World's smallest farmers: just a single cell

23 January 2011
Posted by Chris Smith.

There are many examples of organisms engaging in agriculture in the natural world, ranging from humans that grow wheat to leaf-cutting ants that nurture edible forms of fungi. But now scientists have discovered what is possibly the smallest of nature's farmers - a single-celled social amoebal species called Dictyostelium discoideum.

These microscopic soil-dwellers engulf and consume bacteria, multiplying in the process.  But when food becomes scarce, Rice University researcher Debra Brock has discovered, they do something very interesting - about one third of the amoebae prepare a packed lunch.

Dictyostelium Fruiting BodySo, rather than exhaust the supply of bacterial food locally, they forego finishing their meal to instead sequester inside themselves some of the microbes they have been consuming.  They then join up with other fellow amoebae to form a migratory multi-celled "slug" that moves up through the soil in search of pastures new.  Close to the surface, the slug-like structure then produces fruiting bodies, called sori, which contain amoebal spores - complete with pre-packaged bacteria - that can be disseminated to colonise a new terrain.  And when the bacterially-equipped amoebae emerge from their spores, the bacteria are used to "seed" new microbial colonies that can be consumed subsequently.

What's intriguing is that only a fraction (36%) of the amoebae behave as "farmers" like this.  The trait does appear to be genetic, because if mixtures of amoebae including both farmers and non-farmers are given a dose of antibiotics to sterilise them of any bacteria they may be carrying, those identified as farmers previously then pick up fresh stocks of bacteria.

Now it remains to identify the genes that underpin this interesting behaviour, which may lead to a better understanding of how bacterial and other cells interact, including in a number of important diseases such as TB where carriage of bacteria within cells is a critical step in the pathological process.

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