Half-century-old sperm still up to the job

Sheep sperm frozen in liquid nitrogen for half a century has been used to successfully impregnate 34 ewes.

Researchers from the University of Sydney in Australia report that the reactivated semen, preserved since 1968, was used to inseminate 56 ewes, with only 12 failures.

The success rate was equal to that typically achieved by using sperm frozen for 12 months.

“We believe this is the oldest viable stored semen of any species in the world and definitely the oldest sperm used to produce offspring,” says researcher Jessica Rickard, a member of the university’s Animal Reproduction Group.

The sheep produced were all Merinos, a breed created to maximise wool growth and selectively crossed over decades to maximise desirable traits. They showed characteristics typical of earlier iterations.

“The lambs appear to display the body wrinkle that was common in Merinos in the middle of last century, a feature originally selected to maximise skin surface area and wool yields,” says researchers Simon de Graaf.

“That style of Merino has since largely fallen from favour as the folds led to difficulties in shearing and increased risk of fly strike.”

The old semen had been stored in small pellets placed in a large vat of liquid nitrogen and held at minus-196 degrees Celsius.

At the start of the research, Rickard and colleagues thawed some of the material and performed in vitro tests to ascertain its viability and DNA integrity.

“What is amazing about this result is we found no difference between sperm frozen for 50 years and sperm frozen for a year,” she says.

The researchers are currently formally organising their results for journal publication.