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Second rhino pregnancy signals hope for nearly extinct subspecies

Second rhino pregnancy signals hope for nearly extinct subspecies
Barbara Durrant, left, San Diego Zoo Global's director of reproductive physiology, examines Amani, a female southern white rhino, while Dr. Parker Pennington watches an ultrasound screen. (Ken Bohn /)

A second rhino has become pregnant this year through artificial insemination at the San Diego Zoo Safari Park, another milestone for an ambitious program to rescue a rhino subspecies with stem cells.

Researchers hope 10-year-old Amani and five other southern white rhinos will become surrogate mothers of northern white rhinos, which are critically endangered. The pregnancies of Amani, announced Tuesday, and Victoria, announced in May, are dry runs for the goal of implanting northern white rhino embryos.

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Just two northern white rhinos are left in the world, both female and too old to reproduce. This means the subspecies is functionally extinct.

Nola, a northern white rhino at the Safari Park, died in 2015. She was then one of only four left. Sudan, the last male, died in March at the Ol Pejeta Conservancy in Kenya.

So if the northern white rhino is to have any future, it will come from human intervention.

The embryos are to be created from cryogenically preserved northern white rhino cells. These are stored at the Frozen Zoo, a project of San Diego Zoo Global, the zoo’s conservation arm. It was established in the 1970s by the late Kurt Benirschke, who foresaw that these cells might one day be turned into complete animals.

The process is complicated. The frozen cells will be thawed and converted into stem cells. The stem cells will be turned into germ cells — sperm and egg cells. After fertilization creates the embryos, researchers will implant them into the surrogate mothers.

If all goes as planned, these pregnancies will be carried to term, and the northern white rhino will have a second shot at survival.

Team members on the project include Jeanne Loring, a stem cell scientist at Scripps Research. The Frozen Zoo has 12 samples, nine of which have been thawed and “reprogrammed” into lines of stem cells. Five of those nine lines have been well studied, and are the subject of a recently submitted paper, she said.

While it’s unclear if the three remaining samples can be of use, the nine lines already recovered provide sufficient genetic diversity to generate a viable population, Loring said.

There are early signs of germ cell development, Loring said. Stem cells were allowed to spontaneously differentiate into adult cells of various types. Some of them became “primordial germ cells,” precursors of both sperm and egg cells.

“But a lot of the work is ahead of us,” she said. “We need to purify those cells out of the rest and improve the methods for generating more of them.”

In the meantime, the southern white rhino females are being acclimated to this unusual way of becoming moms. They have been carefully and patiently trained with positive reinforcement to voluntarily accept highly intimate medical exams from keepers. This is done to minimize stress, which could endanger the pregnancy.

The hope is that several years from now, each of the southern white rhino females will receive a northern white rhino embryo and carry it to term.

It’s too early to tell if Amani’s and Victoria’s current pregnancies will be successful. Southern white rhino pregnancies typically last 16 to 18 months.

And as for the birth of new northern white rhinos, Loring says she’s optimistic, but it may take a while.

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“I'm hoping it happens in my lifetime,” she said. “I can't really tell you how long it’s going to be. There’s a lot of work to be done, but it just seems really feasible to me. We can see the path.”

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