Parthenogenesis: undivided motherhood
But there is a way back. Sometimes, for various reasons, a female individual, giving birth to offspring, manages, so to speak, without a dad ... This phenomenon has received the name "parthenogenesis" in scientific usage - from the two Greek words "parthenos" (virgo) and "genesis" (generation). Generally speaking, we have known since childhood that new living organisms may well arise without any stamens and pistils. A strawberry bush, for example, scattering its long mustache in the garden, will grow offspring in the form of exactly the same bushes. A branch stuck into the ground (the fashionable word "clone" is Greek for "stalk") will turn into a new tree. The historically preceding asexual reproduction is based on the process of mitosis - a simple division of a living cell. As a result, two absolutely identical cells with the same set of genes are formed - exact copies of the parent, preserving, figuratively speaking, all the advantages and vices. Changes from generation to generation can only occur as a result of genetic mutations. True, microbes perfectly adapt to changing environmental conditions, but they take the number and speed of reproduction. And even they have something like a sexual process - the exchange of genes between cells of the same, and sometimes completely different species. And in the vast majority of plants and the most primitive representatives of the animal kingdom (like earthworms and starfish), capable of vegetative propagation, it complements, but does not replace, the sexual process.
Game of nature
During sexual reproduction, a drawing of a new organism, embedded in its DNA, is created by a random combination of genetic material from two parents. Dice thrown on the table. What combination will come out in the end - increasing or decreasing the chances of winning in the struggle for existence - is a matter of chance, but without the constant shuffling of genes and the selection of their optimal combinations, complex multicellular organisms could not have evolved. If we continue the association with gambling, the gene of eukaryotic (having a cell nucleus) organisms, from yeast to humans, is made up of two “decks” - paired genes (alleles) located on paired chromosomes. Somatic (body) cells during growth and replacement of dead cells multiply by the same simple division - mitosis, in which a set of chromosomes is transmitted unchanged from the parent cell to the daughter. Mutations in somatic cells can lead to various (usually unpleasant) consequences, but they are not transmitted to the next generation.
Two moms - one dad
For experiments with species of reproduction of living beings, nature has been almost an eternity. Today's masters of genetic engineering are trying to meet deadlines by offering their sensational solutions.
Researchers from the University of Newcastle (UK) announced that they have found a way to eradicate a number of serious diseases transmitted by inheritance through mitochondria. Mitochondria is one of the most important organoids of a living cell, which is responsible for the energy supply of intracellular processes. It is believed that mitochondria comes from an ancient bacterium that once rooted in a eukaryotic (nuclear) cell. The organoid inherited its own DNA from its progenitor. If malfunctions occur in it, the mitochondria does not function correctly, which affects the life of the whole cell. To cope with this problem, Scottish scientists suggest transplanting the core of a fertilized egg with mitochondrial defects into a healthy egg from a donor. Previously, all genetic information is removed from the donor cell, except for that which is responsible for the production of mitochondria. Thus, a fetus will arise in which the genetic material of two women and one man will be present. Experiments on mice show that the remains of donor DNA in no way affect the development of the cub and only help to relieve it of diseases associated with mitochondria. Using this method, it was possible to obtain human embryos, which normally developed, but were destroyed on the sixth day after fertilization. Legal and bioethical considerations still hinder going further in this direction.
Germ cells are formed as a result of a much more complex division process - meiosis, in which oocytes and spermatozoa form respectively from primary germ cells - oogonia (female) and spermatogonia (male). In this case, the diploid (carrying a complete set of genetic information in two pairs of chromosomes) the primary germ cell turns into a haploid cell, with one of each pair of chromosomes and, accordingly, one of each pair of parental genes. In this case, chromosomes exchange sites, and each egg or sperm gets a random set of genes received from the grandparents of the unborn baby.
Two germ cells merge into one - the zygote, which some time later begins to divide in the mitotic way, developing into an embryo. But an unfertilized egg can begin to divide - this is what is called parthenogenesis. It should be immediately clarified: parthenogenesis is not asexual reproduction, but a kind of sexual (with its inherent biological processes), but with the participation of only female germ cells.
