It is commonly known by most people that for fertilize an egg in sexual species -like the human being- and for life to occur, the intervention of a male gamete is necessary, namely, a sperm. However, recently science has revealed that, in some species, this process is not necessary.
In some laboratories fatherless mice have been developed. This is what the biotechnologist assures Juliet Hamzewho researches in the field of reproduction at the University of Murcia (UMU) and is also a Margarita Salas postdoctoral researcher at the INIA-CSIC Animal Genomic Engineering Laboratory, where she studies various processes involved in reproduction through gene editing. fertilization and early embryonic development from a multispecies approach.
Hamze states, in a recent popular science article that he has published, that this milestone has been achieved “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,” explains the UMU researcher.
It has also been seen, according to Hamze, that “the Life expectancy of these mice that have only genes of maternal origin is higher: They live on average 186 days more, that is, 30% more”.
How to activate the egg
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 mother and father,” explains the biotechnologist.
Along these lines, there are various methods to “activate” the egg without the need for a sperm to penetrate itsuch as the addition of chemical agents or the use of small electrical currents, which produce “a similar effect,” in Hamze’s words.
As a result, “a cascade of intracellular activities is generated, which ends up obtaining a ‘partenote’ or ‘gynogenote’, which is nothing other than an embryo that only has 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,” which is the type of cell division that ensures that humans have the same number of chromosomes in each generation. It is a two-step process that reduces the number of chromosomes by half, from 46 to 23, to form sperm and eggs. To complete it, the oocyte needs to be fertilized or activated. “If this second division does not occur, a ‘diploid parthenote’ embryo can be generated.”which means that “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,” explains the UMU researcher.
Regarding the question of 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, a copy of the maternal genome and another of the paternal one”, according to Hamze. The only exception in which parthenotes develop beyond implantation is the aforementioned case of mice, at least for now. Although, “according to some studies, parthenotes are capable of developing in vitro just like classic embryos,” adds the expert.
According to Julieta Hamze, 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, she asserts, ““Oocyte activation is a key process in cloning”. “In addition, 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 made it possible to generate great advances in the field of cell therapy,” highlights the researcher from the University of Murcia.
‘Androgenotes’, embryos that only have paternal genome
‘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. It has been seen that androgenotes have lower development rates than parthenotes or gynogenotes, and the offspring die in the first 48 hours of life. From this it follows that the female gamete is essential for life,” explains UMU researcher Julieta Hamze.