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 (

Further Reading:

Green Islands

Autumn is here and all across the northern hemisphere deciduous trees are putting on a show unlike anything else in the natural world. The range of colors are spectacular both from afar and up close. If you're like me then every single leaf is worth investigation. The trees are shedding their leaves in preparation for dormancy. The leaves aren't dying outright. Instead, the trees are reabsorbing the chemicals involved in photosynthesis as a way of getting back some of the energy investment that went in to producing them in the first place. 

If you look closely at some leaves, however, you may notice green spots in an otherwise senescent leaf. Why is it that certain parts of these leaves are still photosynthetically active despite the rest of the photosynthetic machinery shutting down around them? The answer to this question is way cooler than I ever expected. 

These "green islands" as they are called are almost always associated with an insect. If you look closely towards the base of these spots you will usually find a tiny leaf mining larvae of a moth busy munching away at the remaining photosynthetic tissue. The most obvious conclusion at this point would be to say that the moth larvae are the cause of the green islands. However, it is not that simple. 


When researchers raised the moth larvae under sterile conditions, they did not produce the green island effect. This proved to be a bit of a conundrum. Why would this happen in the wild but not under sterile conditions in a lab? The answer is bacteria. 

It would appear that the moth larvae have a symbiotic relationship with bacteria living on their bodies. These bacteria interact with the tissues of the leaf and alter the production of cytokinins. In the leaf, cytokinins inhibit leaf senescence. When the plant switches into dormancy mode, cytokinin production is shut down. The bacteria, however, actually ramp up cytokinin production throughout the tissues surrounding the larva. The result of which is a small region or "island" of tissue with prolonged photosynthetic life. 

Because of this, the larvae are able to go on feeding well into the fall when food would otherwise become nonexistent. By harboring these bacteria, the moths are able to get more out of each seasons reproductive efforts instead of simply stopping once fall hits. This is the first ever evidence of insect bacterial endosymbionts have been shown to manipulate plant physiology, though it most certainly will not be the last. 

I would like to thank Charley Eiseman for the use of this photo as well as inspiring this post. Charley is the man behind one of my all time favorite blogs Bug Tracks so make sure to visit and like Northern Naturalists.

Further Reading: [1] [2] [3] [4] [5]