EGGS fertilised by more than one sperm are supposed to be doomed. But biologists have now found that pig eggs can engage in a ménage á trois with two sperm and still produce a perfectly healthy embryo.
Polyspermy, as the fusion of an egg with more than one sperm is known, should create chaos by introducing an extra set of chromosomes. "Many people thought polyspermy must be a dead end," says Yong-Mahn Han of the Korea Research Institute of Bioscience and Biotechnology in Taejon. "But that isn't what we found."
Working with Randall Prather and colleagues at the University of Missouri in Columbia, Han conducted IVF on pig eggs and implanted those containing multiple sperm. About a twelfth of these embryos grew into healthy pigs with a normal complement of chromosomes.
To understand how this was possible, the researchers took a closer look at what happens to fertilised eggs after they fuse with two sperm. They discovered that the three pronuclei-the nuclei from egg and sperm that normally unite before the egg divides-could assume two different groupings. In one, the pronuclei form a threesome at the centre of the embryo. These are presumably almost certain to die while attempting development with an extra set of chromosomes.
But in more than half of eggs fertilised by two sperm, two pronuclei pair up, while the other is left to one side. The researchers believe this allows the extra set of chromosomes to be segregated into a cell of its own. Han suspects that this cell shrivels and dies, leaving the rest of the embryo with a normal number of chromosomes.
Author: Philip Cohen
New Scientist magazine, issue 23rd Jan 99