Walkingstick Molecules

Johnny over at Ecographica has a post on defense molecules secreted by Walkingstick insects.  Head on over to Johnny's post for pictures of the Walkingsticks.

There are several species of Walkingstick, and the one described in this paper produces three different defense molecules that are stereoisomers of one another: anisomorphal, dolichodial, and peruphasmal.  Being an organic chemist, I wanted to know that these molecules look like so I did a little digging around.

The first place I tried was PubChem - only dolichodial was listed, although PubMed does have papers listed for all three compounds. Next I tried ChemSpider, and again only dolichodial was listed.  Time for a more specialized database.  Rich Apodaca is compiling a list of 64 free chemistry databases on his Zusammen Blog, and one of the databases he has profiled is a collection of pheromones and similar molecules called Pherobase.

Pherobase has a search box at the top of the page, and typing the name of each compound produced a list of google-search results.  The titles and descriptions of the search results were not completely obvious, but the pages with a file name containing "compounds-detail-" seemed a good place to look.  This gave me both a 2D drawing and a 3D structure for each of the compounds.  Go ahead an check out the Pherobase pages for anisomorphal, dolichodial, and peruphasmal.  The Pherobase page for each compound includes a 3D structure with the Jmol applet - if you want to turn of the auto-rotation, right-click in the molecule window and set Spin to "off."

Here are all three molecules side by side:

They are stereoisomers - the difference between them is the 3D orientation of the three side groups attached to the ring: dark wedges are "up" and dashed wedges are "down."  At a glance, I can't say why they might specifically be "defense" molecules.  However, much of molecular signalling just boils down to shape rather than chemical reactions - these compounds may just smell or taste bad to predators.

According to the article, the specific compounds produced by the bugs depends on their developmental stage, and even location.  The researchers raised 14 Walkingsticks and observed the types of defense molecules they produced as they grew.  The bugs they used produced a mixture of anisomorphal and dolichodial as hatchlings and the amount of  dolichodial increased after 2 months.  However, when they reached maturity they stopped producing either anisomorphal or dolichodial, and produced peruphasmal exclusively.  As adults, other populations of walkingsticks produce anisomorphal, or a mixture of anisomorphal and peruphasmal.  None of the adults produce dolichodial.

I'm a chemist, I don't know what all this means.  Something seems to happen at 2 months that changes the amount of  dolichodial they produce.  And something else happens when they reach maturity that they stop producing dolichodial.  It's interesting that adults produce one or both molecules that have the aldehydes on opposite sides of the ring - and the compound produced only by the immature insects is the compound with both aldehydes on the same side of the ring.

Dossey, A., Walse, S., & Edison, A. (2008). Developmental and Geographical Variation in the Chemical Defense of the Walkingstick Insect Anisomorpha buprestoides Journal of Chemical Ecology, 34 (5), 584-590 DOI: 10.1007/s10886-008-9457-8