BY GIVING up sex 10,000years ago, a parasite was able to spread from just a few host species to a wide range of birds and mammals, including people.
Today almost all infections of the single-cell parasite Toxoplasma gondii are caused by just three strains. By looking at mutations in the parasite鈥檚 鈥渏unk DNA鈥�, a team led by David Sibley at Washington University in Missouri has shown that the three strains appeared around 10,000 years ago. They were all the product of a mating between closely related parental strains. Crucially, the reshuffling of the genome during this mating somehow made the offspring more infectious.
It is thought the ancestral form of Toxoplasma had a two-stage life cycle, just as some strains still do. The parasite reproduces sexually only in cats, its main host, producing millions of eggs that are then excreted. The eggs infect intermediate hosts such as rodents and form cysts in tissues. It can also affect the brain: infected rats often behave more recklessly and are more likely to be eaten by cats.
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The cysts of ancestral strains probably affected only cats that ate infected rats. But Sibley鈥檚 team showed that the three modern strains are more orally infectious, infecting up to 100 per cent of mice that eat them (Science, vol 299, p 414). What鈥檚 more, the modern strains can spread from one intermediate host to another without ever reproducing sexually, instead of having to cycle between cats and rats.
Toxoplasma now infects between 30 and 60 per cent of people. There is no treatment or vaccine. It was long thought to be harmless except in pregnant women, as it can cause miscarriages or damage babies鈥� brains in rare cases. But recent studies suggest it may have subtle effects on our brains, reducing reaction times and making people more likely to be involved in accidents (New 杏吧原创, 26 October 2002, p 40).
Sibley points out that the origin of the modern strains coincides with early agriculture and the domestication of the cat, developments that brought together potential host species. He warns that similar changes in related parasites, such as Neospora caninum, which mainly infects dogs and cattle, could allow them to expand their host range as well. 鈥淐omplex life cycles can change drastically over a short period of time,鈥� he says.