A Animal That Does Not Have A Head Or A Brain?

(Phys.org) —A fundamental question in the evolution of animal body plans, is where did the head come up from? In animals with a clear centrality of correct-left symmetry, the bilaterians, the head is where the brain is, at the inductive pole of the body. Piddling is known about the possible ancestor of bilaterians. Fortunately their sister grouping from that aforementioned progenitor, the cnidarians, can exist studied in parallel today to give some clues. Cnidarians are creatures like jellyfish, hydra, and body of water anemone which possess rudimentary nervus nets, but no clear brain. They all have just a single orifice to the external world, which basically does it all. In a recent paper published in PLOS Biology, researchers from the University of Bergen in Kingdom of norway compared gene expression patterns in body of water anemone (Nematostella vectensis, Nv) with that from a variety of bilaterian animals. They institute that the head-forming region of bilaterians is actually derived from the aboral, the reverse-oral, side of the bequeathed body plan.

Pioneering developmental biologist Lewis Wolpert, is oft credited with having observed: "It is non nascence, marriage, or decease, but gastrulation which is truly the most important time in your life." Almost all animals undergo a similar gastrulation process early on in their development. The betoken where the cells first invaginate during gastrulation, the blastopore, uniquely defines an embryonic axis. Subsequently this phase however, all bets are off—attempts to ascertain phyla co-ordinate to hardline criteria, similar blastopore = anus, are invariably met by counterexample where information technology instead becomes the mouth. Factor expression, while non ever constrained into single contiguous areas, therefore provides a baggage-costless way to assign homology across species.

Wolpert's concept of positional information in development has been largely vindicated by the discovery of hox gene codes in a wide diverseness of animals. While hox genes are the critical regulators of centric patterning, in most bilaterians they are not expressed in the anterior caput-forming region. The researchers focused instead on the genes six3 and FoxQ2, transcription factors which accept been shown to regulate inductive-posterior evolution. Six3 knockouts in mice, for instance, fail to develop a forebrain. In humans six3 regulates forebrain and eye development.

Sea anemone, like Nematosella, are curious creatures. As larvae they swim about with their aboral pole frontwards. As adults they plunge this region into the ocean flooring, and permanently anchor themselves in. Their bodies then undergo various changes but their oral pole remains intact for feeding. Past using knockdown and rescue experiments in Nemostella, the researchers were able to show that six3 is required for the evolution of the aboral region, and the expression of further regulatory genes. This suggests that the region distal from the cnidarian mouth parallels evolution of the bilaterian head.

The researchers also looked at the expression of the forkhead domain protein foxQ2, which functions downstream of six3. Forkhead box genes are an important class of transcription factors which often lack the signature homeodomains and zinc-finger regions common to other transcription factors. Instead they take a unique Deoxyribonucleic acid-bounden region that has the shape of a winged helix. The forkhead gene, fox2p, in humans has recently garnered a lot of media attention for its apparent role in neural evolution, and in fifty-fifty more esoteric functions like voice communication development.

FoxQ2 is known to be a well-conserved marker for the most anterior tip of a variety of bilaterians including sea urchines, drosophila, and cephalochordates. The researchers established that before gastrulation in cnidarians, foxQ2a was expressed in the aboral pole, and in a small number cells resembling neurons. After the expression of this "band gene" was excluded from a key spot.

In conclusion, the expression of genes for anemone head development, away from the mouth region, suggests that head development came first and was a split outcome from oral fissure development. Secondarily, the head and a coalescing brain appear to have merged to get a centralized control eye.

More data: Sinigaglia C, Busengdal H, Leclère L, Technau U, Rentzsch F (2013) The Bilaterian Head Patterning Gene six3/6 Controls Aboral Domain Development in a Cnidarian. PLoS Biol xi(2): e1001488. doi:ten.1371/periodical.pbio.1001488

Abstract
The origin of the bilaterian head is a cardinal question for the evolution of brute body plans. The head of bilaterians develops at the anterior terminate of their chief trunk axis and is the site where the brain is located. Cnidarians, the sis grouping to bilaterians, lack brain-like structures and information technology is not clear whether the oral, the aboral, or none of the ends of the cnidarian chief body axis corresponds to the anterior domain of bilaterians. In order to understand the evolutionary origin of head development, we analysed the function of conserved genetic regulators of bilaterian anterior development in the sea anemone Nematostella vectensis. We show that orthologs of the bilaterian anterior developmental genes six3/6, foxQ2, and irx have dynamic expression patterns in the aboral region of Nematostella. Functional analyses reveal that NvSix3/half-dozen acts upstream of NvFoxQ2a as a central regulator of the development of a broad aboral territory in Nematostella. NvSix3/6 initiates an autoregulatory feedback loop involving positive and negative regulators of FGF signalling, which subsequently results in the downregulation of NvSix3/six and NvFoxQ2a in a small-scale domain at the aboral pole, from which the apical organ develops. We show that signalling by NvFGFa1 is specifically required for the evolution of the apical organ, whereas NvSix3/half-dozen has an earlier and broader part in the specification of the aboral territory. Our functional and gene expression data advise that the head-forming region of bilaterians is derived from the aboral domain of the cnidarian-bilaterian ancestor.

Synopsys: www.plosbiology.org/article/in … journal.pbio.1001484

Citation: Which came first the head or the encephalon? (2013, March 28) retrieved iv July 2022 from https://phys.org/news/2013-03-brain_1.html

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