Insect wings may have evolved from crustacean legs

The first wings on Earth may have evolved from the sunken legs of an ancient flightless crustacean.

Today, modern crabs, lobsters, shrimp and shrimp are sometimes called sea bugs, and as part of the arthropod family, which is characterized by strong armor and segmented joints, the name makes sense.

Scientists currently believe that the first insects emerged about 480 million years ago, evolving from aquatic forms of crustaceans. As terrestrial ecosystems became more complex, some 80 million years later, the wings would allow them to take flight.

If they are right, it means that the first insects were buzzing on our planet long before birds, bats and pterosaurs, so where did they get this ability?

It’s a simple question that has been bothering experts for centuries. A century-old hypothesis suggests that insect wings are an evolutionary novelty that is popping up yet from random tissue buds during development.

A more recent view is that they arose from existing structures already present in ancient crustaceans, slowly transforming over time into something more useful out of the water.

The gills of these ancient crustaceans are one of the best candidate appendages because they possess joints and muscles. In some crustacean larvae, they even look like mini wings.

But two new papers on a distant relative of winged insects suggest their legs are a better match.

Eliminate some genes on shrimp Parhyale hawaiensis, the first study shows that a genetic network similar to that of the insect’s wing operates both in the crustacean’s armor and in the leg segment closest to its body. This suggests that both are somehow squeezed through the body wall and come back to form wings.

The second study found something similar. By eliminating some genes, the researchers compared how the six leg segments on a fruit fly and other insects line up with the seven or eight leg segments found on P. hawaiensis.

Eventually, the first six leg segments of the crustacean, from the “tip” towards the body, matched perfectly with the first six segments found on the insect legs. But this begs the question: where did segments seven and eight in crustaceans in insects go?

The authors found their answer in an article written in 1893. It suggested that these proximal “lobes” on the crustacean leg had fused into the insect’s body wall. Since then, it has been noted that in many insect embryos, the leg segment closest to the body actually fuses with the body wall during development.

“But I didn’t have the story wing part yet,” explains molecular biologist Heather Bruce of the Woods Hole Oceanographic Institute.

“So I kept reading and reading, and I came across this 1980s theory that not only did insects embed their proximal leg region into the body wall, but the small lobes of the leg later moved onto the back. and they formed wings “.

Using genomic and embryonic data, Bruce and his colleagues found evidence to support this.

First, they say, the proximal lobes of the legs integrate into the body wall. Then, once there, the closest segment moves “to the back, to later form the insect wings”.

“The complementary perspectives of the leg and wing genes lead these groups to agree on answers to several key questions about the transformation of the crustacean winged insect,” two independent experts write in a review of the two studies for Nature Ecology and evolution.

“They agree that the lateral body wall of the insects is homologous to the most proximal leg segment of Parhyale. They also agree that the wing incorporates components of the body wall derived from the legs of crustaceans. “

However, the studies do not agree on everything. The first study supports what is known as the “dual origin” hypothesis, according to which the most proximal leg segments and the body wall both contributed to the development of the wings. Or, as the authors say, “novelty through the fusion of two distinct fabrics”.

The second paper proposes a more gradual and complex transformation that mainly concerns the leg segments. According to their findings, the two most proximal leg segments first fused into the insect’s body wall, and then only the closest leg segment squeezed the back to form wings.

(Bruce & Patel, Nature Ecology and Evolution, 2020)

The difference is subtle, and more research is needed to show which – if either – is more correct. But the similarities between the studies provide a compelling solution to the question of which of the earliest theories of insect wing evolution is right.

Bruce has argued for several years that ancestral crustaceans once contained eight leg segments. In today Parhyale, he argues, one of these was incorporated into the body wall, while in fruit flies, one was incorporated into the body wall and the other into its wing later.

This gives the insect wings the simple appearance of “double origin”, where the body wall and the leg merge to form wings, when in fact, the authors say, the insect body wall itself derives from the segments of the more proximal legs.

“Although the wings are a consequence of what is now the insect’s body wall, they owe their origin to the leg segment of an ancestral arthropod,” the authors conclude.

It’s a clear idea that helps bring together many competing hypotheses, but in all likelihood it won’t put an end to the mystery. In the last 10 years alone, we have come to learn a lot more about the evolution of insects.

Before genomic research, we didn’t even realize that crustaceans and insects were so closely related in the arthropod family, which is why many people thought insect wings came out of nowhere.

The gills, segmented legs, and body armor of the crustaceans have now given us direct targets to study.

“People are very excited about the idea that something like insect wings may have been a new breakthrough of evolution,” says Patel.

“But one of the stories that emerges from genomic comparisons is that nothing is brand new; everything came from somewhere. And you can, in fact, figure out where.”

Agreeing where is another matter.

“Although the origin of the insect wings remains mysterious, the research of both groups reveals exciting paths towards the definitive solution of this mystery”, concludes the Nature revision.

The first and second documents were both published in Nature, ecology and evolution.