“Insects are not supposed to make their own pectinases,” explains Dr. Matan Shelomi, a postdoctoral fellow in the Department of Entomology and lead author of the study. Yet the stick insects make lots, and their genome contains multiple pectinase genes!” Based on DNA similarity, the source was a gamma-proteobactera, the most common bacteria type in the stick insect microbiome, but commonly found on the leaves they eat too. “We are not sure how it happened, but one or two pectinase genes from a gut bacterium or even just something in the food clearly jumped into the stick insect genome, and then evolved along with the insects,” explains Shelomi. Tests show some of the new pectinases retained their original job degrading pectin, while others have yet unknown functions. But when did the transfer happen?





An international collaboration solves the puzzle



To find out, the team first tested seven different species of stick insect, including a primitive and short species found only in California called Timema cristinae, in the sister group to all the other stick insects. Timema do not have pectinases, while the others, the “Euphasmatodea,” do. It was not clear, however, whether Timema never had the genes or simply lost them. The team then collaborated with the 1K Insect Transcriptome Evolution Project (1KITE). Using 1KITE’s genetic databases from 1000 insect species, including nearly 50 Phasmatodea, the researchers could quickly search multiple groups for these enzymes. The results showed that the gene jump occurred sometime after Timema and Euphasmatodea split, but before the latter diverged into the 3000 or so species it has today: between 110 to 60 million years ago.