Parthenogenesis in wildlife cannot be called something exceptional. Rotifers - tiny (from 40 microns to 2 mm) inhabitants of freshwater reservoirs, allocated in a separate type of animal kingdom - for 40 million years are represented only by females, producing offspring exclusively through parthenogenesis.
Despite the progressiveness of sexual reproduction, the variant with the origin of offspring from one individual has its advantages. For example, when the environment favors the rapid propagation of the species and there is enough food around for numerous offspring, parthenogenesis gives a gain in the rate of population of this environment. In this case, genetic diversity can be sacrificed (the offspring carries only the mother’s chromosomes), but mobilize the potential of the species to perform only maternal functions. As soon as the conditions change in an unfavorable direction, one can return to fertilization again, creating less numerous, but more adapted organisms. But rotifers are more likely an exception to the rule. In many plant species, arthropods, amphibians, reptiles, and even birds, there is not an obligate (obligatory) form of parthenogenesis, but an optional one under suitable circumstances. For example, in some species of aphids, the transition to parthenogenesis and vice versa is seasonal in nature.
It is surprising, however, that sometimes species of living organisms that have not previously been noticed in this resort to parthenogenesis. In recent years, several striking cases of parthenogenesis in sharks have been described, for which this method of reproduction is usually not typical. In 2001, at the Henry Doorly Zoo in Nebraska (USA), a small-headed hammerhead fish (a species of shark) gave birth to a baby after a long stay in a water tank where there were no males. This "immaculate conception" at first confused scientists. Among others, an option was considered with long-term preservation of sperm from long-term sexual intercourse - such a phenomenon of "false parthenogenesis" is sometimes observed in nature. An accident helped to dot the “i”: a grown cub died from a stingray injection. The result of DNA analysis clearly showed that in the cells of the cub there was no genetic material other than maternal. A certain program hidden in shark DNA, which in vivo reproduces exclusively through fertilization, has included a reserve mechanism for the conservation of the species - parthenogenesis. Thus, the absence of males may become the cause of parthenogenetic reproduction, for example, at the border of the species range.
A similar case occurred in 2002 at the Detroit Aquarium (USA), and then in Hungary. In 2006, a parthenogenetic cub hatched from a female Komodo lizard female in a London zoo. Outside, Komodo lizards also do not resort to parthenogenesis.
In discussions about parthenogenesis, the theme of one of the main tenets of Christianity - the Immaculate Conception of the Virgin Mary - inevitably emerges. Do the gospel traditions store evidence of parthenogenetic birth of a person? Opponents of this version point out that, if so, the baby Jesus should have been born as a girl - of course, due to the lack of Y chromosomes in the egg. It is difficult to seriously discuss this topic, because if you take a religious point of view, you need to remember about participating in the immaculate conception of the Holy Spirit, to whom the impossible is subject to his omnipotence. However, if the intervention of supernatural forces is neglected, then not only humans, but also any other mammal, are incapable of “immaculate conception”. On the path to parthenogenesis of man by nature, a powerful barrier was erected, whose name is genomic imprinting.
Half to half
Both sharks and monitor lizards had parthenogenetic calves.
This is natural for living organisms in which the male sex is transmitted through the Y-chromosome located only in male germ cells. In some species of living creatures, sex is inherited differently: for example, in one of the turkey breeds, many eggs develop parthenogenetically, and only males appear from them. In bees and ants, parthenogenesis is used to control the sex ratio: females develop from fertilized eggs, and males develop from unfertilized eggs. And some species of bony fish “seduce” males of another species. The sperm does not penetrate the egg, but only stimulates the doubling of its chromosomes and division. However, can we say that these cubs were clones, an exact genetic copy of maternal organisms? No, in this case it is not.
