hugfish) have no mineralized tissues (figure 2a). The earliest mineralized tissue was found in the feeding apparatus of extinct jawless fishes, the conodonts. Cartilaginous fish produce calcified cartilage and dermal bone, including teeth, dermal denticles and fin spines, but their cartilage is not replaced with endochondral bone (figure 2a). Endochondral ossification is established by a highly complex process unique to bony vertebrates. Recent decoding of the elephant shark genome suggests that the lack of genes encoding secreted calcium-binding phosphoproteins in cartilaginous fishes explains the absence of bone in their endoskeleton [96].
Adaptive immune system. All metazoans protect themselves against pathogens using sophisticated immune systems. Immune responses of invertebrates are innate and usually stereotyped. On the other hand, vertebrates adopted an additional system or adaptive immunity using immunoglobulins, T-cell receptors and major histochompatibility complex (MHC) molecules [105]. The adaptive immune system enables more rapid and efficient response upon repeated encounters with a given pathogen. Surveys of cephalochordate and urochordate genomes failed to detect genes encoding immunoglobulins, T-cell receptors or MHC molecules, indicating that the adaptive immune system is another vertebrate innovation [106,107]. The recent discoveries of alternative antigen receptor systems in jawless vertebrates suggest that the cellular and molecular changes involved in evolution of the vertebrate adaptive immune system are more complex than previously thought [108,109].
Genome constitution. It has been revealed that a high grade of synteny is conserved between cephalochordate and vertebrate genomes [45]. The vertebrate genome has experienced both quantitative and qualitative alterations during evolution, clearly distinguishing vertebrates from invertebrates, including lancelets and tunicates. Quantitatively, it has been argued that two rounds of genome-wide gene duplication (2RGD) occurred in the lineage leading to vertebrates [110,111]. Indeed, numerous gene families, including those encoding transcription factors (Hox, ParaHox, En, Otx, Msx, Pax, Dlx, HNF3, bHLH), signalling molecules (hh, IGF, BMP) and others (dystrophin, cholinesterase, actin, keratin) were expanded by gene duplication in the vertebrate stem lineage [111]. This yielded an increase in genetic complexity, which is one of the key events underlying increased morphological complexity under developmental control. Recent decoding of the lamprey genomes suggests that duplication occurred in the early phase of vertebrate divergence [96,112] (electronic supplementary material, table S1). However, the mechanism and exact timing of 2RGD still remain to be elucidated. 2ff7e9595c
Comments