The scientists, from the University of Sheffield, used 3D imaging techniques to reveal a previously unknown structural feature in the zebra finch - a constriction at the opening of its sperm storage tubules.
This constriction may act as a gate, allowing female birds to perform ‘quality control’ on sperm that enter their reproductive tract after mating, influencing the paternity of their offspring and increasing the success rates of breeding attempts.
The advantage of having an influence on paternity would be to ensure healthy offspring with the best chance of survival.
Dr Tania Mendonca
Conductor of research
The discovery of the constricted opening suggests that sperm storage tubules act as a filtering system, allowing sperm that meet certain criteria to enter, while rejecting others. Ultimately, when the egg is ready to be fertilised, the pre-selected set of stored sperm is released.
By understanding how the birds perform this quality control on sperm after mating we can develop strategies to incorporate these selection criteria into artificial insemination and IVF in humans, thereby improving their success.
This may also be particularly important in captive breeding programmes where the availability of fertile individuals is low and assisted reproductive technology plays an important role in sustaining the population.
Dr Tania Mendonca, who conducted this research at the University of Sheffield, said: “The advantage of having an influence on paternity would be to ensure healthy offspring with the best chance of survival.
"Unlike sperm, eggs are a limited resource and it only takes one sperm to fertilise the egg - so there is an incentive for the female to ensure that the available egg is not fertilised by ‘poor quality’ sperm which could potentially result in an unsuccessful breeding attempt.”
This is the first time these storage organs have been imaged in 3D.
It was made possible using a novel application of an imaging technique called selective plane illumination microscopy, or ‘SPIM’, developed for this purpose by a collaboration between the University of Sheffield’s Departments of Animal and Plant Sciences and Physics and Astronomy.
Although this study was conducted on birds, a number of other female animals across the animal kingdom - from reptiles to mammals - can store sperm in specialised organs. The findings therefore shed light on why this ability to store sperm evolved and how the reproductive cells interact with each other during sperm storage.