In evolution, the primate eyes have it

CORNELL (US)—Researchers studying vision differences in two monkey species have indentified an evolutionary mechanism that enables changes in brain structure to alter the way primates see.

It turns out the owl and capuchin monkeys, which evolved from a common ancestor about 15 million years ago, have minor differences in the timing of cell proliferation, which explains major differences in their eyesight, says Barbara Finlay, neurobiologist and psychologist at Cornell University.

In both monkey species, the specialized eye cells develop in the growing embryo from a single retinal progenitor cell. In their basic design, the eyes of these primates have the capability and necessary architecture to be either nocturnal or diurnal, based on a species’ ecological niche and needs, Finlay says.

Finlay and her colleagues compared the developing eyes in fetuses of the two species to better understand how the nocturnal owl monkeys developed retinas with many more rod cells than cones, while capuchin monkeys, which are active during the day (diurnal), developed more cone cells than rods. Cones help distinguish color during the day, while light-sensitive rods are needed for night vision.

“So we believed that comparing how their eyes develop during embryonic growth could help us understand what evolutionary changes would be required to evolve from a diurnal to a nocturnal eye,” explains Finlay.

By comparing the timing of retinal cell proliferation in the two species, the researchers found evidence that an extended period of progenitor cell proliferation in the owl monkey gave rise to an increased number of rods and other associated cells that make its eyes adept at night vision; the eyes also evolved to be large, with bigger light-gathering and light-sensing structures needed for nocturnal sight.

“The beauty of the evolutionary mechanism we have identified is that it enables the eye to almost toggle back and forth between a nocturnal and a diurnal structure,” says neurobiologist Michael Dyer of St. Jude’s Hospital in Tennessee. “It is an elegant system that gives the eye a lot of flexibility in terms of specialization.”

This research was funded by the National Science Foundation and Brazil’s NSF equivalent, National Counsel of Technological and Scientific Development. Researchers from Cornell, St. Jude’s Children’s Research Hospital, and the Federal University of Para, Brazil, were involved in the study.

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