One of the best-known facts in biology is that, in sexual species like ours, to generate a new living being, a female and a male gamete are necessary. That is, an egg and a sperm.
The theory is simple: during fertilization, the sperm penetrates the egg or oocyte and activates it. The oocyte is the one that has the necessary machinery to carry out the first stages of early embryonic development, until the embryo itself is activated. From that moment on, it is capable of generating its own proteins, encoded by genes inherited from the mother and father.
But what happens if what activates the oocyte is not the sperm?
It is not magic, but it is feasible to achieve in the laboratory. Various methods (addition of chemical agents, use of small electrical currents) produce a similar effect on the oocyte to the penetration of the sperm.
As a result, a cascade of intracellular activities is generated. And in the end, we obtain a “partenote” or “gynogenote”, which is nothing more than an embryo that has only a single copy of the genome inherited from the mother.
On the other hand, when the oocyte is ovulated, it has not completed the second meiotic division . To complete it, it needs to be fertilized or activated. If this second division does not occur, a “diploid parthenote” embryo can be generated. What does this mean? It will have the two copies of the genome that would be expected in a classic embryo… with the difference that both will be of maternal origin.
The million-dollar question is whether these parthenotes that have two copies of the genome of maternal origin can develop and give rise to a living animal. The answer is negative: to obtain offspring, one copy of the maternal genome and another of the paternal genome are necessary. Although, according to some studies , parthenotes are capable of developing in vitro just like classic embryos. Furthermore, in the case of mice, when parthenotes are transferred to the uterus of a female, they develop beyond implantation .
But if parthenotes have the same gene pool as classical embryos (they are diploid, they have two copies of the genome), why does their development stop?
The importance of imprinted genes
In mammals, a phenomenon called “genomic imprinting” occurs that escapes the rules of classical genetics. Normally we have two copies of each gene (alleles), one inherited from the mother and one from the father. These two copies are expressed at the same time, but one dominates the other. There are genes that are not governed by these rules and only one of the two copies is expressed. These genes are said to be imprinted. This means that they have epigenetic marks that cause only one of the two alleles to be expressed. These imprinted genes are essential during early embryonic and placental development.
There is the answer: if a parthenote does not have the copy of the paternal genome, those imprinted genes in which only the paternal copy is expressed will not be present. Therefore, there will be failures in the development of the embryo and the placenta that will make the embryo without a father not viable.
Despite everything stated so far, science has managed to develop fatherless mice.
Thanks to the development of gene editing tools, in recent years it has been possible to obtain offspring from parthenotes, generated from unfertilized eggs. This has been achieved by modifying the expression of some key imprinted genes for development. It has also been seen that the life expectancy of these mice that only have genes of maternal origin is longer. They live on average 186 days longer , that is, 30% longer!
Is the female gamete essential for life?
So far we have only talked about gynogenotes, but “androgenotes” also exist. These are embryos that contain only copies of the paternal genome. In this case, to generate them an egg without a nucleus is needed. Let us remember that the oocyte is the cell that has the necessary machinery for initial embryonic development, until the embryo itself is capable of generating its own proteins.
From this it follows that the female gamete is essential for life.
The study of parthenotes serves not only to understand the function of imprinted genes in the first stages of embryonic development, but is also relevant in many areas of knowledge. For example, oocyte activation is a key process in cloning. Furthermore, various studies have managed to isolate stem cells from parthenogenic embryos and differentiate them into different cell types (such as nerve and muscle cells). This has allowed great advances to be generated in the field of cell therapy.
Author Bio: Julieta G Hamze is a Postdoctoral Researcher in the area of Reproduction at the University of Murcia