INTRODUCTION
The diversity of life on Earth is breathtaking. Underwater shots of a coral reef amaze the imagination with a variety of shapes, colors and sizes of animals. Even more surprising is that each of these bizarre creatures arises from a single cell - a zygote. Moreover, the zygote of the sea urchin does not have long needles, and the zygote of the great white shark does not have a terrible mouth full of teeth. These signs arise later as a result of the development of the organism. Why does the zygote of a sea urchin eventually turn into a ball of needles, and the zygote of a white shark into the most dangerous predator of the seas? How does a developing embryo decide on which side it will have its head and which side will have its tail? Where is the plan of the future organism stored? These are all questions that developmental genetics is concerned with. One of the most impressive achievements of developmental genetics is the realization of the fact that despite the huge variety of animal forms, the genetic mechanisms of morphogenesis are rather conservative.

EMBRYO COORDINATE SYSTEM
All cells of the body contain the same set of genes and differ only in the set of active and inactive genes. This means that the main task of development can be reduced to ensuring that the necessary genes are activated in specific parts of the body. To do this, the cells must know in which part of the body they are located. To set the location of something, we need a coordinate system. For example, the coordinates of the city of Novosibirsk on the globe can be set as follows: 55 ° N. sh. and 82° E. e. In the body, the position of individual organs can also be set using a coordinate system relative to the main axes of symmetry of the body. Determining the axes of symmetry is one of the very first tasks that the embryo faces in the process of development. It is necessary to decide where the head will form in the future, and where the tail will form (anteroposterior axis), which side the back will be on, which side the stomach will be on (dorsoventral axis). The axes of symmetry of the embryo create a coordinate grid that serves as a guide for the cells. It is convenient to consider how this happens on the example of a Drosophila embryo. The main axes of symmetry in Drosophila are laid during the formation of an egg. Even before fertilization, during egg maturation, transcripts of the bicoid and nanos genes accumulate in it. Moreover, these transcripts are not just evenly distributed throughout the egg, but are localized in a strictly specific way: The RNA of the bicoid gene accumulates in the anterior pole, while the nanos RNA accumulates in the posterior one (Fig. 1). Such segregation is carried out by special proteins that recognize specific sequences in RNA and deliver transcripts to the desired pole along the cytoskeleton, “like on rails”. At the poles, transcripts are anchored in actin filaments. Later, after fertilization, translation of the accumulated transcripts begins and bicoid 104 H.P. proteins appear in the egg. Battulin and nanos, but since all bicoid RNA molecules were in the anterior pole, the maximum concentration of the bicoid protein is observed at the anterior pole, gradually decreasing towards the posterior pole. This is how the bicoid protein gradient is established (Fig. 1). Similarly, a nanos protein gradient is formed at the opposite pole. Thus, primary heterogeneity is created in the egg, now any cell of the embryo can find out at what distance from the anterior and posterior poles it is by the concentration of these proteins. This means that she can decide whether to activate the head development program or the abdomen development program. Here it is worth making a small digression. As mentioned above, determining its location is one of the most important tasks of the cell during development. And you can determine your position only by measuring the distance to any landmark. To measure the distance, special devices are used, a ruler for measuring something small, radars and echo sounders for large distances. In any case, to measure the distance between objects, something must cross the space between them: it is either a physical body (ruler), or a wave (when measured with an echo sounder). In the microscopic world of cells, characteristic distances to landmarks are hundreds of micrometers. Obviously cells cannot use rulers, and cells do not have radio or sound wave receptors. Therefore, the only way available for cells to estimate the distance is to measure the concentration of a specific substance (morphogen) released by the landmark. The farther the cell is from the source of the gradient, the fewer morphogen molecules reach it. Gradients of various morphogens throughout the developmental process serve as a source of information about the spatial position of cells in the developing embryo. In reality, the formation of two differently directed gradients at the poles is only the beginning of the deployment of the embryo's coordinate system. Based on these gradients, new gradients are established that have a different spatial distribution. In Drosophila cells in the anterior half of the embryo, a high concentration of the bicoid protein activates the expression of the hunchback gene, forming another gradient in the embryo. In addition, now all the cells of the anterior half of the embryo are qualitatively different from the cells of the posterior half by the presence of the hunch-back protein. The formed gradients of bicoid, nanos, hunchback and other proteins serve as the basis for further marking of the embryo along the anteroposterior axis. As a result of the work of early development genes, the entire embryo is divided into segments, each of which has a