This really intrigued me, so I spent some time looking into leaf-mining insects, and what I found was truly incredible. First, there are a lot of leaf miners. Some are moths, some are flies, some are beetles, and even some are wasps. The adult lays her eggs in within the tissue of a leaf, and when the eggs hatch, the larvae begin eating the insides of the leaf. This, of course, protects the larvae, because they are not exposed to predators. They stay inside the leaf until they are ready to pupate (start metamorphosis into their adult form).
Now, of course, if a larva is “unlucky” enough to hatch shortly before or after the leaf falls from the tree, this could be a problem. After all, the larva is eating the living tissue inside the leaf and therefore needs the leaf to stay alive while it is feeding. What happens if the leaf dies before the larva is ready to pupate? Well, that’s where the “green islands” come in. It seems that the larva can keep the portion of the leaf it is eating alive so that it can continue to eat and develop, and that’s why the area around the larva stays green!
So how does the larva do this? How does it keep its part of the leaf alive, even when the rest of the leaf is dead? Well, for a long time, this was quite a mystery. However, in 2007 David Giron and his colleagues published a study in which they analyzed dead leaves with leaf miner infestations. They looked at the tissues near the leaf miner larvae (which were still green) and compared them to dead tissues in the same leaf, living tissues in an uninfected leaf, and dead tissues in an uninfected leaf. They found that certain plant hormones (called cytokinins) were in abundance in the areas around the leaf miner larvae. In some cases, there were more cytokinins in the area around the leaf miner larvae that there were in the living tissue from an uninfected plant!
So the conclusion was clear. The leaf miner larvae were doing something to increase the amount of cytokinins in their area of the leaf, which was keeping that area alive. But what exactly were they doing? The authors weren’t sure. As they say:
First, high levels of cytokinins in mined areas could result from the overexpression of the plant genes which produce cytokinins in the infected zone. Oral secretions of feeding insects contain many specific compounds able to interact with plant gene expression…Second, cytokinins in mined tissues could result from an accumulation process rather than a synthesis per se. Feeding activity of leafminers is known to alter biophysical properties of leaves and consequently respiration and transpiration rates…Such processes strongly impact the flow of nutrients within a leaf and could potentially result in the accumulation of cytokinins in mined areas. Third, insects themselves could potentially synthesize cytokinins.
Well, it turns out that one of the co-authors (Wilfried Kaiser) on that study recently did another study that helps to narrow down the possibilities. He found that leaf miner larvae are infected with bacteria from the genus Wolbachia. It turns out that lots of insects are infected by these bacteria, so that wasn’t surprising.
The surprising thing happened when Kaiser cured the leaf miner larvae of their infection by administering an antibiotic. The leaf miner insects remained healthy, but they could no longer produce green islands. Thus the bacteria make the “green islands.” What’s still not completely clear is whether the bacteria make the cytokinins responsible for the “green islands” or they do something to help the leaf miner make them. We know that bacteria (the simplest living things on the planet) are significantly better chemical producers than the most high-tech chemistry lab we have, so my bet is that the bacteria are making the cytokinins.
Regardless of exactly how the bacteria help the leaf miner insects, this is yet another stunning example of mutualistic symbiosis. Quite some time ago I blogged about mutualism in nature. While simple forms of mutualism are easy to explain in an evolutionary framework, the complex ones are extremely hard to understand in an evolutionary framework. They are all over creation, however, so any theory that tries to explain origins must account for them at some point.
I personally think that mutualism shows us that lots of creatures were made for each other, and they are just one more facet of the amazing design that we see all around us.