Researchers in China and the US were the first to generate apparently healthy mammals through so-called parthenogenesis, a process in which only the mother’s egg gives rise to offspring, without the need for a father.
The feat, carried out with mice, may have applications in animal husbandry in the future, but its main impact should be on the understanding of the process that goes from fertilization to the development of the adult organism in living beings like us.
The mechanism of “virgin birth” (meaning from the Greek term “parthenogenesis”) is described in detail in an article recently published in the scientific journal PNAS.
The work was coordinated by Yanchang Wei, from the Shanghai Central Laboratory for Assisted Reproduction and Reproductive Genetics, and also involved researchers from the University of California at San Francisco.
One of the parthenogenesis-spawned females even reached adulthood and produced offspring normally, according to the study.
Although the ability to generate offspring without the participation of a male is present, in one form or another, in several groups of invertebrates and vertebrates, nothing like it has been seen in mammals until today. Everything indicates that this is due to the importance of a process such as “imprinting”, or genomic imprinting.
Most living things that reproduce sexually inherit half of their DNA from their mother, while the other half comes from their father. In “imprinting”, there is a biochemical marking of certain stretches of paternal and maternal DNA, causing certain regions to be activated or deactivated (in general, these regions are different, depending on the parent who transmitted them to the new organism).
It’s as if the genes are tagged with a tag, saying something like, “This copy of the gene is of paternal origin; please use only the corresponding version that is in the maternal DNA.”
The main hypothesis used to explain this curious phenomenon has to do with a kind of evolutionary arms race between fathers and mothers in mammal species. As the puppy develops inside the mother’s body, the interests of each of the parents are not exactly the same.
On the paternal side, it is more advantageous for the baby to absorb as much nutrients as possible, so that it has a high chance of having its own offspring in the future. On the maternal side, however, fetuses that are too “greedy” are a risk to the health of the pregnant woman, who could lose her ability to bear more babies in the future or even die.
From this point of view, the standard form of “imprinting” would be a kind of biochemical peace agreement between fathers and mothers, with a compromise between the genetic interests of the two sides. And indeed, there is evidence that when genomic imprinting doesn’t work right, babies can be born with developmental problems or an increased risk of death during pregnancy.
To get around these barriers, Yanchang Wei and his colleagues used molecular biology techniques that work like an eraser, erasing the “imprinting” of genes already known to be important for development, both on the maternal and paternal sides. This was done in mouse eggs, a process of trial and error that eventually worked.
The next step was to unite the eggs with their “sister” cells, the polar bodies, which are formed in the process of cell division that gives rise to the eggs. It was as if fertilization with sperm was imitated by this fusion of female cells from the same female.
The process is extremely inefficient: it took hundreds of eggs for just three parthenogenetic offspring to be born, and only one of the females managed to develop and reproduce. Even so, it is an important achievement, says Lygia da Veiga Pereira, from the National Laboratory of Embryonic Stem Cells, at USP.
“The ability to make multiple, simultaneous and specific changes in oocytes [óvulos] it’s an impressive point,” she says. “The same goes for the fact that altering just eight imprinting centers was enough to resolve the question of maternal and paternal genome equivalence. That’s a small number, given that in mice there are about 150 ‘imprinted’ genes.”
On the other hand, she recalls, the fact that an adult and fertile female was generated does not mean that the cognition or other characteristics of the animal were not altered as a result of the process. It is this drawback that should prevent any possibility of using the technique in human beings for a long time.
