Cuckolds

The solution to theoretical problems requires adequate model objects. Those who are interested in the emergence and evolution of new signs should pay attention to horned beetles. Their horns are certainly evolutionary innovation. This sign cannot be called invisible or insignificant in any way: the horns, which sometimes make up more than 30% of the body weight of the beetle affect its appearance and behavior. The horns are widespread: they are worn by several thousand species of beetles, belonging to at least five families. The horns are diverse in shape and location. There is dimorphism of males, in which only large individuals have horns, and small ones are content with an incomplete kit or completely do without them. There is also sexual dimorphism - females are usually spray.

The variety of horns and their carriers gives scientists a rich material for the study of the mechanisms of the emergence, development and change of the sign, and they achieved significant success in this. Armin P. Moczek, who is now working at the University of Indian, and Douglas J. Emlen, were especially successful. For many years, they have been studying the horned beetles of the plate-eared family (Scarabaeidae)-in this family, in particular, include May beetles, bronzes, beetles-carriers, nasal beetles ... and dung-in-laws, which will be discussed.

The main conclusion of the researchers is that it is not difficult to acquire horns. They grow when a very old genetic mechanism is turned on in the wrong place, usually regulating the development of limbs in insects. Moreover, the activity of some of these genes is changed. However, one mutation is enough, and we get a horn. The horns appear in five areas of the body: three on the head and two in the thoracic region (Fig. 1), but their combination can be any. The horns of beetles are formed at a very late stage of larval development, when the insect is about to turn into a doll. The process takes about 48 hours. At this time, muscle tissue cells in the head or thoracic region die, and the epidermis exfoliates from the external chitin cuticle, its cells are divided and formed one or more outgrowths. They are folded under the cuticle, but when the larvae turns into a doll, the outgrowths will deal and turn into horns. A small change in the work of genes that form the horns so that they change the shape or location on the head, so that the variety of the sign is also easily achievable (Fig. 2).

Many species of beetles are characterized by males of dimorphism: only large beetles have horns, and the larger they are, the longer they have a horn relative to the size of the body (Fig. 3). Small individuals have no horns. It turns out that allometric ratios depend on the power of the larva. This is proven by observations of bugs-dumbfounds, in which the size of the body of the larva depends on the availability of food. In a large horned beetle, only a large, sewn larva turns. If she was starving, then she will be a small, and an adult insect will come out of it small. The work of many development genes is regulated by insulin, the level of which depends on the wealth of nutrients: the more the larva eats, the more insulin produces. A change in the level of insulin or sensitivity to it affects the relative size of wings, legs and horns (but not genitals). Another important factor affecting the formation of horns is a juvenile hormone that determines the development of insects. If on a small larva of Kalo-Bolka Onthophagus Taurus the latest age to act with a metroprene - an analogue of a juvenile hormone, then a beetle from it will turn out to be small, but horned. Armin Mozhki suggested that the synthesis of the juvenile hormone depends on the weight of the larva and only the well -fed individual in the state to ensure the concentration of the hormone sufficient for the formation of horns. If a group of cells changes sensitivity to insulin or juvenile hormone or the timing changes when the level of juvenile hormone in the body of the larva reaches a maximum, this will affect allometric relationships in an adult.

Why do you need dimorphism of males and what factors determine the critical size of the body that allows the beetle to have horns (we call it threshold of horn)? Males use horns for tournament battles. The small insect has no chance of defeating a stronger opponent, so it uses another tactic: it makes its way to places where it can meet the female without protection and tries to mate with it stealthily. From this point of view, horns need only large individuals who have a lot of chances to win in the tournament, and in secret affairs they will be useless or even harmful.

Suddenly it turned out that the threshold of hornedness easily and very quickly changes depending on the conditions of the environment. In the 1960s, the Mediterranean dung beams O. Taurus accidentally hit the USA. They were brought to Australia specifically in 1965-1975, so that they disposed of cow manure, which were refused to eat local lands (Bornemissza, 1976). Over the three decades, the West Australian beetle has significantly increased the threshold of rogatism, despite the fact that the insects there were on average smaller than American ones. In the shortest possible time, differences arose between American and Australian populations, which are usually formed only between species. It turned out that in Western Australia the density of O. Taurus Two orders of magnitude higher than in the USA. With high density, only part of the females is involved in reproduction, and the competition between the males intensifies. The second factor that has aggravated competition in Australia is a lack of manure. In such conditions, only the largest males have chances for winning. The rest of the horns are not needed, and allometric ratios in the population are shifted.

As we recall, the size of the adult beetle is determined by the amount of food that was in the larva. This circumstance does not depend on genes. An experimental audit showed that the larvae of the Australian population pass longer the last stage of development, their horn appears 24–48 hours later than that of American brothers in appearance, and requires a higher concentration of juvenile hormone. So the elementary difference in the habitat of different populations leads to a significant divergence of the sign.

But back to the larvae that turned into horned dolls. At this stage, the horns “ripen”, gain the final shape or resolve, so that a sleeveless beetle may well come out of a horned doll. The horn begins to form in all the larvae of bumps, and then in the smaller development its development turns back. These observations suggested that the ancestors of modern bumps were horned. Consequently, not large males acquired this feature, but small ones - they lost secondly. As for sexual dimorphism, that is, the absence of horns in females, not everything is so unambiguous here. If the horns are crowned with the head, as, for example, in the same O. Taurus , they grow only in males. Obviously, the females of these species were initially (or in the foreseeable past) were rash. The situation is different from the species with horns on the thoracic region, for example , O. Nigriventris : In females in the late stages of development of the larva, a horn grows, comparable in shape and size with horns of small males. However, unlike them, the female at the doll stage loses this beauty and enters into adulthood already with a splendor (Fig. 4a).

The amazing case of horn loss is described for O. Taurus (Fig. 4b). In males of this type of horn on the head, but in the larval stage, both males and females also develop a horn in the front -line, and it is more than in many species wearing it in adulthood. However, at the doll stage, he is safely lost. Perhaps this weapons were worn by the ancestors of O. Taurus And then for some reason they lost.

Scientists are now exploring genetic mechanisms that regulate the development of horns in the doll. The results of these works confirm that different types of horns during evolution arose independently. The beetles in the formation of horns on the chest segment of the body participate in the Aristaless transcription factor ( Al ), but it never regulates the formation of horns on the head. In addition, horns of different localization are distinguished not only by the regulation of their formation, but also by subsequent absorption mechanisms.

It is difficult to study evolutionary events, because we usually see the result, in extreme cases, the final stage of the process, but not its beginning. Horned beetles, in which changes occur literally before our eyes, allow you to explore the process. This is what it means to choose the right model object.

Natalia Reznik

G.​ F.​ Bornemissza (1976). The Australian Dung Beetle Project 1965-75 . Australian Meat Research Committe Review 30: 1-30. DJ Emlen , LC Lavine, B. Ewen-Campen (2007). On the Origin and Evolutionary Diversifcation of Beetle Horns. PNAS 104: 8661 - 8668. AP Moczek (2005). The Evolution and Development of Novel Traits, Or How Beetles Got Their Horns. Bioscience 55: 937 - 951.