He, she, it: how animals choose their gender
Such an unusual way of determining gender, in fact, is very typical of the animal world. Moreover, the division into males and females in nature occurs by more exotic methods.
Before talking about how sex determination occurs in animals, it is necessary to define the term "sex" itself. Sex is a set of morphological, physiological, biochemical and other features of the organism that ensure sexual reproduction, by which we will mean fertilization, that is, the fusion of male and female germ cells (gametes) into a zygote from which a new organism develops.
Familiar
In humans, sex depends only on chromosomes – tightly packed DNA molecules. Chromosomes bring gametes to the future zygote: a sperm from the father's side and an egg from the mother's side. A man or woman grows out of a zygote and does not depend on all chromosomes, but only on sexual ones. In humans, the sex chromosomes are called X (female chromosome) and Y (male). The combination of two X chromosomes determines the female sex. Males carry one X and one Y chromosome in their cells.
This method of sex determination is called chromosomal. It is characteristic of many animals, but not all of them have a female sex determined by a combination of the same chromosomes, and the male – different. A similar scheme with humans "works" in some insects, fish and reptiles. True, the same insects, fish and reptiles (but others) determine the female and male sex in reverse. Another option: females (or males, in different species in different ways) carry two identical chromosomes, and males (or, respectively, females) – the same chromosome, but in the singular. This is how the sex is determined, for example, in a water bug, some butterflies and roundworms.
There are several other possible combinations, but there seem to be enough examples to illustrate the thesis that chromosomal sex determination can be implemented in various ways. It should be noted that in order to ensure the formation of those "morphological, physiological and biochemical features of the organism", different genetic mechanisms work in each of the variants.
Unusual
In search of the optimal option, nature has developed fundamentally different ways of determining gender. In order to ensure maximum adaptability of the organism to the living conditions, she decided to take a direct part in the choice between males and females. For example, in the hatteria mentioned above, at temperatures below 22.1 degrees Celsius, females hatch from eggs, and at temperatures from 22.2 degrees and above, the probability of males appearing sharply increases. In red-eared turtles, on the contrary, with an increase in temperature, the number of males decreases.
"Sex addiction" in vulture turtles and alligators is more complex. At the minimum temperature that the eggs of these animals withstand, they develop mainly females. As the temperature increases, the number of males also increases. However, the growth curve of their number has a maximum. That is, with an increase in temperature, the number of males in the population first increases, and then begins to decrease again.
How does temperature affect the sex of the embryo? The determining role here is played by the balance of sex hormones. During the formation of the body, sex hormones are responsible for the development of sex signs. In many organisms, female hormones are derivatives of male hormones. Certain enzymes are responsible for the transformation. It is their activity that may depend on temperature. Therefore, heating or cooling the eggs of, say, reptiles will have an effect only during a certain period of embryo development – at the stage when there is a "choice" between male and female sex hormones.
The question arises, what happens in animals whose sex depends on temperature, with male and female sex chromosomes? The answer is as follows: they simply do not have sex chromosomes. That is, the production of sex hormones is programmed in non-sex chromosomes, identical in males and females, and sex is determined only by physiological and morphological, not genetic features.
However, it turned out that there are exceptions to this rule. In 2007, Australian scientists discovered that in the bearded agama Pogona vitticeps – lizards living in dry woodlands and rocky semi–deserts of Australia - sex is determined by both chromosomes and temperature. At elevated temperatures, agama eggs hatched mainly females, and some of them had the male genotype ZZ (female agamas carry sex chromosomes ZW). Researchers have suggested that a certain factor encoded in chromosome Z. is responsible for the formation of the male sex. When a lot of it is synthesized (genotype ZZ), male sexual characteristics are formed, when few (genotype ZW) - female. Perhaps, at elevated temperatures, the intensity of synthesis of this factor or its activity in males decreases, and the development of the embryo is directed according to the female type.
By the way, such a sex determination mechanism – dependent on the amount of a certain factor – is characteristic of many insects, for example, the famous Drosophila fly Drosophila melanogaster. Her sex is determined not by the combination of sex chromosomes, but by the ratio of the number of sex chromosomes X (they also have sex chromosomes Y) to the number of non-sex chromosomes. The greater this ratio, the more pronounced the female characteristics. When it is 1:3, a "super male" is formed – an individual with hypertrophied male characteristics, when it is 1:2, a normal male is formed. The ratio of the number of sex chromosomes X to the number of non–sex chromosomes 2:3 gives "supersamok", a ratio equal to 1 – normal females, and intermediate variants between 1:2 and 1 – "intersex" - flies, which have partially developed male and partially female characteristics.
Quite unusual
In some organisms, completely paradoxical, at first glance, events are associated with the definition of sex. For example, the sex of the marine worm Bonellia viridis depends on the meeting of the larva with the female. If the date takes place, the larva will develop into a male parasitic in the uterus of this female. If not, the larva will have to become a female herself.
Sex determination in coral fish Labroides dimidiatus, better known as sponges, or cleaners, also depends on the relationship of males and females. As one of the zoologists who studied these fish put it: "The cherished dream of a female guban is to become a male." The male guban "contains" a harem of several females. In the event of his death, one of them begins to change sex and eventually turns into a male. True, the female cleaners do not turn out to be quite real males: the "reborn" fish turn out to be hermaphrodites.
In the saucer snails Crepidula fornicata, each individual manages to be both male and female during their lifetime. These mollusks live in colonies: individual organisms "sit" on top of each other, forming a kind of pyramid. New individuals are formed in colonies from top to bottom. "Newborn" saucers are always males. Gradually, the male reproductive system of mollusks degrades, and they turn into females. The newly minted female is fertilized by the male formed above her.
Some organisms can change gender "at will". So, sea cucumbers Polycheira rufescens do this several times in their lives. During the breeding season, some individuals become pronounced females, some become males, and some combine both qualities. After fertilization has occurred, most sea cucumbers remain hermaphrodites.
In general, a clear distinction between male and female individuals is not characteristic of all living beings. Many of them prefer not to determine their gender at all. Hermaphroditism is especially characteristic of organisms that are quite low on the evolutionary ladder. Animals that have only one sex include many sponges, coelenterates (jellyfish, coral polyps), many worms, as well as some crayfish and fish.
Hermaphrodites are self-sufficient living organisms: they form both male and female germ cells and they do not need a pair to reproduce.
(From the editorial board of "VM":
Here, the author of a generally good article made a mistake, which we consider necessary to correct. In the animal world, self-fertilization with the so-called synchronous hermaphroditism occurs in helminths, hydra and mollusks, as well as some fish (Rivulus marmoratus). However, since the main advantage of sexual reproduction is the recombination of the genetic material of parental individuals, during the evolution of most species, various mechanisms have been developed to prevent self-fertilization. Cross-fertilization with hermaphroditism is ensured by the simultaneous development of different-named sex glands, the structure of the sexual apparatus, which does not allow the entry of semen into the female organs of the same individual or the impossibility of the formation of a zygote from male and female cells of the same individual, etc.)
There are many more examples of nature's very free treatment of gender. In each case, one or another way of determining it allows the body to adapt to the environmental conditions surrounding it. The creative approach that is being implemented at the same time cannot but cause admiration: in search of the optimal mechanism, nature has tried the most unexpected options. In the course of evolution, transitions from one method to another have been made repeatedly, and perhaps only chance has determined that people are quite definitely divided into men and women and change gender only with the help of surgeons.
Portal "Eternal youth" www.vechnayamolodost.ru03.07.2008