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![]() Why are there only two sexes?
by BBC Science's Corinne Podger
Scientists in Britain believe that the reason there are only two sexes is due to a bacterial infection our ancestors caught about two billion years ago.
Mushrooms have as many as 36,000 sexes, and a strange growth called slime mould has about thirteen. But these are rare exceptions to the almost universal rule that life on earth is divided into two sexes. An evolutionary mystery
Professor Laurence Hurst, of Bath University, England, explains the problem: "For example, imagine that you are in a disco and the lights have gone out. You're looking for somebody to go home with and the law is the first person you bump into is the person you choose. Bacteria in the genes So the question is why we only have two sexes, if it appears to make the survival of species more rather than less difficult. Professor Hurst believes it's all down to how we inherit a particular set of genes, called mitochondrial genes.
"It looks as though there used to be bacteria where mitochondria come from. So we think that we got them about two billion years ago from bacteria taken into ourselves. So they became part of us, and that ability to replicate at will is left over from their bacterial ancestry." Fewer mutations Because mitochondrial DNA can reproduce so quickly, any mutation in it could spread rapidly if 99% of a population could mate with any other members. If the mutation was damaging the result could be catastrophic. Instead, almost every species on Earth only inherits mitochondria from its mothers. Exceptions like mushrooms have evolved to avoid exchanging mitochondrial DNA when they reproduce. For the rest of us, finding a partner may be a little difficult, but in evolutionary terms, this is set against the benefits of fewer mutations. |
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