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Safety in numbers - For our ancestors, ganging together was the only hope of survival

点击量:   时间:2017-11-13 03:01:13

By Stuart Blackman THE voracious appetites of some single-celled predators may have triggered the evolution of multicellular animals, biologists suggest. For 3 billion years after the appearance of life on Earth, the world was dominated by relatively simple, single-celled organisms. Around 900 million years ago, something kick-started the transition to multicellular life. Why that happened has been debated for decades. One idea was that natural selection favoured cells that clumped together because they would be too big a mouthful for single-celled predators. To test this, Martin Boraas and his colleagues at the University of Wisconsin in Milwaukee studied cultures of the green alga Chlorella vulgaris. The researchers had already shown that populations of the alga will remain single-celled for more than two decades as they multiply, except for the occasional appearance of loose clusters of the cells. However, when the scientists inoculated the cultures with Ochromonas vallescia, a predatory single-celled flagellate, it was a different story. The algae initially went into decline, but then recovered. At this stage the alga population included colonies made up of anything from four to hundreds of cells, as well as single cells. Sixteen days after inoculation, during a second cycle of flagellate growth and Chlorella decline, the colonies persisted while the single cells dropped in frequency to less than 1 per cent of all cells in the culture. The number of cells in the algal colonies then gradually declined until they reached a steady state only 10 to 20 generations after the predator was introduced. At this point, the bulk of the Chlorella cells were part of eight-cell colonies. The researchers’ observations confirmed that while the flagellate could ingest single cells and young colonies, mature colonies were simply too big. So predation conferred a selective advantage on the rare mutation that for some reason caused the algal cells to stick together. “Our study provides evidence of what we think is a reasonable hypothesis for one possible origin of multicellularity: survival or resistance to ingestion,” says Boraas. During normal cell division, Chlorella divides into between 2 and 16 daughter cells, which then split from the mother cell wall. But in colonies, the cell wall remains intact, holding the daughter cells together. When the daughter cells divide, they bud off from the parent colony, making the multicellular colonies self-replicating units. A similar process might have set the scene, 900 million years ago, for the subsequent evolution of complex multicellular animals. That the initial large colonies gradually give way to the eight-cell pattern probably reflects the trade-off between the pressures of predation and of maximising nutrient uptake, as cells within a large colony have less surface area exposed to the nutritive medium. When unicells and colonies were cultured in the absence of the predator, the unicells again became dominant ( Evolutionary Ecology, vol 12, p 153). The study provides “remarkable present-day evidence that predation pressure induces multicellularity”, says Nick Barton,