Russian and Polish scientists have discovered a mechanism by which hybrid frogs “throw out” half of their genes in order to be able to reproduce. It turns out that the genetic material is gradually removed from the nuclei of the progenitor cells of the ovum and sperm during the development of the tadpole using special structures – micronuclei. An open mechanism can be the key to developing new genome editing techniques. Article published in Scientific Reports. The research was supported by a grant from the Presidential Research Projects Program of the Russian Science Foundation.
“We first described a similar pattern of genetic material removal in vertebrate hybrids. Earlier, the gradual removal of genetic material was found only in plants, ”said Dmitry Dedukh, Ph.D.
Maintaining the integrity of the genetic material is necessary for the normal functioning of cells and the transfer of genes from generation to generation. Any damage to the genome integrity caused by stress factors, such as ionizing radiation, toxic substances, aging, or cancer transformation, usually leads to cell death. However, some living creatures are able to remove a significant part (sometimes up to 90%) of genetic material during development, and at the same time live and reproduce normally. Such organisms were found among interspecific hybrids: the result of the crossing of two parent species. One of the most common examples is the edible frog Pelophylax esculentus, whose legs are popular in French cuisine.
Half of the set of genes of these animals belongs to the lake frog, half to the pond. As a rule, interspecific hybrids are incapable of reproduction, since chromosomes of different species are very different from each other, and are not capable of recombination – the exchange of their regions, necessary for the formation of germ cells. But edible frogs “learned” to manipulate their genome to avoid problems in the formation of germ cells and reproduce normally. The genes of one of the species, that is, half of all the chromosomes of the animal, are completely removed from the precursor cells of egg cells and sperm (the so-called germ line cells). Chromosomes of the second parental species double to restore their original number.
As a result, the exchange of sites takes place without problems, and the germ cells contain the genes of only a pond or lake frog. The transmitted set of genes depends on the habitat: edible frogs maintain their numbers, living with one of the parent species. But how exactly is the removal of half of the genetic material to date was not known.
Scientists from St. Petersburg State University together with colleagues from the University of Wroclaw (Poland) have shown that removal of chromosomes from germ line cells occurs even in tadpoles. They observed the development of a hybrid of artificially fertilized eggs from the appearance of the forelimbs to metamorphosis, weekly analyzing the chromosome set of germ cell precursors. It turned out that at each stage of development in the nuclei of the cells of the germ line, there was less and less genetic material of one of the species, while the second genome was completely preserved. The researchers found that chromosomes are removed through special structures – micronuclei that are present only in the cells of the germ line of hybrid animals, but not the parent species. Each micronucleus contains one chromosome, which indicates their gradual, rather than simultaneous removal.
“Selective chromosome utilization most likely occurs directly during germline cell division. Chromosomes are located in the center, or equator, of the cell to attach to the spindle of division, which distributes them between two daughter cells, but part of the chromosomes belonging to one of the parent species, “lags behind.” Individual chromosomes are not able to attach to the fission spindle and form micronuclei, which later degrade. During each division, one or several chromosomes are excluded, and thus the process occurs gradually, ”Dmitry Dedukh explained.
In the future, scientists plan to find out exactly how chromosomes are recognized, which must be removed or, conversely, left in the cell. Similar mechanisms have been studied on unicellular organisms – ciliates, but they are unknown in vertebrates and are of considerable interest. The conclusions made by scientists will help in the development of new methods for editing the genome. Their use will be useful in agriculture and aquaculture, as well as in the treatment of trisomy – congenital genetic mutations in which a person has an extra chromosome, for example, Down’s syndrome.