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The dodo has been extinct for more than 300 years, but that isn't stopping Dallas' Colossal Biosciences from trying to resurrect the 3-foot-tall, flightless bird.

On Sept. 17, the "de-extinction" biotech company announced it cleared an early hurdle by growing primordial germ cells—the precursors to eggs and sperm—from the rock dove, also known as the common pigeon.

Colossal said the next phase is to gene-edit primordial germ cells from the Nicobar pigeon, the dodo's closest living relative. The Nicobar genome will serve as the scaffold for reconstructing a dodo-like genome. The company said it has established a breeding colony of Nicobar pigeons in Texas and is working to develop that bird's primordial germ cells for editing.

"Our avian team's breakthrough in deriving culture conditions that allow pigeon primordial germ cells to survive long-term is a significant advancement for dodo de-extinction," Ben Lamm, CEO and co-founder of Colossal Biosciences, said in a press release.

"This progress highlights how Colossal's investment in de-extinction technology is driving discovery and developing tools for both our de-extinction and conservation efforts."

Cloning birds is trickier than cloning mammals. In mammals, scientists take an unfertilized egg, remove its nucleus and replace it with the nucleus from a donor body. Birds' large, opaque eggs are laid after embryos begin developing, which complicates the approach.

Scientists have previously been able to culture and gene-edit primordial germ cells of chickens and geese, a technique that has been used to create a chicken fathered by a duck. But the "recipe has not worked on any other bird species tested, even closely related species like quail," Anna Keyte, Colossal's avian species director, said in the press release.

Colossal said it screened more than 300 "recipes" before landing on one that kept pigeon primordial germ cells growing for 60 days. The methods and data were on the preprint server bioRxiv and have yet to be peer-reviewed.

Colossal plans to inject the gene-edited Nicobar germ cells into chicken embryos. The chickens would grow up with pigeon-making cells in their ovaries or testes, acting like surrogates. In that way, a chicken could lay an egg that hatches a pigeon and, after additional gene edits, potentially a dodo-like bird.

Colossal aims to rewild the resurrected dodo in Mauritius, an island country off the coast of East Africa where the birds were endemic before going extinct in the 17th century.

The development follows the company's announcement in July to bring back the giant moa, a 10-foot-tall, flightless bird that vanished from New Zealand roughly 600 years ago.

Beth Shapiro, Colossal's chief science officer, called the ability to grow the primordial germ cells a "transformative tool for avian conservation."

"By developing these protocols," Shapiro said in the press release, "we're establishing crucial biobanking capabilities and opening new possibilities for genetic rescue of endangered species."

Some endangered birds include the Mauritian pink pigeon, which is expected to go extinct in the next 50 to 100 years due to inbreeding.

Colossal is also pursuing de-extinction projects for mammals including the woolly mammoth and Tasmanian tiger. In April, the company announced the birth of three dire wolves, modern canids engineered to resemble the long-extinct species popularized by HBO's Game of Thrones.

Critics counter that true resurrection isn't possible. Without intact genomes, they say, the best outcome is a genetically modified proxy or hybrid.