Heliconius butterflies are unusual in consuming pollen — unlike butterflies that live on nectar — a task made more difficult by the fact that they lack the mouthparts needed to chew it. How do these insects manage to devour pollen grains too big to swallow, without the ability to chew them? The answer lies in their saliva: Heliconius butterflies have evolved special enzymes to break down pollen into edible components. In a recent collaboration between the Briscoe, Martin, and NCASD labs, the team has identified the enzyemes (called cocoonases) used by the butterflies to pre-digest pollen and modeled their structures. The team found numerous different cocoonases used by Heliconius butterflies, but there is catch: while evolutionary conventional wisdom would suggest that different species would have different enzymes, in fact each species has the whole complement. The mystery deepened when the researchers discovered that the different cocoonases had identical active sites, implying that they did not substantially differ in their preferred chemical targets. Why maintain a whole spectrum of enzymes that all do the same thing? The solution to the mystery lies in the outside of the enzyme. The team found that while their active sites were the same, the Heliconius cocoonases varied systematically in terms of their surface properties. These differences appear to have evolved to cope with one of the challenges of pollen-eating: it’s heterogeneous stuff. To get digestive enzymes into every nook and cranny of a pollen grain, a butterfly needs an arsenal of biomolecules, each of which being ideal for diffusing into a different type of chemical environment. Armed with these “chemical teeth,” Heliconius butterflies are able to tap into a rich foodstuff that would otherwise be too tough to swallow.
Key to unlocking the mysteries of the cocoonase was a novel protocol for modeling biological molecules developed as a collaboration between the NCASD and Martin labs. In addition to solving basic problems in evolutionary biology, this work holds the potential to lead to new classes of enzymes with the ability to work in a wider range of medical or industrial settings. The research appears in the Journal of the Royal Society, and can be found at http://rspb.royalsocietypublishing.org/content/285/1870/20172037.