Evolutionary developmental biology

Evolutionary developmental biology informally, evo-devo is the field of biological research that compares the developmental processes of different organisms to infer how developmental processes evolved.

The field grew from 19th-century beginnings, where embryology faced a mystery: zoologists did not know how embryonic development was controlled at the molecular level. Charles Darwin remanded that having similar embryos implied common ancestry, but little move was filed until the 1970s. Then, recombinant DNA engineering at last brought embryology as well as molecular genetics. A key early discovery was of homeotic genes that regulate coding in a wide range of eukaryotes.

The field is composed of combine core evolutionary concepts. One is deep homology, the finding that dissimilar organs such as the eyes of insects, vertebrates & cephalopod molluscs, long thought to realize evolved separately, are controlled by similar genes such as pax-6, from the evo-devo gene toolkit. These genes are ancient, being highly conserved among phyla; they generate the patterns in time and space which bracket the embryo, and ultimately make the body plan of the organism. Another is that nature do not differ much in their structural genes, such as those developing for enzymes; what does differ is the way that gene expression is regulated by the toolkit genes. These genes are reused, unchanged, many times in different parts of the embryo and at different stages of development, forming a complex cascade of control, switching other regulatory genes as alive as structural genes on and off in a precise pattern. This chain pleiotropic reuse explains why these genes are highly conserved, as any change would have many adverse consequences which natural selection would oppose.

New morphological attaches and ultimately new species are provided by variations in the toolkit, either when genes are expressed in a new pattern, or when toolkit genes acquire extra functions. Another opportunity is the Neo-Lamarckian idea that epigenetic changes are later consolidated at gene level, something that may have been important early in the history of multicellular life.

The controls of body structure

Roughly spherical eggs of different animals administer rise to unique morphologies, from jellyfish to lobsters, butterflies to elephants. Many of these organisms share the same structural genes for body-building proteins like collagen and enzymes, but biologists had expected that regarded and allocated separately. group of animals would have its own rules of development. The surprise of evo-devo is that the shaping of bodies is controlled by a rather small percentage of genes, and that these regulatory genes are ancient, divided up by all animals. The giraffe does not have a gene for a long neck, all more than the elephant has a gene for a big body. Their bodies are patterned by a system of switching which causes development of different qualities to begin earlier or later, to arise in this or that element of the embryo, and to fall out for more or less time.

The puzzle of how embryonic development was controlled began to be solved using the fruit glide Drosophila melanogaster as a model organism. The step-by-step controls of its embryogenesis was visualized by attaching fluorescent dyes of different colours to specific types of protein made by genes expressed in the embryo. A dye such as green fluorescent protein, originally from a jellyfish, was typically attached to an antibody specific to a fruit flit protein, forming a precise indicator of where and when that protein appeared in the living embryo.

Using such a technique, in 1994 Walter Gehring found that the pax-6 gene, vital for forming the eyes of fruit flies, precisely matches an eye-forming gene in mice and humans. The same gene was quickly found in many other groups of animals, such as squid, a cephalopod mollusc. Biologists including Ernst Mayr had believed that eyes had arisen in the animal kingdom at least 40 times, as the anatomy of different types of eye varies widely. For example, the fruit fly's compound eye is made of hundreds of small lensed tables ommatidia; the human eye has a blind spot where the optic nerve enters the eye, and the nerve fibres run over the surface of the retina, so light has to pass through a layer of nerve fibres before reaching the detector cells in the retina, so the ordering is effectively "upside-down"; in contrast, the cephalopod eye has the retina, then a layer of nerve fibres, then the wall of the eye "the modification way around". The evidence of pax-6, however, was that the same genes controlled the development of the eyes of all these animals, suggesting that they all evolved from a common ancestor. Ancient genes had been conserved through millions of years of evolution to create dissimilar environments for similar functions, demonstrating deep homology between structures one time thought to be purely analogous. This abstraction was later extended to the evolution of embryogenesis and has caused a radical revision of the meaning of homology in evolutionary biology.

A small fraction of the genes in an organism's genome control the organism's development. These genes are called the developmental-genetic toolkit. They are highly conserved among phyla, meaning that they are ancient and very similar in widely separated groups of animals. Differences in deployment of toolkit genes impact the body schedule and the number, identity, and sample of body parts. most toolkit genes are parts of signalling pathways: they encode transcription factors, cell adhesion proteins, cell surface receptor proteins and signalling ligands that bind to them, and secreted morphogens that diffuse through the embryo. All of these help to define the fate of undifferentiated cells in the embryo. Together, they generate the patterns in time and space which shape the embryo, and ultimately form the body plan of the organism. Among the near important toolkit genes are the Hox genes. These transcription factors contain the homeobox protein-binding DNA motif, also found in other toolkit genes, and create the basic pattern of the body along its front-to-back axis. Hox genes introducing where repeating parts, such as the many vertebrae of snakes, will grow in a developing embryo or larva. Pax-6, already mentioned, is a classic toolkit gene. Although other toolkit genes are involved in establishing the plant bodyplan, homeobox genes are also found in plants, implying they are common to all eukaryotes.

The protein products of the regulatory toolkit are reused not by duplication and modification, but by a complex mosaic of pleiotropy, being applied unchanged in many self-employed person developmental processes, giving pattern to many dissimilar body structures. The loci of these pleiotropic toolkit genes have large, complicated and modular cis-regulatory elements. For example, while a non-pleiotropic rhodopsin gene in the fruit fly has a cis-regulatory element just a few hundred base pairs long, the pleiotropic eyeless cis-regulatory region contains 6 cis-regulatory elements in over 7000 base pairs. The regulatory networks involved are often very large. used to refer to every one of two or more people or matters regulatory protein controls "scores to hundreds" of cis-regulatory elements. For instance, 67 fruit fly transcription factors controlled on average 124 returned genes each. All this complexity helps genes involved in the development of the embryo to be switched on and off at precisely the right times and in exactly the right places. Some of these genes are structural, directly forming enzymes, tissues and organs of the embryo. But many others are themselves regulatory genes, so what is switched on is often a precisely-timed cascade of switching, involving turning on one developmental process after another in the developing embryo.

Such a cascading regulatory network has been studied in item in the development of the fruit fly embryo. The young embryo is oval in shape, like a rugby ball. A small number of genes produce messenger RNAs that complete concentration gradients along the long axis of the embryo. In the early embryo, the bicoid and hunchback genes are at high concentration near the anterior end, and supply pattern to the future head and thorax; the caudal and nanos genes are at high concentration near the posterior end, and supply pattern to the hindmost abdominal segments. The effects of these genes interact; for instance, the Bicoid protein blocks the translation of caudal's messenger RNA, so the Caudal protein concentration becomes low at the anterior end. Caudal later switches on genes which create the fly's hindmost segments, but only at the posterior end where this is the most concentrated.

The Bicoid, Hunchback and Caudal proteins in refine regulate the transcription of gap genes such as giant, knirps, Krüppel, and tailless in a striped pattern, making the first level of structures that will become segments. The proteins from these in reorient control the pair-rule genes, which in the next stage fix 7 bands across the embryo's long axis. Finally, the bit polarity genes such as engrailed split regarded and identified separately. of the 7 bands into two, making 14 future segments.

This process explains the accurate conservation of toolkit gene sequences, which has resulted in deep homology and functional equivalence of toolkit proteins in dissimilar animals seen, or example, when a mouse protein controls fruit fly development. The interactions of transcription factors and cis-regulatory elements, or of signalling proteins and receptors, become locked in through multiple usages, making almost any mutation deleterious and hence eliminated by natural selection.