Chromosomes of the grandmother of the boa

Most people are much more interested in sex relations than genetic systems for determining the floor in the embryo - this is the destiny of specialists. But even for half a century they were mistaken regarding snake sex chromosomes. It turned out that the system for determining the floor of pythons and boas differs from the existing in other snakes. Truth was revealed due to the development of DNA analysis methods.

The floor of animals can depend on external factors, such as the ambient temperature or the amount of cytoplasm in cells. But much more often it is determined by sexual chromosomes. There are several systems of chromosomal definition of sex, we are now interested in two. A system in which male cells contain two different sex chromosomes (male gender heterogamous), and female male - two identical ones, is called XY. This is the usual system of mammals. If the female gender is heterogameted, the sex chromosomes are designated by the letters Z and W (the genotype of males ZZ, the WW cells are non -viable).

The evolution of sexual chromosomes occurred independently of different taxa - mammals, reptiles, birds, insects, but in all cases, sex chromosomes were formed from an autosom. At first, genes that determine the floor arose in them, then structural restructuring took place in one of the paired chromosomes, which made a impossible recombination - DNA exchange between homologous chromosomes. According to this model, the sex chromosomes at the beginning of evolution are outwardly similar to each other, but then perestroika, as well as the loss and acquisition of genetic material, lead to the appearance of differences, including in the size and set of genes. The chromosome, which determines the heterogamous floor (Y or W), mainly contains genes responsible for determining the floor, it is smaller in size, and the larger (X or Z) retains many sequences that are of general value. Thus, the sexual chromosome, which determines the heterogamous floor, turns into an extremely stable gene complex.

Different sexual chromosomes are clearly visible against the background of paired autosomes. Such obvious differences allow researchers to quite simply recognize the floor determination system. If different -caliber chromosomes are found in the cells of the male, then the floor determination system in this type is XY, and if in females, then ZW.

However, sexual chromosomes do not always differ. For example, in the lower snakes (Henophidia’s treasure) - pythons and boas - sexual chromosomes are so similar that it is impossible to determine a heterogamous floor on the eye, analyzing the chromosomal set. Strictly speaking, no one really did it. There are many other snakes in which the floor determination system is identified as ZW. And the boas, what, aren't the snakes? Snakes, and even what! So, they have the same.

From time to time, researchers began to study the sex chromosomes of pythons and boas, but they could not confirm the existence of the ZW system. Meanwhile, along with the lack of arguments for the “For”, good arguments “against” began to appear. Texas geneticists Christopher Mallery Jr. and Maria M. Carrillo studied the inheritance of coral coloring in the royal python python regius [1]. As a result of a series of crosses, it was possible to show that a mutation that determines the color of Coral Glow is present on both sexual chromosomes and is inherited, as it should be under the XY system, but not ZW.

Warren Booth, a geneticist and ecologist from the University of Talsa (Oklahoma, USA), has been investigating the optional parthenogenes of vertebrates for several years [2]. But and his colleagues showed that with parthenogenesis, the offspring of only the Horogamet Paul is born. In the case of the ZW system, these are males ZZ, and in the case of XY - females XX. Several types of successes were also among the observation facilities, they exclusively females with an optional partenogeal, which is strange with the ZW floor determination system. If the partenogenetic offspring is hijacking, females of boas should have two identical sex chromosomes, and since the WW individuals are non -viable, this is possible only with a system for determining the XY floor.

To resolve this contradiction, Warren Boot turned to a specialist in the replication chromosomes, Associate Professor of Markett University (Wisconsin, USA) Tony Gamble. He re -read scientific publications and was surprised to find that there was no evidence of the ZW system of the ZW system. And then scientists joined forces [3]. They used new methods of the genome study that allow us to establish a floor determination system in cases where sex chromosomes have no visible differences. Scientists sequenced the sequences of several thousand short fragments of the DNA of males and females and chose fragments among them, which are found only in individuals of the same sex. Such markers should be located on chromosomes Y or W. In the case of a predominance of men's markers, the view has a XY system of floor definitions if female markers prevail-the ZW system.

Researchers worked with the central American imperial boats of the Boa Imperator and the dark tiger pythons of Python Bivittatus , the inhabitants of tropical Asia. As a control, they used the Texas rattles of Crotalus Atrox - the representative of the highest snakes, in whom the system for determining the floor ZW does not cause doubts. Indeed, specific markers at the rat -scientist were found in DNA of female individuals, and in boas - in the DNA of males, which gives the reason to assume a system of determining the XY floor. These results were confirmed for another type, the boas of the ordinary Boa ConstricTor . The sequence of the Y-chromosomes of both species is conservative. As for the pythons, they have a floor determination system, most likely, also XY, but these data should also be checked, since there were few copies at the disposal of the researchers - only three females and four males.

The sequences of the genomes of boas and pythons are known. Scientists used these data to identify sex chromosomes. It turned out that in BOA and pythons they arose independently, from different pairs of autosomes. The lizard of Anolis Carolinensis (North American Red Red Anolis) acted as a hypothetical ancestor. In the highest snakes, sexual chromosomes are homologous with the sixth chromosome of anolis. In boas, the sex chromosomes are homologous with the lizard microchromosome, which is called the clutch group F (Linkage Group F, LGF). As for the pythons, their sex chromosomes XY, like ZW of the highest snakes, are homologous with the sixth chromosome of Anolis. So one pair with autosomes gave rise to different systems for determining the floor. In conclusion, researchers write about what a wonderful model to study the evolution of sex chromosomes are snakes. It can be assumed that the XY system is characteristic of all pythons and boars, and not only three studies studied, but this hypothesis still needs to be tested. Scientists also plan to investigate other types of primitive snakes, including the ferretable Scolecophidia. And Tony Gamble reflects on how often researchers are uncritical of assumptions that are considered to be granted, but are not supported by experimental data. As Sherlock Holmes used to say, nothing is so deceptive as the facts are too obvious.

Natalia Reznik

1. Mallery CS, Carrillo Mm A Case Study of Sex-Linkage in Python Regius (Serpentes: Boidae), With News INTO Sex Determination in Henophidia // Phyllomedusa. 2016. 15. 29–42. DOI: 10.11606/ISSN.2316-9079.V15I1P29-42.

2. Booth W., Schuett Gw the Emering Phylogenetic Pattern of Parthenogenesis in Snakes // Biol. J. Linn. SOC. Lond. 2016. 118. 172–186. DOI: 10.1111/bij.12744.

3. Gamble T., Castoe Ta, Nielsen SV, Banks JL, Card DC, SchuELD DR, SchUETT GW, BOOTH W. The DISCOVERY OF XY SEX Chromosomes in A Boa and Python // Current BioLOGY. 2017. 27. 2148–2153. DOI: 10.1016/J.Cub 2017.06.010.