In 1995, a boy made medical history when scientists discovered that his birth was involved in parthenogenesis, the process that occurs when offspring result from an unfertilized egg. More than two decades later, researchers are still unraveling the mystery of parthenogenesis.
Mouse pups without sex or sperm
Now, a new study has managed to reveal part of this enigma by verifying that it can occur in mammals, at least under laboratory conditions . Who needs a male when the egg can spontaneously develop into an embryo? A team of scientists from Shanghai Jiao Tong University (China) has managed to produce fatherless mice, without any male genetic DNA, using only unfertilized mouse eggs . Some fish, reptiles, amphibians, scorpions or bees have offspring in this way, but it was thought that in mammals it was impossible.
How have they achieved it?
They added DNA that is ejected in the early stages of egg development and used the CRISPR gene-editing technique to target seven imprinted gene regions previously identified as important in embryo development and change epigenetic imprinting. Targeting these sites requires the precise addition or removal of methyl groups while leaving the underlying DNA code unchanged, a process that has so far proven difficult. The result is that of the two females used in the experiment, the second copy of the mother’s genetic code ‘trickled’ the ovum as if it were a male in order for it to become an embryo. So it was.
Epigenetic rewriting
“After parthenogenetic activation, these edited regions showed maintenance of methylation as naturally established regions during early preimplantation development,” the authors wrote in their paper published in the Proceedings of the National Academy of Sciences.
This method does not change the actual DNA of an animal, but instead alters gene expression and activity, to create unfertilized parthenogenetic embryos without male sperm. The researchers compared the development of the parthenogenetic animals with a control group of mice.
They edited 227 unfertilized ovules, which ultimately resulted in 192 embryos. After transferring the embryos to female mice, only 14 of these embryos reached term; although, finally, only three survived. Of these three surviving mice, only one parthenogenetic mouse managed to reach adulthood, which then successfully bred, with a male, to create viable offspring.
“These data demonstrate that parthenogenesis can be achieved by specific epigenetic rewriting of multiple critical imprinting control regions,” the experts continue.
As only one of the pups survived to adulthood, the success rate would need to be improved . Further research will help clarify why more offspring did not survive and how to successfully edit parthenogenetic mammalian embryos to better ensure their survival.
But the mouse born of parthenogenesis had its own completely normal offspring, demonstrating the first case of parthenogenesis from an unfertilized mammalian egg; results that could have even broader applications in medical research, agriculture and much more.
More research will be needed to understand whether parthenogenesis will ever be possible in humans.
Referencia: Yanchang Wei et al, Viable offspring derived from single unfertilized mammalian oocytes, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2115248119
