ASU Research E-Magazine: Galling Matters

Species formation was thought to be a product of strict barriers between populations of the parent species. New research suggests much more subtle interactions with the environment can play a role.

One of the puzzles of evolution is how, exactly, one species becomes two. Molecular biologists routinely compare the relatedness of species by methods such as comparing the similarity of DNA sequences. But exactly how one species splits in the first place is not well understood.

For example, scientists know that the life span of various species of mammals can be quite long. As a result, researchers are lucky to be able to observe even a few generations of reproduction in any one species. Insects, however, are a much different story. The huge variety of insects may offer some clues as to how different species actually come into being.

There are more species of insects on Earth than any other type of living thing. Timothy Craig would like to know how this tremendous diversity occurred.

Craig is an associate professor of life sciences at Arizona State University’s West Campus. He has spent his scientific career studying the behavior of plant-eating insects. A tiny fly that likes goldenrod plants may offer some clues to the intriguing question of biodiversity.

The most widely accepted idea detailing the creation of species is called the theory of allopatric speciation. In basic terms, the theory says that for new species to form, a population of organisms must be geographically split from itself. Oceans and mountain ranges provide handy barriers.

A mountain range can prevent two populations of the same type of organism from interbreeding. Given enough time, each separate population becomes more adapted to its respective environment. For example, a population of creatures living on the ocean side of a mountain range would adapt to salty air and a temperate climate, while a population of the same creatures living on the desert side of those same mountains would adapt to a lack of water and intense heat.

Over time, the theory says that these populations would change so much that they could no longer interbreed. In essence, they become separate species.

"The allopatric model may not explain how speciation proceeds in all situation. Under some conditions, sympatric speciation may have occurred," Craig says.

Theories of sympatric speciation have been around for a long time. Those theories do not require geographic isolation or physical barriers for speciation to occur.

"There was a sort of revival of interest in these theories in the late 1960s," Craig explains. "A researcher named Guy Bush did studies and said, basically, that it was possible to have speciation without geographic isolation, but only if certain criteria are met."

Bush argued that sympatric speciation could occur in plant-eating insects if those insects tended to specialize on particular plants. The insects would eat only those plants. They also would most likely mate and lay their eggs on those same plants.

Prior to joining the ASU West faculty, Craig worked for a year with Warren G. Abrahamson at Bucknell University. They studied the goldenrod plant. Craig says s particular species of fly seems to fit the criteria outlined by Guy Bush.

The fly is known as Eurosta solidaginis. It lives in Minnesota and infests and parasitizes two particular species of goldenrod, Solidago altissima and Solidago gigantea. Fly infestations cause large round galls to form on the stems of these plants.

Craig and his colleagues were able to show that there are subtle differences, but differences nonetheless, between flies living on the two varieties of goldenrod.

The initial differences occur in the biochemical makeup of the flies.

Craig’s wife and colleague, Joanne Itami, is an associate research professor at ASU West. Chemical tests she has run on the flies indicate that proteins present in their bodies are, in fact, somewhat different.

"As yet, we haven’t found a way yet to distinguish between the flies on appearance," Craig says. But they have discovered differences in behavior that are profound.

"We found, somewhat to our surprise, that these flies are extremely reluctant to lay eggs on anything but their host plant," he explains. Often, the flies actually will die rather than lay eggs on the other species of goldenrod.

Craig says that this particular species of fly has formed what are called "host races." Host races are independent populations of flies that are reproductively isolated from each other due to their preference for a particular species of host plant.

The ASU West research group also found that when the flies were given only plain sticks, not from the goldenrod plant, they would mate with each other indiscriminately. The particular species of goldenrod, altissima or gigantea, allows the flies to choose the right mate.

The researchers discovered that the hybrid offspring of laboratory matings did not survive as well as pure host-race flies.

The two species of goldenrod vary in tiny ways. Craig has tested his hybrid flies on various plants. He found that the offspring of hybrid flies actually do fairly well on a few varieties of plants, but not on the majority of them.

"We think we have found what could be an intermediate stage in the process of speciation," Craig says. "The flies become differentiated and maintain some degree of differentiation even though they are mixed together."

Craig and Itami continue to spend summers in Minnesota, since Arizona is one of the few places where these specific species of goldenrod plants do not thrive. But much of Craig’s other research is being conducted in Phoenix. He and teams of both graduate and undergraduate students have started studies to investigate gall midges, tiny flies the size of gnats. They belong to the genus Asphondylia. Gall midges parasitize the salt bush, a plant common to the Sonoran Desert.

Gall midge infestations create large bulbous growths (galls) on salt bush stems and branches. The galls contain midge larvae. In turn, these larvae-filled galls are routinely attacked by a several different kinds of parasitoid wasps. The wasps penetrate the gall and lay their eggs inside individual fly larva. When they hatch, the wasp larvae proceed to eat the fly larva from the inside.

"Parasitoid wasps are like the creatures in the movie Alien," Craig says. "The film makers actually got the concept for their monster from these wasps."

The gall midge, salt bush, and wasps form what scientists call a three trophic, or three-level ecological system. The success of each individual participant species depends in some form on actions of the other participants in the system.

All of Craig and Itami’s earlier work in Flagstaff focused on insect/plant interactions on willow trees. A few years ago, they were able to show that certain female sawflies actually varied the sex of their offspring depending on the vigor of the plant on which they chose to lay eggs. Bigger, healthier plants got female eggs; weaker plants got male eggs.

The ASU West researchers found that female size plays a large role in how successful the sawflies are at producing offspring. The bigger the plant, the bigger the female fly. On the other hand, males sawflies do not require great size or strength to mate. As a result, they tend to come from much smaller plants.

Ecology is an intensely complex subject. Researchers must work hard to understand even the simplest ecological system. Craig and Itami believe that big ideas can come from small animals. Charles Darwin thought the same way. The English naturalist spent a number of years writing a book on earthworms.

From their investigations, Craig and Itami are devising ideas that could someday help us better understand our own place on this planet.—John Svetlik