Injection of CRISPR solution into crustacean embryos (Parhyale hawaiiensis). Photo credit: Heather Bruce
It sounds like a “Just So Story” – “How the insect got its wings” – but it really is a puzzle that has puzzled biologists for over a century. Fascinating and competing theories about the evolution of insect wings have emerged in recent years, but none have been entirely satisfactory. Finally, a team from the Marine Biological Laboratory (MBL), Woods Hole, settled the controversy, using evidence from previous scientific work as well as state-of-the-art genomic approaches. The study, conducted by MBL Research Associate Heather Bruce and MBL Director Nipam Patel, will be published in this week Natural ecology & evolution.
The team confirmed that insect wings developed from an outgrowth or “lobe” on the legs of an ancestral crustacean (yes, crustacean). After this marine animal passed into the rural population about 300 million years ago, the leg segments closest to its body were built into the body wall during embryonic development, possibly to better support its weight on land. “The leg flaps then moved onto the insect’s back, and these later formed the wings,” says Bruce.
Insects built two ancestral crustacean leg segments (7 in red and 8 in pink) into the body wall. The lobe on the leg segment 8 later formed the wing of insects, while this corresponding structure forms the tergal plate in crustaceans. Photo credit: Heather Bruce
One of the reasons it took a century to find out, according to Bruce, is that it wasn’t recognized until around 2010 that insects are most closely related to crustaceans within the arthropod tribe, as demonstrated by genetic similarities.
“Previously, everyone had insects classified in the myriapod group based on morphology, along with the millipedes and millipedes,” says Bruce. “And if you look for the origin of the insect wings in Myriapods, you won’t find anything,” she says. “Insect wings were seen as ‘novel’ structures that arose in insects and had no corresponding structure in the ancestor – because researchers were looking for the insect ancestor in the wrong place.
“People are very excited about the idea that something like insect wings could have been a novel innovation in evolution,” says Patel. “But one of the stories that genomic comparisons tell is that nothing is brand new. everything came from somewhere. And you can actually find out where from. ”
Parhyale hawaiensis, shown here eating, has sequenced its genome and is the most genetically manageable crustacean for biological research. Photo credit: Heather Bruce
Bruce smelled their now-reported discovery as he compared the genetic instructions for the segmented legs of a crustacean, the tiny beach funnel Parhyaleand the segmented legs of insects, including the fruit fly Drosophila and the beetle Tribolium. Using CRISPR-Cas9 gene editing, she systematically deactivated five common genes for leg patterning in Parhyale and in insects, and found these genes to correspond to the six leg sections furthest from the body wall. Parhyalehowever, has an additional seventh leg segment next to its body wall. Where has this segment gone, she wondered? “And so I started looking through the literature and found this really old idea suggested in 1893 that insects had their proximal ones built in [closest to body] Leg region in the body wall, ”she says.
“But I still didn’t have the wing part of the story,” she says. “So I read on and read and came across this theory of the 1980s, according to which insects not only built their proximal leg region into the body wall, but the small lobes on the leg later moved onto the back and formed the wings. I thought, wow, my genomic and embryonic data support those old theories. ”
It would have been impossible to solve this longstanding mystery without the tools now available to study the genomes of a wide variety of organisms, including Parhyale, which the Patel laboratory developed as the most genetically manageable research organism among the crustaceans.
Reference: “Cancer leg pattern genes knockout suggests that insect wings and body walls evolved from old leg sections” by Heather S. Bruce and Nipam H. Patel, December 1, 2020, Natural ecology & evolution.
DOI: 10.1038 / s41559-020-01349-0
