Are the mechanisms of Dorsal/Ventral axis specification, similar in all animals?
………. Perhaps, the difference lies only in the location of the mouth!
Recently, the molecular components of the BMP-Chordin system, that specifies the Dorsal/Ventral axis in Bilaterates, have also been found and characterized in the Radiate Nematostella vectensis. These morphogens, which are asymmetrically distributed in Bilaterates across the presumptive dorsal/ventral axis (which they specify), are also found to be asymmetrically distributed along an “invisible” directive axis in Nematostella. Moreover, these molecules have also been proved to be similar to their Bilaterate counterparts in sequence and function. This was revealed by studies employing sequence homologies to construct phylogenetic trees, and to test the effect of administering Radiate morphogens on developing vertebrate embryos.
A recent study employing on in-situ hybridization assays revealed elaborate patterns of expression of expression of the BMP agonist-antagonist system orthologues in the sea anemone Nematostella vectensis. The study (1) found the orthologues in Nematostella vectensis, of vertebrate anterioposterior patterning NvHox genes; BMP antagonists NvChordin, NvFollistatin, NvNoggin, NvGremlin etc.; the BMP family genes Nv-dpp, NvBMP5-8 etc.; and the homeodomain transcription factors NvGbx and NvGsc (Nv = Nematostella vectensis).
NvAnthox8 gene is expressed along the entire length of the oral aboral axis in the endoderm of the Nv planula larva, however the expression is limited to only one side of the directive axis. NvChordin is initially expressed in the ectoderm around the entire oral pole, but becomes restricted to a W-shaped patch on the side opposite to the expression domain of NvAnthox8 in the planula. NvNoggin is expressed in the pharyngeal endoderm on the same side of the directive axis as the NvChordin.
NvGbx, whose vertebrate orthologue is expressed in parallel domains flanking the neural tube, in Nv is expressed in parallel domains flanking the directive axis on either side in the pharyngeal endoderm of the planula larva. NvGsc whose vertebrate orthologue, goosecoid is expressed in the organizer region of the blastopore lip, in Nv is expressed initially in both the ectoderm and endoderm, but later becomes confined to the pharyngeal endoderm in the planula, along the length of the directive axis, in two diametrically opposed domains such that the major domain lies in the same region as the Hox gene.
When the Nv planula embryos were treated with Lithium Chloride, it was found to have a radializing effect on the expression domains of the two genes NvNoggin and NvGsc, an effect similar to the dorsalization effect of LiCl seen in Xenopus embryos. This radializing effect disrupted the expression domains such that they lost their asymmetry across the directive axis, and became evenly distributed radially. This effect is thought to be due to the inhibition of the canonical Wnt pathway by LiCl.
In addition, in experiments focused on the functional effect of Nv morphogens, it was found that the injection of NvNoggin into the presumptive ventral aspect of the early Xenopus blastula resulted in the development of a secondary embryonic axis on the ventral side of the embryo alongside the original axis. This was the same effect as was observed, when the Xenopus Noggin itself was injected in the same region. The rate of secondary axis induction by NvNoggin was very close to what had been observed for Xenopus Noggin.
Furthermore, in similar experiments conducted on 32-hour zebra fish embryos (2), it was found that NvChordin, when injected into a wild-type embryo, dorsalized it such that there is an enlargement of the dorsal neural structures, while the ventral structures decreased in size. NvGremlin also had a similar effect however it was far more intense than that of NvChordin. In contrast, injection of Nv-dpp caused an exaggeration of the ventral tail fin and other ventral structures in the zebra fish embryos.
All these functional assays on zebra fish and Xenopus embryos showed that the radiate homologues of the bilaterate dorsal/ventral patterning system have the functional properties that are required for dorsal/ventral patterning in vertebrates, and hence the two “Groups” must have arisen from a common ancestor. Furthermore the fact that the radiate morphogens despite the evolutionary schism of hundreds of millions of years, still retain their functional properties, indicates that they must have a similar function in the radiates as well. This conservation of function is further proved by the teratogenic effect of even distribution of NvNoggin and NvGsc as is caused due to LiCl treatment of Nv planulae (1).
The hypothesis of all metazoans (radiates and bilaterates) having a common ancestor has long been based on a conserved organization of the expression domains of D/V patterning genes (3) in various bilaterate groups (protostomes and deutrostomes) as well as on the occurrence of the very same genes in the radiates. However, until recently, it had not been proven that the D/V patterning genes found in radiates still retain the functional properties required for D/V patterning in bilateral animals. However one minor problem is that the organization of D/V morphogens and hence that of D/V anatomical structures is exactly the opposite in most protostomes compared to deutrostomes. In protostomes, the heart lies dorsal to the gut and the gut is dorsal to the central nervous system, in contrast the deutrostomes have their hearts ventral to the gut and their guts ventral to the CNS. Nonetheless, in both cases nervous system is always specified by the Sog/Chordin and in each case, they are known to antagonize the BMP/Dpp family morphogens. Although in Drosophila, the nervous system is little affected in Sog mutants, the effects are more prominent in spiders (4) and other arachnids.
One hypothesis that explains the opposite D/V patterns of protostomes and deutrostomes is the one that suggests a rotation of the mouth from the neural side of the D/V axis, to the non-neural side. This hypothesis is rooted in the structural organization of the very close cousins of Chordates, the Hemichordates. The hemichordates have the same dorsal ventral pattern of structures as protostomes, however they have the diagnostic pharyngeal gill slits of the chordates, and the same battery of neural patterning genes…. although not centralized in CNS as in chordates (5). This is because the hemichordates have a diffused nervous system with cell bodies scattered in the ectoderm and only tracts of axons running along the body length as dorsal and ventral cords (which the early zoologists mistook for a CNS). It is hypothesized that sometime during the course of evolving into chordates, the common ancestors of chordates and hemichordates developed adhesion molecules that allowed the development of a CNS. This led to the shrinkage the coeloms in the proboscis and allowed the expansion and elaboration of the forebrain, which in turn might have set the stage for the mouth to migrate to the non-neural side of the D/V axis. Since conventionally it is the mouth that determines which of the two sides is dorsal, once the mouth moved to the opposite side, the whole dorsal ventral scheme was reversed.
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