unique set of active genes. This set of morphogens is a signaling substance that is produced in strictly defined areas of the developing organism and determines the path of development of surrounding cells.105 implement the appropriate development program. Using the example of the formation of the anteroposterior axis of the Drosophila embryo, it can be illustrated that the entire individual development of the organism can be divided into separate, very simple, cell behavior programs. A typical program looks like this: "if the concentration of substance A in the cell is X, then activate gene B." A similar program activates the expression of the hunch-back gene in response to a high concentration of bicoid. Programs can be more complex, taking into account the gradients of several substances at once, for example: "if the concentration of substance A in the cell is X, and substances B and C are absent, then activate gene D." On fig. Figure 2 shows a hypothetical example of the implementation of such a program, as a result of which, based on gradients of three substances, the D gene is activated in three regions in the anterior lower quarter of the embryo. If the product of this gene triggered a limb development program, then our hypothetical embryo would develop into a six-legged ant-like animal. Thus, with the help of a relatively small set of simple programs based on several heterogeneities that served as the beginning of the deployment of a network of coordinates, in our example, a complex six-legged animal developed from a simple egg. Of course, the development of any real animal is much more complicated, because hundreds and thousands of such programs of behavior are simultaneously implemented at once. We have not yet said anything about what is the physical basis of the development program, where are all these thousands of programs of cell behavior recorded? Answer: of course, in DNA. All these instructions are regulatory elements of genes - enhancers, silencers, transcription factor landing sites in the promoter regions of the gene, etc. For example, in the above example, the activation of the hunchback gene occurs because there are 7 landing sites in the promoter region of the gene in Drosophila melanogaster for protein bicoid. If there are bicoid protein molecules in the cell, they bind to the landing sites in the promoter of the hunchback gene and activate its expression. In other Diptera, the number of bicoid landing sites in the promoter may differ. Thus, Musca domestica has 10 of them, so the Musca domestica hunchback gene is activated by lower concentrations of the bicoid protein. Such differences in the organization of the regulatory part of developmental genes lead to a slight change throughout the entire development program, since with the same bicoid gradient in Musca domestica, the zone of activity of the hunchback gene turns out to be wider, which means that the gradients built on the basis of hunchback also change their spatial arrangement (Fig. 3). Thus, the main the task of the genes of early development is to establish the axes of symmetry and determine the boundaries of the segments of the embryo. What this or that segment will eventually turn into is determined by another group of genes - Hox genes.

HOX GENES
Many animals are characterized by body segmentation. The brightest are strongly differentiated and differ from each other in structure. The head of insects is formed from several segments, which carry elements of the mouthparts and antennae. 2. Scheme for the implementation of a typical development program.106 N.R. Battuliny, then follows one thoracic segment with a pair of legs and two thoracic segments with legs and wings, followed by abdominal segments without limbs at all. Deciphering the genetic mechanisms for controlling the development of individual segments began with the discovery of mutations in Drosophila that turn one part of the body into another. Such mutations were called homeotic. A classic example of a homeotic mutation is the transformation of the third thoracic segment bearing the haltere into an additional second thoracic segment with one more pair of wings in Ultrabitho-rax (Fig. 4). A typical property of homeotic transformations is a change in the development of an entire segment according to the type of development of the segment ahead. It turned out that all of them encode transcription factors, proteins that regulate the activity of other genes. And they all contain a homeobox, a sequence that encodes a specific DNA-binding domain (homeo-domain). Based on this feature, homeotic genes were grouped into the group of Hox genes. Each Hox gene has its own area of responsibility. For example, the Ultrabithorax gene is active in the third thoracic segment and in all abdominal segments. The Antennapedia gene is active in the second thoracic segment and in all subsequent segments. Thus, it can be said that Antennapedia is responsible for the differentiation of the second thoracic segment, while Ultrabithorax is responsible for the differentiation of the third thoracic segment. In total, 8 Hox genes were found in the Drosophila genome, and they are combined into two genes. complex (Fig. 5). The Antennapedia complex contains 5 genes responsible for the development of Fig. Fig. 3. Scheme of the hunchback gene promoter. Rectangles indicate bicoid landing sites in the promoter. A greater number of landing sites in the promoter of Musca domestica results in gene activation by low concentrations of bicoid (modified from: (Simpson, 2002. P. 907–917)). Hox genes are a group of genes encoding homeodomain-containing transcription factors. Hox genes play an important role in the formation of the anteroposterior axis of the body in all