A model of pattern formation in living systems is presented. The pattern is achieved by the sequential interaction of two signaling pathways. The coupling of the pattern to the (thick) epithelial sheet changes is given, when the Gauss curvature'K'enters. The model suggests a novel point-like placement for organs such as eyes, antennae, limbs, and wings. In the case of plant patterns, 'leaf' outgrowths from a stem are predicted to be under the control of a yet to be discovered "master regulatiory gene", analogous to the animal case, when various outgrowths from the animal body are generally accompanied by activation of the 'distal-less' (Dll) gene.
Deep Dive into On Pattern and Evolution.
A model of pattern formation in living systems is presented. The pattern is achieved by the sequential interaction of two signaling pathways. The coupling of the pattern to the (thick) epithelial sheet changes is given, when the Gauss curvature’K’enters. The model suggests a novel point-like placement for organs such as eyes, antennae, limbs, and wings. In the case of plant patterns, ’leaf’ outgrowths from a stem are predicted to be under the control of a yet to be discovered “master regulatiory gene”, analogous to the animal case, when various outgrowths from the animal body are generally accompanied by activation of the ‘distal-less’ (Dll) gene.
It has now become clear that virtually all developmental regulatory genes control several different processes, acquiring new developmental roles. Clusters of Hox genes, as well as Pax-6, Dll and Tinman proteins, along with many others, shape the development of animals as diverse as flies and mice. These genes and their proteins are just a part of the collection that make up the genetic 'tool kit' for animal development. Transcription factors are proteins that bind to DNA and directly turn gene transcription on or off, and comprise a large fraction of the regulatory tool kit. The present view is that although developmental regulatory genes are remarkably conserved, their interactions are not (Carroll et al., 2001;Davison, 2001;Wilkins, 2002;Carroll, 2005). The recent impressive progress in unraveling the genomic basis of development and evolution has led to a great advance in understanding of animal development.
However, what is further clear is that elucidation of the actual cell shape changes along with an understanding of the causes of changes in tissue shape and cell number is necessary to obtain a fuller grasp of morphogenesis. Further, such issues as the positioning (patterning) of stem cells, as well as designation of precise positioning of eyes, antennae, legs, wings, gills remain to be addressed by a patterning model.
Communication between cells must play a decisive role in development. Natural selection is the principal influence driving evolution. The argument here is that, acting along with natural selection, are generative ‘rules’, from the very origin of multicellularity, which lead to bias or constraint on natural selection. A number of authors have previously argued that this is the case (Arthur, 2002;Webster and Goodwin, 1996; J.M. Smith et al., 1985). It seems possible or even probable that remnants of the action of such constraint remain today, even after the extensive elaboration of more than 600 million years of evolution. Such elaboration upon the primitive rules would suggest that while such rules have become obscured, they may be still accessible.
Thus we argue that sophisticated eukaryotic cells found a way to form multicellulars before the Cambrian, discovering ‘rules’ that were ‘adaptive’ at that time, and although extensively elaborated since then, have left evidence as to their form and origin. Of interest here is to propose a possible key component in the origin of the metazoan, one still operative today. The rules or model proposed in what follows are no doubt too simplified to be realistic, but it is hoped that they will introduce less familiar concepts, and will act to stimulate further investigation along these lines.
It has become customary to assume that patterning occurs by way of small diffusible molecules. These are assumed to diffuse over long enough distances (say, thirty cells) so that cells at different positions in the resultant gradient somehow read the concentration of the diffusible molecules, allowing cells to determine patterns of gene expression. Such diffusible ‘morphogens’ have long been the standard framework for interpretation of experiments involving pattern formation (Kerszberg and Wolpert, 2007). In fact, the term ‘morphogen’ has even come to specify such long range diffusible molecules, basing the meaning of the very term on a particular model, rather than on its Greek root meaning “shape genesis”.
The gradient model has recently come under careful and sharp experimental criticism; at best, such gradients are not the whole story (Gregor et al., 2007;Kerszberg andWolpert, 1998, 2007;Kornberg and Guha, 2007). It is argued in the present work that long range diffusion is even unnecessary for pattern formation, and only short range diffusion, as short perhaps as nearest neighbor diffusion, can establish most needed patterns. Many of the problems related to morphogen propagation and gradient establishment were recognized long ago by Wilson and Melton (1994).
Among the necessary information that patterns must supply, cells must be able to acquire gradient specification in the plane of the epithelium, that is, acquire planar polarity. Hair cells of the fly wing, for example, are polarized in the plane of the epithelium (Lawrence et al., 2004). Another demanding condition is that boundaries separating determined regions may be specified within a single cell diameter, occurring perhaps during insect segmentation at the antero-posterior parasegment boundaries. Cellcell interactions are then strongly indicated as essential players (Kerzberg and Wolpert, 2007). Also, growth must be included as an integral Aspect of a reasonable patterning model.
Further, Kornberg and Guha (Kornberg and Guha, 2007) have argued convincingly (on the basis of experiments on fly wing imaginal discs) that in the absence of constraining impenetrable physical barriers, gradient-generating dispersion of morphogens cannot be achieved by passive long range diffusion. This method of p
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