Insect wings could have evolved from crustacean legs

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

Today, modern crabs, lobster, shrimp and crayfish are sometimes referred to as the insects of the sea, and as part of the arthropod family – marked by strong body armor and segmented joints – the name has a some sense.

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

If they’re right, that means the first insects buzzed around our planet long before birds, bats, and pterosaurs – so where did they get this ability?

It’s a simple question that has bothered experts for centuries. A century-old hypothesis suggests insect wings are an evolutionary novelty that is emerging again from random tissue buds during development.

A more recent opinion is that they originate 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 appendix candidates because they have both joints and muscles. In some crustacean larvae, they even look like mini wings.

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

Knock Out Certain Genes On Shrimp Parhyale hawaiensis, the first study shows that a network of genes similar to that of the insect wing functions both in the body armor of the crustacean and in the segment of the leg closest to its body. This suggests that the two have somehow squeezed through the body wall and come out to form wings.

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

Ultimately, the first six segments of the crustacean’s leg, from the toe to the body, matched perfectly with the first six segments found on the paws of insects. But that begs the question – where have segments seven and eight gone in crustaceans in insects?

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

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

“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. ”

Using genomic and embryonic data, Bruce and his colleagues found supporting evidence.

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

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

“They agree that the side wall of the insect body is homologous to the most proximal leg segment of Parhyal. They also agree that the wing includes body wall components derived from crustacean legs. “

Studies do not agree on everything, however. The first study supports what is called a “dual origin” hypothesis, whereby the most proximal leg segments and body wall both contributed to wing development. Or, as the authors say, “novelty through the fusion of two distinct fabrics”.

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

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

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

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

This gives the wings of insects the simple appearance of “double origin”, where the body wall and the leg intermingle to form wings, when in fact, say the authors, the body wall of the insect itself. same is derived from the most proximal leg segments.

“While the wings are an outgrowth 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 good idea that brings together a lot of competing hypotheses, but in all likelihood it won’t end the mystery. In the last 10 years alone, we have learned a lot more about the evolution of insects.

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

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

“People are very excited that something like insect wings may have been a breakthrough evolutionary innovation,” Patel says.

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

Agreeing on where is another question.

“Although the origin of insect wings remains mysterious, the research of the two groups reveals exciting ways to finally solve this mystery”, concludes the Nature review.

The first and second articles were both published in Nature’s ecology and evolution.