Joint Congress on Evolutionary Biology. By the skin of their teeth.

Abstract

C R E D IT S ( T O P T O B O T T O M ): D A P H N E S O A R E S ; R O B IN H O P K IN S /U N IV E R S IT Y O F T E X A S , A U S T IN doctoral fellow at the University of Texas, Austin, has unearthed evidence that this red form persists because it helps keep hybrids from forming between the annual and pointed phlox, and at the meeting she reported that the eastern annual phlox is not as well adapted to its pollinators as the original variety. Colleagues say that Hopkins’s series of results illustrates how two species stay apart through a process called reinforcement. “She went soup to nuts,” says Mohamed Noor, an evolutionary biologist at Duke University in Durham, North Carolina. Not only did she show how reinforcement operates, “she went and identifi ed the individual genes that are responsible for it.” In 1889, Alfred Russel Wallace proposed that if hybrids resulting from the mating of two species were inferior, their lack of fi tness would favor the evolution of traits in the two species that eliminate hybrids. (And when reinforcement occurs between two subspecies, it can help push them to become fully distinct species.) But documenting this reinforcement process has been diffi cult. Some researchers have found evidence of reinforcement in laboratory fruit fl ies, but few studies had been done in a natural setting. “[Hopkins] has identifi ed a system in which she could study it in real time and in nature,” says Janette Boughman, an evolutionary biologist at Michigan State University in East Lansing. In 2011, seeking to understand why the eastern Texas annual phlox was red, Hopkins tracked down the genetic basis of the hue change. She found that the variety had mutations that altered the activity of two genes involved in producing the blue color. One genetic change makes the color more intense, leading to darker blooms, though still blue. The second mutation ramps down the production of the blue pigment, resulting in a reddish hue. Together, they produce the deep-red fl ower. Hopkins also found that the color change is recessive and the intensity gene is dominant, so any hybrids between any annual phlox and the pointed phlox are dark blue. Previous studies have shown that this dark blue is less attractive to butterfl ies that pollinate these fl owers, so Hopkins looked more closely at how pollinators used these different-colored blossoms. She crossbred the central and eastern Texas annual phlox varieties to get plants with light or dark red or light or dark blue fl owers and grew these plants under the same conditions in the fi eld. Sometimes she grew pointed phlox with the annual phlox. When pointed phlox was present, about 40% of the offspring of the light-colored annual phlox were hybrids with the pointed phlox, while only 10% of the offspring of the dark-fl owered forms were, indicating that color intensity, not color per se, was most important for reinforcement to occur. “There’s been selection for a fl ower-color difference in order to reduce hybridization,” Boughman says (Science, 2 March, p. 1090). Hopkins has now tackled the question of why the red form hasn’t spread beyond eastern Texas. When she grew red and blue annual phlox under the same conditions, the two varieties did equally well in terms of the number of fruits set, she reported at the meeting. But when she looked at pollinator preferences, butterflies favored the light blue over either the dark blue or dark or light red, suggesting an advantage to being light blue. “That may explain why the light blue stays” and why genetic variation is maintained in the annual phlox, she concluded. Although there is a cost for having a dark color, that cost is justifi ed when two phlox species are present. “She’s given us a level of insight into the process of reinforcement that few other studies have done,” Boughman says. The work “helps us to understand the process of speciation and the multiple ways in which speciation can happen.” –ELIZABETH PENNISI In a fast-fl owing underground stream in a cave in Ecuador, a 7-centimeter-long catfi sh holds its own thanks to an unusual sense organ. Astroblepus pholeter relies on teeth to sense the water fl ow around it. Not the teeth in its mouth, but ones sticking out all over the catfish’s body, sensory neurobiologist Daphne Soares of the University of Maryland, College Park, reported at the meeting. Soares has long puzzled over this Ecuadorian fi sh, which has no functional eyes and doesn’t even have a lateral line, a row of specialized hair cells along the sides of most fi sh that sense vibration and the movement of water. Scanning electron microscopy studies she recently conducted revealed that the catfi sh’s body is covered with bumps that proved to be teeth—complete with enamel and dentine. The teeth protrude through the skin enough that they move back and forth as the animal swims, relaying the surrounding water’s movement to the catfi sh’s brain, Soares reported. When she scraped the teeth off such fi sh, they swam crookedly, didn’t orient to the water fl ow, and failed to stick to rocks as they normally would. The teeth hook up to a large nerve that in other fi sh is connected to the lateral line. When Soares examined museum specimens of other catfi sh, she found that they too have these dermal teeth, but only around the mouth. She thinks that when surface-dwelling catfi sh fi rst became cave-locked, their lateral line was ill-suited for the high fl ow and eventually gave way to these dermal teeth. Soares has looked in other caves in Ecuador but thus far has found no other fi sh with this adaptation. Worried that these teeth-wielding fi sh may be limited to just one cave, Soares has stopped studying this population and is instead trying to get some protection for it. “It’s not listed as endangered, but I think it should be,” she said. –E.P. Phlox of a different color. Annual phlox (top) evolved a red variety (bottom) so as to keep its species separate from another phlox (inset). By the Skin of Their Teeth >>

DOI: 10.1126/science.337.6093.409

Cite this paper

@article{Pennisi2012JointCO, title={Joint Congress on Evolutionary Biology. By the skin of their teeth.}, author={Elizabeth Pennisi}, journal={Science}, year={2012}, volume={337 6093}, pages={409} }