The 20th anniversary of cloning Dolly the sheep has marked two decades of scientific advancement but many cloned embryos still fail to make it to term. Researchers from the U.S. and France studied gene expression in developing clones and they've discovered that majority of losses are due to embryonic death, a failure during the implantation process, or the development of a defective placenta.

Harry Lewin from UC Davis Department of Evolution and Ecology used RNA sequencing to study the gene expression in cloned cows during implantation in to understand the molecular mechanisms that resulted in a high rate of pregnancy failure for clones since the survival rate is at most 10 percent. A result of a 12-year collaboration, the study published in the journal Proceedings of the National Academy of Sciences was able to combine the French team's expertise in cloning and reproductive biology with the U.S. team's expertise in functional genomics.

Lewin stated, "The study has resulted in the redefinition of our understanding of how nuclear reprogramming affects gene expression in extraembryonic tissues of cloned cattle embryos and the exquisite communication between clones and their recipient mothers."

Olivier Sandra, the team leader for the study at the Institut National de la Recherche Agronomique in France, believes the team has made a significant discovery. "The large amount of data our collaboration has generated sheds light on mechanisms that account for embryonic losses at implantation. They also provide new insights on how events taking place at implantation drive the progression of pregnancy and shape the post-natal phenotype of the progeny, in cattle as well as in other mammalian species."

By studying tissue from cloned cow embryos at the 18 and 34 days of development and utilizing RNA sequencing, the researchers noted multiple genes with and abnormal expression that resulted in the high fatality rate for cloned embryos. Looking at the extraembryonic tissue of the cloned cows at day 18, the researchers discovered anomalies in expression of more than 5,000 genes. Non-cloned cows conceived using artificial insemination were used as a control group.

Upon reaching day 34 of development, the pattern of gene expression was much more similar to control cows derived from artificial insemination. This suggests that surviving clones were able to implant in the uterus and begin to form a placenta. These results indicate that the loss of cloned cows before implantation could be caused by problems with critical developmental genes in the extraembryonic tissue.

"Our data confirm that the interactions between the uterus and the extra-embryonic tissues is critical during implantation, making this step a major hurdle for the progression of pregnancy," shared Sandra.

"We now understand why clones fail, which can lead to improvements in the process of cloning of animals," said Lewin, adding as a warning, "Our discoveries also reinforce the need for a strict ban on human cloning for any purposes."