For the first time, researchers have directly created mice with two dads.
The mice weren't able to survive for more than a day or so after birth, but coaxing an embryo made from the DNA of two dads all the way through fetal development was no small feat. It was much harder, the researchers found, than making mice with two moms.
The findings help explain why mammals can't reproduce with only a single parent. They also may help explain why those animals that can reproduce alone, such as some amphibians, are almost always female. [10 Amazing Things Scientists Just Did with CRISPR]
The barrier to single-sex reproduction, said senior study author Wei Li, a stem-cell researcher at the Chinese Academy of Sciences, is something called genomic imprinting, or the molecular tagging of DNA that regulates how the instructions in the genome are carried out. But the new study, published online Oct. 11 in the journal Cell Stem Cell, reveals that the barrier can be crossed.
Genomic imprinting is known to be important for embryonic development, but exactly how it works and what controls it is a "black box," said Mellissa Mann, a researcher in reproductive science at the University of Pittsburgh School of Medicine and the Magee-Women's Research Institute who was not involved in the new study.
In sexual reproduction, every offspring inherits half its DNA from its mom, and half from its dad. But the DNA from each parent has unique molecular tags that are added during the formation of the original sperm and egg. These tags are the instructions that dictate whether a gene is expressed or stays dormant.
"You could inherit a copy of a specific gene from mom, and that gene with these molecular 'marks' would enable it to be transcribed to do its function," Mann said. "But the same gene from dad would have different tags, preventing it from being expressed."
These tags have a lot of real-world consequences. They're crucial in early embryonic development, Mann said. Previous studies have found that when scientists try to engineer offspring from two sperm, the embryo fails to develop, while the cells that grow the placenta flourish; offspring from two eggs result in a developing embryo and a placenta that fails to grow. [5 Reasons the Placenta Is Amazing]
There are also genetic disorders that depend on genomic imprinting. Angelman syndrome, a neurological disorder that causes intellectual disability and seizures, is caused by a mutation of the UBE3A gene on the mother's chromosome. When that same gene mutation is passed down by dad instead of mom, the result is a completely different neurological disorder called Prader-Willi syndrome.
It takes two
Some animals are capable of reproducing solo. Most of these cases are when a female reproduces without a male, a process called parthenogenesis. Various fish, reptiles and amphibians can reproduce this way. Only one fish (Danio rerio) is known to reproduce with males alone, and only in certain laboratory conditions, said Baoyang Hu of the Chinese Academy of Sciences, another senior author of the new paper.
The researchers behind the new study wanted to understand why mammals can't reproduce alone. So to start, they set out to see what it takes to create parthenogenesis in mammals. They used mouse stem cells engineered to have only one set of maternal chromosomes, like an egg, and injected them into a normal egg cell to create an embryo with two sets of maternal DNA. They then transferred the resulting embryo into a surrogate mother.
To create a viable embryo, the researchers had to delete three imprinted segments of the genome from the engineered stem cells. The resulting mice appeared normal, though Li told Live Science that the mice were tested for only a limited time under experimental conditions, and so they may not have been as healthy as mice produced the old-fashioned way.
But mice with two moms are nothing new; similar mice with two moms were first produced in 2004, the authors wrote. [How Do Ancestry DNA Tests Really Work?]
So, they turned to a harder problem: Making mice with two dads. That's been done only once, by researchers at M.D. Anderson Cancer Center in Texas. In that case, researchers created male stem cells with X but no Y chromosomes, injected them into female blastocysts, the early stage of development in which some cells have differentiated into specific types. They then let those females develop into adults. They then mated the females carrying only male X-DNA with males, creating offspring that carried only genes from two dads.
This time, the researchers wanted to see if they could make mice with two dads without the intermediary step of a genetically engineered female, Hu told Live Science. To do so, they injected a sperm cell and stem cells with only paternal DNA into an egg cell that had its nucleus (and thus all its maternal DNA) removed. They then extracted stem cells, which are capable of becoming any cell in the body, that developed in the resulting embryo. Those stem cells were then put into a separate blastocyst. The blastocyst was necessary to develop a placenta, Hu said. Finally, this stem-cell-injected blastocyst was put into the uterus of a surrogate mother.
"To our knowledge, it is the first time that bi-paternal mice with two direct fathers have been produced," Hu told Live Science.
But the mice didn't fare well. Indeed, successful births were in the minority. Of 1,023 attempts, only 12 live pups were born. And the pups weren't normal. They were swollen with fluids and more than twice as big as regular mice pups. All of them struggled to nurse and breathe and died soon after birth.
"People are getting ahead of the science with this to say there's a potential that two males could potentially have a child," Mann told Live Science. "This is a very long ways off."
What the study does do, however, is hint at why parthenogenesis is more common in nature than male-only reproduction. The researchers had to delete seven imprinted regions of the genome to make male-only reproduction work, four more than they had to delete for females. (The additional loss of genetic material could explain why the mice with two fathers were so abnormal, Mann said.)
In the future, the method can be used to more closely study the effects of imprinting on various developmental disorders, said study senior author Qi Zhou, also of the Chinese Academy of Sciences. The mice could be used to test genetic modifications that might correct inherited disorders, Zhou said.
Originally published on Live Science.