For decades, researchers thought that wood-feeding insects relied entirely on their helpful microbes to break down the tough cell walls of plants. Recently, cellulase, an enzyme that digests the cell wall polymer cellulose, was discovered in stick insects themselves. Now, according to a new Insect Biochemistry & Molecular Biology study, walking sticks in the order Phasmatodea produce cellulases that can take down several types of cell wall polymers equally.
The complex polymers in plant cells walls – such as cellulose, xylan, and xyloglucan – typically require several different enzymes (cellulases, xylanases, and xyloglucanases, for example) to fully break them down into their component sugars. Yet all stick insects express multiple, different cellulase enzymes from the glycoside hydrolase family 9 (GH9) – nearly half a dozen gene copies per species. Why inherit multiple copies of genes for cellulase, and why have Phasmatodea genomes preserved all these GH9s over evolutionary time? Maintaining redundant enzymes doesn’t make much sense if they all have the same function.
A Max Planck Institute for Chemical Ecology team led by Matan Shelomi hypothesized that some copies evolved new or additional functions to cellulase activity that would increase the insects' ability to digest leaves. So they isolated cellulase genes from six stick insect species – who live as far as Australia, Vietnam, Borneo, Peru, and Madagascar – as well as one sister group called Timema cristinae. The team expressed their individual GH9 enzymes, then tested them against various plant cell wall polymers.
Turns out, in addition to the glucose backbone of cellulose, some of their enzymes can also break down the xylose backbone of xylan, while others target the xylose-glucose backbone of xyloglucan. "If we hadn't tested these enzymes on other substrates besides cellulose, there was no way we could have discovered these functions," Shelomi says in a statement. "It was good that we did: Nobody found these kind of powerful enzymes in an animal before." This is the first time xyloglucanase of any kind was found in a multicellular animal, and it suggests that stick insects can derive more nutrition from the same plant diet than any other herbivore.
Furthermore, because these multifunctional enzymes were also found in Timema cristinae, that means an ancestral cellulase gene – from before the Phasmatodea evolved – had duplicated into several genes, and some went on to evolve new abilities.