for National Geographic News
Dads, in the mammalian branch of the animal kingdom, are often out of the loop when it comes to producing progeny. After that initial contribution of sperm, they are excluded from pregnancy and are all but superfluous even after birth, when nurturing falls to Mom. Now Japanese scientists have streamlined reproduction even further—they have eliminated fathers entirely.
Scientists led by Tomohiro Kono, a biologist at the Tokyo University of Agriculture, have created baby mice without the introduction of sperm. They combined the genetic contents of two mouse eggs—one of which had been genetically altered—to produce a live mouse that reached adulthood and reproduced.
The results are published in the April 22 issue of the journal Nature and could have profound implications for disease and the role of males in reproduction.
Amphibians, fish, and insects are able to reproduce from eggs alone—a process called parthenogenesis. But under normal circumstances mammals, including humans and mice, cannot. They need genetic contributions from mom and dad.
"The goal of our study was to discover why sperm and eggs were required for development in mammals," Kono said.
Imprinting Eggs and Sperm
During normal sexual reproduction, mammals inherit two copies of each gene—one set from mom, the other from dad. It was thought that both components were essential, because a subset of genes, described as "imprinted" genes, behave differently depending on whether they were delivered via sperm or egg. Both are needed to produce healthy offspring.
The Japanese team believed that they could use two eggs to create a viable mouse embryo. The challenge was to get the imprinted genes in one of the eggs to behave as if the genes had come from a sperm.
"Imprints" are chemical additions to the DNA—they don't alter genetic code but block various genes from turning off or on. The critical point is that only one copy of an imprinted gene should be active—if the mother's copy is on, the father's copy is off, or vice versa.
To get genes in one of the two eggs to "act like genes in a sperm," Kono's team had to find a way to switch off the maternal imprints.
Tricking the Embryo
The scientists couldn't use eggs from adult mice since adult eggs are already imprinted. If two such eggs were combined the product would be an embryo with a poorly developed placenta. The embryo would die during early development.
