A Precarious Partnership


In the hot desert regions of southwestern California and Arizona it seems that life has few options in its struggle to endure. Organisms are forced to make strange and intimate partnerships, one of the most interesting being the Joshua tree (Yucca brevifolia) and its sole pollinator: the yucca moth. The tree must trade "seeds for seeds" as the yucca moths are not only its pollinator, but its parasite as well.

For this Blue Monday, I interviewed Jeremy Yoder from Olle Pellmyr's lab. He and others have studied thoroughly this strange pairing of yucca and yucca moth as it may provide insight into elements of how co-evolution and ecological partnerships work.

Here is a little more on how their relationship works:
The moth pollinates the tree as it lays its eggs into its flowers. On the up side, the tree is fertilized and those seeds that survive may create the next generation. On the down, the moth larvae must eat the seeds to provide new pollinators for the next generation.

This seems like a very precarious relationship. After all, what stops the moth from simply overdoing it and laying far too many eggs, killing off the Joshua trees and later yucca moths all together?

The answer is natural selection. Not just any natural selection either, but selection that is mediated by the Joshua tree itself! With-in the fruit of the Joshua tree is a mechanism that destroys over-egged fruit. The tree can sense when the partnership is being abused by a trigger deep within the fruit. When the eggs or ovipositor of the moth delve too deeply, the fruit is aborted, rejected by the Joshua tree. The Joshua tree misses an opportunity for seeding in this way, but it also eliminates over-egg laying genotypes by destroying the eggs of the abusive moth as well.

This puts the yucca moth and Joshua tree in a constant state of co-evolution that makes for some fairly noticeable morphological differences over evolutionary time. Interestingly, the trees and their corresponding moths have marked differences from region to region as dictated by their interactions with one another. Jeremy has found that trees and their moths in two slightly separated regions have taken separate approaches to the partnership. In the westernmost regions, the trees have a bottle shaped pistil to their flowers for the long ovipositors of their moths, while in the Eastern regions the pistils are more jug-shaped to accommodate smaller yucca moths.

Using genetic analysis, Jeremy and the Pellmyr lab hope to discover if these changes are truly due to inheritance and natural selection among these close knit species or not. It is projects like these that allow us to witness evolution in action and magnifies the delicate, fascinating interactions of organisms in their environments.

Source: The Pellmyr lab web site: http://www.sci.uidaho.edu/biosci/faculty/pellmyr.html