Many animals depend on their microbiome to digest their food. Symbiotic¹ microorganisms produce enzymes² their hosts cannot, and these work alone or together with the animals' own enzymes to break down their food. Many plant-feeding insects need microbial enzymes, such as pectinases, that degrade plant cell walls; yet some insects have overcome this dependency in a surprising way. Researchers at the Max Planck Institute for Chemical Ecology in Jena, Germany, found that stick insects make microbial enzymes themselves. From an ancestral gut³ microbe, the genes⁵ for the essential enzymes simply "jumped" as they are to their insect host. The researchers report this newly discovered "horizontal gene⁴ transfer" in a paper recently published in Scientific Reports. (Scientific Reports, May 2016, DOI: 10.1038/srep26388) "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 DNA⁶ 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 bacterium⁷ 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 collaboration⁸ solves the puzzle

 

To find out, the team first tested seven different species of stick insect, including a primitive⁹ 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 collaborated¹⁰ with the 1K Insect Transcriptome Evolution Project (1KITE). Using 1KITE's genetic¹¹ 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 diverged¹² into the 3000 or so species it has today: between 110 to 60 million years ago.

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