Posts tagged sea anemone
Articles and news from the latest research reports.
Which Came First, the Head or the Brain?
The sea anemone, a cnidarian, has no brain. It does have a nervous system, and its body has a clear axis, with a mouth on one side and a basal disk on the other. However, there is no organized collection of neurons comparable to the kind of brain found in bilaterians, animals that have both a bilateral symmetry and a top and bottom. (Most animals except sponges, cnidarians, and a few other phyla are bilaterians.) So an interesting evolutionary question is, which came first, the head or the brain? Do animals such as sea anemones, which lack a brain, have something akin to a head?
In this issue of PLOS Biology, Chiara Sinigaglia and colleagues report that at least some developmental pathways seen in cnidarians share a common lineage with head and brain development in bilaterians. It might seem intuitive to expect to find genes involved in brain development around the mouth of the anemone, and previous work has suggested that the oral region in cnidarians corresponds to the head region of bilaterians. However, there has been debate over whether the oral or aboral pole of cnidarians is analogous to the anterior pole of bilaterians. At the start of its life cycle a sea anemone exists as a free swimming planula, which then attaches to a surface and becomes a sea anemone. That free-swimming phase contains an apical tuft, a sensory structure at the front of the swimming animal’s body. The apical tuft is the part that attaches and becomes the aboral pole (the part distal from the mouth) of the adult anemone.
To test whether genetic expression in the aboral pole of cnidarians does in fact resemble the head patterning seen in bilaterians, the researchers analyzed gene expression in Nematostella vectensis, a sea anemone found in estuaries and bays. They focused on the six3 and FoxQ2 transcription factors, as these genes are known to regulate development of the anterior-posterior axis in bilaterian species. (six3 knockout mice, for example, fail to develop a forebrain, and in humans, six3 is known to regulate the development of forebrain and eyes.)
The N. vectensis genome contains one gene from the six3/6 group and four foxQ2 genes. Sinigaglia and colleagues found that Nvsix3/6 and one of the foxQ2 genes, NvFoxQ2a, were expressed predominantly on the aboral pole of the developing cnidarian but, after gastrulation, were excluded from a small spot in that region (NvSix3/6 was also expressed in a small number of other cells of the planula that resembled neurons). Because of this, the authors call NvSix3/6 and NvFoQ2a “ring genes”, and genes that are then expressed in that spot “spot genes.” The spot then develops into the apical tuft.
Through knockdown and rescue experiments, the researchers demonstrate that NvSix3/6 is required for the development of the aboral region; without it, the expression of spot genes is reduced or eliminated and the apical tuft of the planula doesn’t form. This suggests that development of the region distal from the cnidarian mouth appears to parallel the development of the bilaterian head.
This research demonstrates that at least a subset of the genes that cause head and brain formation in bilaterians are also differentially expressed in the aboral region of the sea urchin. The expression patterns are not identical to those in all bilaterians; however, the similarities suggest that the patterns of gene expression arose in an ancestor common to bilaterians and cnidarians, and that the process was then modified in bilaterians to produce a brain. So to answer the evolutionary question posed above, it seems that the developmental module that produces a head came first.
Ion selectivity in neuronal signaling channels evolved twice in animals
July 26, 2012
Excitation of neurons depends on the selected influx of certain ions, namely sodium, calcium and potassium through specific channels. Obviously, these channels were crucial for the evolution of nervous systems in animals. How such channels could have evolved their selectivity has been a puzzle until now. Yehu Moran and Ulrich Technau from the University of Vienna together with Scientists from Tel Aviv University and the Woods Hole Oceanographic Institution (USA) have now revealed that voltage-gated sodium channels, which are responsible for neuronal signaling in the nerves of animals, evolved twice in higher and lower animals. These results were published in Cell Reports.
Close-up of nervous system of a transgenic polyp of the sea anemone Nematostella vectensis, in which a red fluorescent reporter gene (mCherry) is driven by the regulatory sequence of the neuronal ELAV gene. The picture shows the diffuse structure of the nervous system, but also reveals the accumulation of longitudinal axonal tracts along the eight gastric tissue folds (mesenteries). Credit: Copyright: U. Technau
The opening and closing of ion channels enable flow of ions that constitute the electrical signaling in all nervous systems. Every thought we have or every move we make is the result of the highly accurate opening and closing of numerous ion channels. Whereas the channels of most lower animals and their unicellular relatives cannot discern between sodium and calcium ions, those of higher animals are highly specific for sodium, a characteristic that is important for fast and accurate signaling in complex nervous system.
Surprising results in sea anemones and jellyfish
However, the researchers found that a group of basal animals with simple nerve nets including sea anemones and jellyfish also possess voltage-gated sodium channels, which differ from those found in higher animals, yet show the same selectivity for sodium. Since cnidarians separated from the rest of the animals more than 600 million years ago, these findings suggest that the channels of both cnidarians and higher animals originated independently twice, from ancient non-selective channels which also transmit calcium.
Since many other processes of internal cell signaling are highly dependent on calcium ions, the use of non-selective ion channels in neurons would accidently trigger various signaling systems inside the cells and will cause damage. The evolution of selectivity for sodium ions is therefore considered as an important step in the evolution of nervous systems with fast transmission. This study shows that different parts of the channel changed in a convergent manner during the evolution of cnidarians and higher animals in order to perform the same task, namely to select for sodium ions.
This demonstrates that important components for the functional nervous systems evolved twice in basal and higher animals, which suggests that more complex nervous systems that rely on such ion-selective channels could have also evolved twice independently.
Source: PHYS.ORG