The fact is that biologically parthenogenesis is carried out mainly in two ways. One of them is that the primary female reproductive cell, without going through the stage of meiosis, begins to share the mitotic pathway, creating its own copy. In the case of highly developed animals, an egg obtained during meiosis, which, as we recall, has a haploid - half - set of chromosomes, merges with another egg, also with a half, but differently combined set of "grandmother and grandfather" chromosomes. A kind of imitation of conception occurs, but the genotype and the properties of the organism that it determines will be different to one degree or another from the mother.
The meaning of this sophisticated term is that for a developing embryo of a mammal, figuratively speaking, it is not indifferent to whom the gene came from — from mom or dad. The gene responsible for the development of some vital vital organ simply will not work, manifest itself if it has the wrong sexual marker. That is why, even if the mammalian egg is forced to share, say, with the help of some external stimuli, there is no chance that a viable organism will appear in the light. Genomic imprinting will block embryo development in the early stages. Unless, of course, genetic engineering intervenes.
The first parthenogenetic birth of mammals was achieved in 2004 by scientists from Tokyo Agricultural University. The Japanese applied the technology of haploidization they developed, that is, the artificial (without meiosis) transformation of somatic cells of a female mouse into haploid (similar to either male or female). Then, under laboratory conditions, it was possible to achieve the fusion of these cells, “deceiving” genomic imprinting using special technologies. And finally, already in the maternal body, the embryo began to develop from the cell.
About how hard it was for geneticists to intervene in the holy of holies of wildlife, figures say. After conducting about six hundred experiments with the fusion of artificially haploidized cells, the Japanese were able to get only 24 pregnancies, only two of which ended in childbirth. Only one cub managed to develop into a full-fledged organism. However, to begin with, the result is not so bad: the unforgettable Dolly at the stage of fertilization had almost three hundred sisters.
Cloning of primates due to the peculiarities of the development of their fertilized eggs during the very first divisions is still technically impossible. And not a single serious scientist set himself the task of reproductive human cloning. Numerous attempts to learn how to grow human embryos by the method of transferring the cell nucleus - the very one with which Dolly was born - are needed for therapeutic cloning. At the same time, as in the case of obtaining cultures of embryonic stem cells from the “waste” of in vitro fertilization (fertilized as a reserve of eggs), the embryo from an egg with a transplanted donor nucleus is supposed to be destroyed at an early stage. Such cells will not be rejected during cell therapy and transplantation of donated tissues and even whole organs to the donor.
But work with ordinary cells of human embryos, and any, even therapeutic, cloning of human cells cause resistance from religious fundamentalists and other guardians of morality. This is one of the reasons why some experts tried to go around the road - for example, to get chimeric embryos from the nucleus of a human somatic cell and the egg of a cow or rabbit.
All these roundabout paths led to deadlocks, except for one: in July 2007, a group of scientists from the Moscow Center for Obstetrics, Gynecology and Perinatology of the Russian Academy of Medical Sciences and the American corporation Lifeline Cell Technology managed to grow six lines of polypotent (capable, like embryonic, turn into any body tissues) stem cells from unfertilized human eggs. The achievement is impressive, although with this method it is possible to obtain cultures of therapeutic cells only for women of childbearing age. And it seems that this direction will not develop.
Already in November of the same 2007, two groups of scientists from the University of Wisconsin and Kyoto simultaneously announced the development of methods for producing artificially induced pluripotent cells (iPSC) from fibroblasts, ordinary skin cells. In order for adult cells to “fall into childhood”, they had to use working virus carriers to introduce working copies of four genes that were active during embryonic development and were blocked in the adult body. The new technique has already been tested in new laboratories, and in February 2009, human iPSCs were first used to repair the damaged spinal cord - so far in mice, but it is hoped that in a few years it will reach the first clinical studies in humans.
Perhaps these experiments will put an end to bioethical debate, leaving the topic of parthenogenesis to the authors of science fiction novels that portend the emergence of a world without men. Fortunately, it does not seem that women have longed for such a world.
Thank you for your help in writing an article by Alexander Chubenko, editor of the portal “ Eternal Youth ”The article was published in the journal Popular Mechanics (No. 3, March 2009).