ENHS Nature Trails May 98 Article 2

President's column: Soaring through Time: the Origin of the Insect Wing
by Nathan Tublitz

Do you believe in evolution? Darwin's thesis -- that all living organisms arose from a common ancestor through a process of natural selection -- is widely but not universally held by biologists and lay people alike throughout the world. It is the single most important concept in biology if not in all of science, with far ranging implications across all aspects of our culture. As with all revolutionary ideas, evolution is easy to grasp: gradual changes in form and/or function drive the birth of new species. Darwin's classic example of evolution is the multiple types of Galapagos finches, each of which evolved a different beak shape to take advantage of a specific environmental niche. Such small step changes, known as microevolution, have been well documented in many different species -- the evolution of horses and humans are two good examples -- and provide a strong foundation supporting Darwin's theory of evolution.

Evolutionary theorists have done a good job of explaining microevolutionary changes, but the origin of such novel features as the vertebrate brain, the ability to echolocate, lungs, or bird feathers have provided a much greater challenge. The relatively sudden appearance of major anatomical or physiological innovations in new species, termed "macroevolution", is currently the source of much spirited discussion among evolutionary biologists.

One of the most fascinating topics in this field is the origin of the wings of insects. Insects are arthropods, that group of animals that include spiders, lobsters, centipedes and millipedes. Not all insects have wings, but those that do comprise the only arthropods able to fly. How did these winged insects arise from their earth-bound brethren? There are two competing hypotheses that attempt to explain the origin of insect wings: one postulates that wings derive from a leg-like appendage in some closely related ancestor whereas the other postulates that wings arose completely independently as an outgrowth from the thorax, the region of the insect body between the head and abdomen.

One test of these hypotheses is to determine whether insect wings have any similarity to the appendages of other arthropods, and that is what several research groups have been investigating in using both traditional morphological criteria and modern molecular biology techniques. Traditional morphological techniques, rooted in the "this-bit-in- this-organism-looks-and-works-similar-to-that-bit-in- that-organism" approach, falls short when comparing insect wings with the appendages on the same segments in crustaceans, lobster- and crab-like animals who are the closest living relatives to the insects. Crustacean appendages, although somewhat flap-like, are at first glance a far cry from the stiff and scale-covered wings of insects. More detailed observations reveal that insect wings have a fringe of short hairs as do some crustacean appendages. So perhaps the two are more similar that first thought, but there has been little additional morphological evidence to support the notion that crustacean appendages are the ancestral origin of insect wings. However, recent molecular work is beginning to suggest otherwise.

The past 20 years have been nothing less than revolutionary in biology because of the advent of molecular biological techniques enabling biologists to identify and analyze individual genes. Much of this work has been performed in relatively few species, in part due to the difficulty and fickleness of molecular procedures. Perhaps the most analyzed creature from a genetic and molecular standpoint is the little fruit fly, Drosophila melanogaster, and the fly wing has been one of the most intensively studied body parts. The genes controlling wing development and growth have been well described, and recently have been used to determine if the appendages of non-insect arthropods use the same genetic building blocks. Using brine shrimp (Artemia franciscana), researchers have found that the same genes used to build the wings in Drosophila are also involved in the construction of the appendages of brine shrimp. These exciting and unexpected findings are indicative of an evolutionary relationship between these two structures and suggest that the wings of insects might indeed have arisen from a similar structure in a close relative. But here's the rub: all appendages in crustaceans stick out of the belly whereas the wings in insects attach on the back. If insect wings evolved from their crustacean counterparts, they had to have somehow migrated around the body from the belly to the back. This problem, albeit difficult, is not developmentally impossible, however it does require changes in several genes, and researchers are currently looking into this possibility by comparing the appropriate genes in flies and brine shrimp.

Regardless of the outcome, insect wings are clearly the product of genes used in ancestral species for other purposes, a conclusion that supports the basic premise underlying evolution. Molecular analyses such as the one discussed here are providing new data for many of the macroevolutionary changes that have been previously unexplainable using traditional anatomical techniques. Darwin would have been proud that his theory has remained intact after such rigorous testing.
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