Host Coercion

Moving from one host to another can be difficult for parasites, especially for those specializing on plants. Because they rely on other organisms for their survival, they have evolved some amazing strategies at getting what they need. A recent study published in PLOS Biology has shed some light on one interesting strategy.

Phytoplasma are bacterial parasites of a variety of plant species. In order to get from one host to another, these bacteria utilize insect hosts. How they do this is quite incredible. These bacteria produce specialized proteins that have some strange effects on plant tissues.

The proteins actually sterilize the host plant. They do this by interfering with the proteins responsible for flower development. Instead of producing normal flowers, the plants produce mutated leaf-like structures. You can see an example of a healthy plant on the left and an infected one on the right. So, why does the bacteria do cause such mutations?

This is where the insects enter the picture. Researchers found that infected plants that produced these mutated leaf-like structures were more attractive to leaf hoppers. The leaf hoppers readily feed and reproduce on these infected plants at a higher rate than they do healthy plants. In feeding, the leaf hoppers inevitably suck up bacteria in the sap.

When the leaf hoppers go on to feed on healthy plants, some of the bacteria get transferred in their saliva, thus completing the parasitic lifecycle. This is what parasitologists call "host coercion." The parasite, in this case phytoplasma bacteria, alter their host in some manner that increases the fitness of the parasite. This is one of the first examples in which researchers have been able to identify the exact mechanism by which a parasite makes this happen.

Photo Credit: John Innes Centre (

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