Solve a mystery that has puzzled biologists for over a century

It sounds like a “fair story” – “How the insect got its wings” – but it really is a mystery that has puzzled biologists for over a century. Intriguing and competing theories of insect wing evolution have emerged in recent years, but none have been entirely satisfactory. Finally, a team from the Laboratory of Marine Biology (MBL), Woods Hole, settled the controversy, using clues from long-standing scientific papers as well as cutting-edge genomic approaches. The study, conducted by MBL research associate Heather Bruce and MBL director Nipam Patel, is published this week in Nature’s ecology and evolution.

Insect wings, the team confirmed, evolved from a growth or “lobe” on the legs of an ancestral crustacean (yes, crustacean). After this marine animal’s transition to land around 300 million years ago, the leg segments closest to its body became incorporated into the body wall during embryonic development, possibly to better support its weight on earth. “The lobes of the legs then moved to the back of the insect, and those then formed the wings,” says Bruce.

One of the reasons it took a century to figure this out, Bruce says, is that it wasn’t understood until 2010 that insects are most closely related to crustaceans in the arthropod phylum, as revealed genetic similarities.

“Before that, based on morphology, everyone had classified insects into the myriapod group, as well as millipedes and millipedes,” says Bruce. “And if you look in the myriapods where the insects’ wings come from, you won’t find anything,” she says. “Thus, insect wings were seen as ‘new’ structures that arose in insects and did not have a corresponding structure in the ancestor – because the researchers were looking in the wrong place for the ancestor of the insect.

“People are very excited that something like insect wings may have been an innovative evolutionary innovation,” Patel says. “But one of the stories that emerges from genomic comparisons is that nothing is new; everything came from somewhere. And you can, in fact, know where it came from.

Parhyale hawaiensis, shown here eating, has had its genome sequenced and is the most genetically treatable crustacean for biological research. Credit: Heather Bruce

Bruce picked up the scent of his now-reported discovery while comparing the genetic instructions for the segmented legs of a crustacean, the tiny beach-hopper Parhyal, and the segmented legs of insects, including the fruit fly Drosophila and the scarab Tribolium. Using CRISPR-Cas9 gene editing, she systematically turned off five shared genes Parhyal and in insects, and found that these genes corresponded to the six leg segments that are furthest from the body wall. Parhyal, however, has an additional seventh leg segment next to his body wall. Where has this segment gone, she wondered? “And so I started to dig into the literature, and I found this very old idea that had been proposed in 1893, that insects had incorporated their proximal [closest to body] leg area in the body wall, ”she said.

“But I still didn’t have the wing for history,” she says. “So I kept reading and reading, and I came across this theory from the 1980s that not only did insects incorporate their proximal leg region into the body wall, but the small lobes of the leg then grew. moved to the back and formed the wings. I thought, wow, my genomic and embryo data supports these old theories. “

It would have been impossible to solve this long-standing conundrum without the tools now available to probe the genomes of a myriad of organisms, including Parhyal, which the Patel laboratory has developed as the most genetically treatable research organism among crustaceans.

Reference: “Destruction of Crustacean Leg Modeling Genes Suggests Insect Wings and Body Walls Evolved from Ancient Leg Segments” by Heather S. Bruce and Nipam H. Patel, December 1, 2020, Nature’s ecology and evolution.
DOI: 10.1038 / s41559-020-01349-0