bilaterally symmetrical animals. A homeotic mutation is a mutation in a gene that leads to the transformation of one part of the body into another. Rice. 4. Homeotic mutation of Ultrabithorax in Drosophila.107 Genetics of development of head and thoracic segments; the Bithorax complex contains 3 genes responsible for the development of the last thoracic and abdominal segments. Surprisingly, the order of the genes in the complexes coincides with the sequence of their expression in the segments of the embryo. This property of Hox genes has been called collinearity. The desire of Hox genes to be close to each other is now explained by the existence of a complex system of regulation of coordinated gene expression. This system includes segment-specific enhancers, boundary elements, insulators, and other cis-regulatory elements located in the intergenic regions of the Hox gene complexes. Surprisingly, Hox genes have been found in all animals, from coelenterates to humans. But what are Hox genes responsible for in mammals? At first glance, mammals can hardly be called segmented animals. Nevertheless, for example, let's recall our spine. It consists of 32-34 repeating elements - vertebrae, and in different parts of the spine the vertebrae are different: in the cervical region the vertebrae are small, in the thoracic region they bear ribs, the lumbar vertebrae are the largest, and in the sacral region the vertebrae fuse. One of the tasks of Hox genes in vertebrates is the differentiation of vertebrae. However, this regulation is much more complicated than the regulation of fly segment identity, since mammals have 39 Hox genes combined into 4 complexes (HoxA, HoxB, HoxC, HoxD), which arose from a single ancestral complex as a result of two genome-wide duplications in the course of evolution. branches of vertebrates. Each of the complexes includes from 9 to 11 Hox genes. Homologous genes located in different complexes are called paralogs (Fig. 6). Unlike flies, in which the development of a particular segment is controlled by one Hox gene, in vertebrates, the development of a segment is controlled by 2, and in some cases, 4 paralogs. But, like in Drosophila, the Hox genes of vertebrates exhibit the property of collinearity, i.e., the genes located at the beginning of the complex are responsible for the development of the anterior sections of the spine.For example, the HoxC5 gene is expressed in the last cervical vertebra, and the Hox C6 gene is expressed in the first thoracic vertebra. Due to the presence of 4 complexes of Hox genes in the vertebrate genome, homeotic mutations in them are very rare; make up for the shortage. For example, in mice, the HoxA3 gene is expressed in the first cervical vertebra and marks the skull–neck boundary. The deletion of HoxA3 slightly changes the shape of the vertebra, but does not affect the connection of the skull with the vertebra. HoxA3 has a paralog, the HoxD3 gene. The deletion of HoxD3 leads to more serious consequences: the first cervical vertebra partially fuses with the base of the skull. If, however, the HoxA3 and HoxD3 genes are removed at the same time, then the first cervical vertebra is not formed at all, and the tissue from which it should have arisen is part of the base of the skull. That is, a typical homeotic transformation occurs, and the segment develops according to the type of the anterior segment. Vertebrate Hox genes are not only involved in the development of the axial skeleton, but also participate in the specification of other segmented body structures. The limbs of mammals are composed of several segments. For example, the human arm consists of a proximal section - the shoulder, consisting of one bone, followed by the bones of the forearm, wrist, metacarpus and phalanges of the fingers. Hox genes are involved in the specification of different segments. 5. Drosophila Hox genes and their areas of responsibility. 108 N.R. Battulin extremities. So, HoxD13 is responsible for the development of fingers and metacarpus, HoxD12 - wrists, HoxD11 - forearms, HoxD10 - shoulders. And again, the collinearity of the action of Hox genes is observed: the more distal the section, the more distant member of the cluster controls its development. Another example of the involvement of Hox genes in the specification of body segments is the differentiation of rhombomeres in the rhomboid brain. In adults, the following brain regions develop from rhombomeres: the medulla oblongata, the pons, and the cerebellum. In embryonic development, rhombomeres appear as a series of 8 identical thickenings behind the midbrain. The cells of each of the rhombomeres give rise to different ganglia and neuronal bundles. The very first genes in clusters are involved in the specification of rhombomeres. Thus, the R2 rhombomer is marked by the HoxA2 gene, R3 by HoxB2, R4 by HoxB1, R5-R6 by HoxA3, HoxB3 and HoxD3, R7-R8 by HoxA4, HoxB4 and HoxD4. It is the set of active Hox genes that determines what a particular rhombomere will turn into. As can be seen, in this case there is an example of collinearity violation, since HoxB1 is expressed in the region next to the one in which HoxB2 is active. In vertebrate development, this example is a rare exception. The HoxA9, HoxA10, HoxA11, and HoxA13 genes control the development of the Müllerian duct into various parts of the female reproductive system. HoxA9 is responsible for the formation of the egg-water, HoxA10 for the uterus, HoxA11 for the cervix, HoxA13 for the vagina. Thus, a small group of homeo-box-containing genes - Hox genes - play a key role in the development of animals, determining the specification of individual segments body. In addition, Hox are involved in the differentiation of segments of individual organs.

