Researchers have pinpointed a key gene involved in the successful formation of a zygote (fertilised egg). The team, based at the University of Bath in the UK and the Centre de Genetique Moleculaire et Cellulaire in France, studied the role of the Hira gene during the first 15 minutes after fertilisation. Although they carried out the research on fruit fly eggs, the scientists - who published their findings in the journal Nature - say it is relevant to all sexually reproducing animals.

The crucial gene makes a protein called HIRA, which seems to be responsible for 'repackaging' the sperm's genetic material, once it has penetrated the egg. Mutations in this gene could explain why some fertilised eggs fail to develop into embryos, say the scientists. Team leader Tim Karr said that sperm DNA needs to be repackaged after it has entered the egg 'so that it can engage in normal cellular activities, including combining with the maternal DNA in the first act of genetic fertilisation'.

The team made their discovery by studying a strain of mutant fruit fly called 'sesame' (ssm), in which a mutation inherited from the female flies stops this process from happening. They looked at sperm DNA during the different stages of fertilisation in eggs from both normal and ssm flies. In the mutant flies, the DNA from the sperm remained coiled up in a tight ball, unable to interact with the DNA from the egg.

During the creation of fruit fly sperm cells, the DNA is tightly packaged up with a special set of sperm-specific proteins. However, after it has entered the egg cell, the DNA needs to be repackaged with ordinary DNA packaging proteins, called histones. 'A single gene, Hira, looks after this repackaging process', said Karr. UK biologist Wolf Reik, of the Babaraham Institute, described the discovery as 'really exciting'. He told the BBC news website that the finding could be 'an explanation for some types of infertility in humans; if there were females that carried this mutation, they would not be able to conceive normally'.