There is a very special way in sexual reproduction. Some animals do not necessarily need males to participate in reproduction, as long as females lay eggs, that is, they can develop into new individuals without sperm participation. Generally speaking, the offspring directly developed from eggs are female individuals. For some organisms, bisexual reproduction and parthenogenesis often coexist, such as bees and aphids.
But the problem is coming again. Many vertebrates, such as frogs and fish, can parthenogenesis, but mammals cannot. Why?
For higher mammals, only the combination of male sperm and female eggs can produce offspring, which have genetic material from their parents. In order to ensure the diversity of genes, mammals have evolved a genome that can distinguish eggs from sperm, which we call imprinted genes.
Some of our genes are expressed by our fathers and some by our mothers. These genes will make some alleles of fertilized eggs express, and some will not express during fertilization. This genomic imprinting is realized by epigenetic methylation, and the existence of genomic imprinting hinders the realization of parthenogenesis.
From the evolutionary point of view, imprinted genes are a delicate balance between eggs and sperm in the game, which cannot be easily broken.
Since the imprinted gene hinders the realization of parthenogenesis, let's get rid of it. As early as ten years ago, the team of Professor You Hong Heye from Dongguan Nanjing Agricultural University knocked out one of the most powerful female imprinted genes and other genetic elements in female mice according to this idea, so that female mice expressed a powerful male imprinted gene.
The next step is to extract eggs from the modified female mice, and then ripen them with another egg, so some eggs with similar gene expression to those of males are obtained. The experiment went smoothly, and the first batch of parthenogenetic mice were born in human history, but I didn't expect that all these mice had serious developmental defects after birth.
In the subsequent research, scientists finally cultivated stable and healthy mice, but in fact these mice are not really parthenogenetic, after all, they are formed by two eggs, and the probability of genetic defects is very high.
Scientists also want to explore how to really realize parthenogenesis, that is, a mouse born with only one mother and no father.
In addition to violent demolition, scientists also found that DNA methylation can chemically modify DNA and directly change gene expression without changing sequence. If we methylate and demethylate some key genes, it is possible to achieve parthenogenesis.
Therefore, on March 7, 2022, Wei Yanchang and others published relevant research papers in the Proceedings of the National Academy of Sciences (PNAS), which means that human beings have achieved parthenogenesis.
Wei Yanchang's team finally identified seven genes that need to be expressed or silenced in oocytes, two of which are paternal imprinted genes and the rest are maternal imprinted genes. Methylating paternal genes, silencing them, and then demethylating maternal imprinted genes to express them. Mice produced under this condition are not very healthy. Scientists continue to make improvements. They found that the expression level of Rasgrf 1 gene in mice was low, so the mice born after demethylation modification of this gene were healthier and had normal reproductive ability.
Parthenogenesis is not uncommon in the whole nature, but it does not exist in mammals. In order to ensure the diversity of genes, mammals have also evolved opposite imprinted genes, making parthenogenesis impossible.
However, the study of parthenogenesis in mammals is of great significance to science. The essence of scientists is to explore the unknown, which is a process in which human beings constantly know themselves and the universe. Exploring the mystery of reproduction can help us unlock the secret of survival. After understanding our reproduction, we may be able to avoid the dead end of human evolution in the future.