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Takes 30 to tango? In birds, embryo survival requires supernumerary sperm

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Bird Eggs Are Fertilized by More Than One Sperm

Bird eggs are fertilized by more than one sperm, a process called polyspermy. Penetration by multiple sperm may be unusual in the animal kingdom, but it is somehow crucial to the development of the bird embryo, a new study finds.

“In humans and mammals, you only need one sperm to penetrate the egg,” said Nicola Hemmings, an evolutionary biologist at the University of Sheffield and one of the study’s authors.

The researchers found that when few sperm penetrated the egg, the bird embryo was unlikely to survive. They report their findings in the journal Proceedings of the Royal Society B.

Exactly why the extra sperm are needed is unknown.

“Although the sperm do seem to be required for the embryo to survive, we don’t know how they contribute to the process of fertilization,” Dr. Hemmings said. Extra sperm may somehow support cell cycles necessary for early embryo development.

Original paper and abstract: 





Polyspermy is a major puzzle in reproductive biology. In some taxa, multiple sperm enter the ovum as part of the normal fertilization process, whereas in others, penetration of the ovum by more than one sperm is lethal. In birds, several sperm typically enter the germinal disc, yet only one fuses with the female pronucleus. It is unclear whether supernumerary sperm play an essential role in the avian fertilization process and, if they do, how females regulate the progression of sperm through the oviduct to ensure an appropriate number reach the ovum. Here, we show that when very few sperm penetrate the avian ovum, embryos are unlikely to survive beyond the earliest stages of development. We also show that when the number of inseminated sperm is limited, a greater proportion than expected reach and penetrate the ovum, indicating that females compensate for low sperm numbers in the oviduct. Our results suggest a functional role for supernumerary sperm in the processes of fertilization and early embryogenesis, providing an exciting expansion of our understanding of sperm function in birds.



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