Biologists Show How Eye’s Neurons Switch Functions During Metamorphosis

By James Devitt

Researchers at NYU’s Center for Developmental Genetics have found that the photoreceptors in an insect’s eye can change their traditional functions during metamorphosis. The study appeared this summer in the journal Nature.
    The researchers found that when photoreceptors responsible for detecting the color green die off during metamorphosis a second class of photoreceptors—those responsible for detecting the color blue—then fill the role of detecting the color green. These rare switches, the authors speculate, are likely the result of changing life patterns.
    The study’s authors, NYU biology postdoctoral fellow Simon Sprecher and biology professor Claude Desplan, note that although examples of such switches are extremely unusual, they may be more common than is currently understood. They point to the Pacific pink salmon and rainbow trout, in which newly hatched fish express an ultraviolet receptor that changes to a blue receptor as the fish ages. As in flies, this switch might reflect an adaptation of vision to the changing lifestyle: the maturing salmon, born in shallow water, later migrates deeper in the ocean where ultraviolet does not penetrate, making detection of ultraviolet unnecessary.    
    Sprecher and Desplan examined the eye of the fruit fly Drosophila, which can be analyzed and manipulated in exquisite detail and can serve as a powerful model system to understand biological processes such as vision. The flies start their lives as a larva and possess two very simple eyes, each composed of 12 photoreceptors, eight of which are devoted to detecting the color green (also used in the functioning of the fly’s biological clock) and four used to detect blue. When the larva metamorphose to become an adult, the larval eye does not disappear but instead becomes an even simpler visual organ called the eyelet that serves exclusively for entraining the biological clock. This eyelet contains only four green photoreceptors and no blue ones. The researchers found that all green photoreceptors degenerate and disappear during early phases of metamorphosis while the blue photoreceptors remain throughout pupation—the process in which an insect reaches maturation—and eventually take over. The researchers believe the catalyst for the switch is the hormone ecdysone.  
    The study was funded by the National Eye Institute of the National Institutes of Health.