THE ROLE OF DEVELOPMENTAL GENES IN THE EVOLUTION OF ANIMALS
As mentioned above, Hox genes play an important role in establishing the general body plan of the animal, being responsible for the marking of segments along the anteroposterior axis. It is not surprising that this group of genes is a favorite tool of evolution, since changes in Hox genes allow the creation of new forms. One of the most radical body plan modifications among vertebrates can be
watch snakes. The ancestors of snakes were similar to lizards, but in the process of evolution they lost first the forelimbs, and then the hind limbs. This two-stage scenario is supported by embryological and paleontological data. To date, fossil remains of ancient snakes have been discovered that do not have front legs, but have hind legs. In addition, modern boas and pythons also have small vestiges of hind limbs. In addition to leglessness, the structure of the spine distinguishes snakes from lizards: snakes have a very long thoracic region with vertebrae that carry ribs. The thoracic spine of the snake begins almost immediately behind the head and stretches to the tail. The acquisition of "snake" traits in the process of evolution, apparently, was associated with Hox genes. Thus, the loss of forelimbs in snakes can be explained by a change in the expression pattern of Hox genes in the anterior part of the spine. In most vertebrates, the forelimb rudiments are formed at the anterior border of the HoxC6 gene expression region. Slightly behind this area, the area of expression of the HoxC8 gene begins; coexpression of HoxC6 and HoxC8 leads to the formation of thoracic vertebrae bearing ribs. In addition, HoxC8 suppresses the development of the limb bud, i.e., the limb can form only in the region that expresses HoxC6 and does not express HoxC8. In snakes, the regions of activity of both genes have greatly expanded. As a result, these genes begin to be expressed immediately behind the head and are active up to the tail. And since both genes are active in the same sections, the forelimb bud is not formed, since there is no section in which only HoxC6 would be active. In addition, rib-bearing vertebrae are formed throughout the region of gene activity (Fig. 7). In the given example, evolutionary changes occurred due to a change in the time and place of expression of Hox genes. Often, in the process of evolution, developmental genes acquire new functions that are unusual for them. One of the most beautiful examples of this kind is the involvement of the Antennapedia gene in the development of eyes on butterfly wings. The pattern on the wings is an important element in the evolutionary success of butterflies, since it allows them to avoid predation and can influence the attractiveness of an individual to the opposite ***. In the pattern, individual elements can be distinguished, such as stripes, spots, chevrons, etc. It is interesting that these Fig. 7. Hox genes and snake evolution. (a) Visualization of HoxC6 gene expression in chicken (left) and snake (right) embryos; b – expression scheme of HoxC6 and HoxC8 genes in lizard and snake (modified from: https://www.naturalhistorymag.com/features/061488/the-origins-of-form?page=4)).110 N.R. Battulin elements are not homologous to the pigment elements of other animals, i.e. the wing pattern is an evolutionary innovation of butterflies. Therefore, studies of the genetic mechanisms that control the development of pattern elements on butterfly wings make it possible to understand how animals develop and develop new traits. It was shown that in butterflies of the species Bicyclus anynana, in larvae, zones of activity of the Antennapedia gene appear in the wing imaginal disk (the rudiment from which the wing of an adult butterfly is formed) (Fig. 8) and it is in the places where the gene is activated that eyes are formed. It is worth emphasizing that in butterflies, as in Drosophila, Antennapedia also plays its role in early development, determining the specificity of the second thoracic segment. But in the process of evolution, he acquired a new function. This example is no exception. It is known that in other butterflies, the development of elements of the wing pattern involves the genes Ultrabithorax (Hox-gene involved in the specialization of the third thoracic segment of insects), Distal-less (one of the key genes in the development of limbs, both in insects and vertebrates), hedgehog (one of the most important morphogens). Thus, these examples illustrate the general principle: the emergence of a new trait is not necessarily associated with the emergence of new genes (such cases are rare), more often existing developmental genes change the time or place of expression or acquire additional functions, which leads to the emergence of new traits.

CONCLUSION
In recent decades, many genetic mechanisms that control development have been deciphered. Studies of these mechanisms began on the most deserving genetic object - the fruit fly Drosophila. It was on Drosophila that the fundamental principles of genetic control of development were established. The role of genes in establishing the axes of symmetry of the organism is shown. It has been proven that cells determine their position in space by morphogen gradients. Hox genes discovered. Further studies have shown that many of the principles of genetic control of development discovered in Drosophila are also valid for other animals. For example, Hox genes have been found in all bilaterally symmetrical animals and even in jellyfish. Moreover, in all animals they play a key role in the specification of different parts of the body. It can be said that developmental genetics began with fruit flies, but today developmental genes are being studied on a variety of animals: butterflies and marine worms, sloths and manatees, snakes and birds, lancelets and ascidians. Probably no other branch of genetics has such a variety of objects. Interest in such unusual objects is due to the fact that developmental genetics is an integral part of evolutionary teaching. All signs of any organism arise in the process of individual development, which means that in one way or another they depend on the genes of development. Recent studies have shown that changes in the control of developmental gene expression underlie the emergence and evolution of new forms. Of course, there are still a lot of unresolved questions, for example, there is still no clear answer to the question of how the symmetry axes are formed in mammals. But let's be optimistic: the more questions remain, the more discoveries await us in the future!