IF HUMANS were mushrooms, finding a date would be much easier. Whereas we muddle by with just two sexes, the fungi have 36,000, all of which can mate with each other, in a mysterious process involving underground fronds.

So why don't humans have such a varied sex life? British scientists now believe they know the answer: two sexes is exactly the right number if you reproduce by fusing cells together. Larger numbers - anywhere from a handful of sexes up to 36,000 - are only permissible if, like slime moulds or mushrooms, you keep your partners' cells at arm's - or frond's - length.

Professor Laurence Hurst of the University of Bath's biology department has now developed and tested a theoretical model that explains that the number of sexes is determined by the method of sexual reproduction. In particular, it depends on the powerhouses of the cell - the mitochondria - which sit outside the nucleus and generate energy for the rest of the cell.

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"Mitochondria are necessary for the capability to convert oxygen to energy," said Professor Hurst at the British Association Festival in Sheffield yesterday. "But they have their own DNA, and they can divide all the time, unlike the cell itself. That's potentially harmful if there is a mutation in the mitochondrial DNA."

If the cells of the mating parents fuse during sex, then the mitochondrial DNA will also be brought together. If either set of DNA has harmful genes, they will be inherited by the offspring and damage its chances of survival.

The best solution for sex involving cell fusion is thus to throw away one of the sets of mitochondrial DNA - precisely what happens in humans. There, the father's mitochondria are carried on the sperm, but destroyed once inside the mother's egg.

That instantly halves the chance of inheriting bad mitochondrial genes. "The difference with mushrooms is that they never allow the two sets of mitochondria to come into the same cell space to compete," said Professor Hurst.

It means that while a mushroom can have sex with almost every companion it meets - decided by its own "sex gene" - any harmful mitochondrial mutations are restricted to the parent carrying them, and cannot easily spread through the population. This "Berlin Wall" strategy is used by many lower species that have multiple sexes, including ciliate bacteria.

However, Professor Hurst's model predicts that those without mitochondrial DNA do not need sexes - and that is what happens in the natural world.

Mitochondria are believed to be the descendants of ancient bacteria captured for their energy-producing ability by cells early in Earth's history. The process described by Professor Hurst's work suggests that cells have evolved methods to ensure the "best" mitochondria are retained.

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So that explains sexes - but what about sex? What's the purpose of that? "It gets rid of bad mutations and lets good ones spread through the population," said Professor Hurst. "But basically it seems to be to avoid parasites."

Dr Martin Hall, of the Natural History Museum, seeking clues in a maggot. He says the study of maggots and corpses can tell forensic scientists the time of death and help to solve murder cases Jayne Emsley

BY CHARLES ARTHUR

Murderes should beware: maggots will find the remains they leave behind, says Dr Martin Hall, of the Natural History Museum.

He has been studying the life cycle of maggots to help forensic scientists determine the times of death of bodies

"For a blowfly, a human body is just as acceptable as a dead hedgehog," he said.

"We are improving our estimates of how long it takes flies to find bodies all the time. For example, it will take them longer if a body is in a third floor float than if its outside."

The study of maggots and corpses now means that scientists can predict with 95 per cent certainty how many days a body has been in the open to within 12 hours. Evidence from Maggots were first used in the UK in 1935 over the double murder by Buck Ruckston of his wife and her maid whose bodies were dumped in a ravine.