Tuesday, September 29, 2009

Papilio’s Unconstrained Phenotypic Flexibility

As with most plant feeding insects, butterfly to host-plant adaptations are highly specialized towards specific plant species or plant groups. The reason for this is that plants characteristically produce chemicals that either deter or attract insects based on the defensive and mutualistic needs of the plant itself. Through natural selection, plants adapt specifically to work with (as with pollinators, like bees), or work against, the other organisms cohabitating in their ecosystem. So, if an insect is to take advantage of a plant as a food source it must first adapt tolerances or immunity to the toxic compounds that the plant produces as a defensive measure.

Sanpshot of P. glaucus taken last week in Hamilton Co, Florida. Notice the fine parallel lines of silk beneath the caterpillar; it will use these lines to slowly pull-up the edges of the leaf (Persea palustris, in this case) for protection.

Given enough time, the chemical tolerances of some herbivorous insect populations can become so specialized as to limit their ability to consume other plants; their body chemistry becomes dependent on a single, or only a few, plant species. When evolution pushes an organism’s adaptations to high levels of specialization and thus reduces its ability to be flexible and responsive in a changing environment, we can say that the organism has become “evolutionarily constrained.” This is the case with many species of butterfly, which have adapted resistance to the phytochemicals of only their favorite plants.

Side view of above larvae; the large eyes may be an adaptation to intimidate predators - possibly to look like a snake???

Called monophagy, dependence on a single plant species can leave butterflies vulnerable when environmental conditions change; when once abundant plants become rarer, food becomes harder to find, and fitness declines. Luckily, the evolutionary history of some butterflies may have imprinted sufficient genetic variability in their genome as to provide them a toolbox from which to draw ecological flexibility.

This was a newly emerged P. glaucus photographed in my backyard; it was posted with others during a discussion on ecological divergence (linked below).

Although often demonstrating monophagy, some species of swallowtail butterfly show rather substantial flexibility when it comes to the ability to physically tolerate the phytochemicals of un-preferred plants. To study the ability of Papilionidae (swallowtails) to tolerate different species of plant, researchers from Michigan State placed females of several Papilionidae species into enclosures containing the leaves of one of several plants families. After observing which leaf types the female utilized as a platform for depositing eggs, and after quantifying the larvae’s growth/mortality response relative to the specific plant species provided, the scientists determined that some butterflies exhibit greater ecological flexibility than others.

For example, the MSU study revealed that species such as Papilio troilus could not tolerate the chemicals of plants other than those of the Lauraceae Family, which it has adapted to prefer. Contrastingly, other swallowtail species, such as Papilio glaucus, demonstrated an ability to eat the leaves of several different plant families, even though it displays monophagy in the wild.

NOTE: The Papilio glaucus used in this experiment were harvested form a population found in Levy County, Florida. This particular Florida population had been previously shown to practice monophagy with preference for sweet bay trees (Magnolia virginiana); Ecographica’s prior Wetland Plant of the Week #25. Not to create conflict with my earlier post discussing Ecological Divergence in the Swallowtail, I wanted to clarify that other populations of P. glaucus are “polyphagous” – they use multiple tree species.

Scriber, J., Larsen, M., Allen, G., Walker, P., & Zalucki, M. (2008). Interactions between Papilionidae and ancient Australian Angiosperms: evolutionary specialization or ecological monophagy? Entomologia Experimentalis et Applicata, 128 (1), 230-239 DOI: 10.1111/j.1570-7458.2008.00691.x

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