Many insects see the world through hundreds of tiny lenses that are grouped together in a visual organ known as the compound eye. Behind each lens sits a cluster of eight light-detecting cells arranged in a pattern that scientists believe has existed for hundreds of millions of years.
Butterflies are the rare exception. They are equipped with nine light-detecting cells, which gives them the ability to visually navigate a much richer world of color, compared with insects such as flies, and allows them to locate nectar and potential mates.
University of California San Diego scientists in School of Biological Sciences Associate Professor Michael Perry's lab have discovered the genetic modification that gave butterflies this visual advantage.
"Butterflies see far more color than flies do because at some point in their evolution they added a photoreceptor to every unit of their compound eye — a rare break from an eye design that has otherwise been conserved across insects for hundreds of millions of years," said Perry, a faculty member in the Department of Cell and Developmental Biology. "We found the genetic switch that did it.". "We found the genetic switch that did it."
In a study published in the journal Science Advances, Perry and his coauthors identified the genetic steps involved in the emergence of the ninth photoreceptor in painted lady butterflies, the most widespread butterfly species in the world. While flies feature light-sensing cells known as photoreceptors R1-R8, butterflies expanded their color vision by adding a second R7 photoreceptor to each unit of the eye, the researchers found. To test whether that genetic change was enough on its own, they recreated it in a fruit fly — switching on a gene in cells that normally keeps it off, and timing it to the brief window when the eye is being built. This produced a "butterfly fly" that grows its eyes on the butterfly plan, with nine cells per eye unit instead of eight.
A new light detector in the eye would be of no use if the brain is not able to connect to the new information provided by that detector. The scientists assumed that adding a sensory input such as a new photoreceptor unit should require the brain to slowly evolve a matching neuron partner that would receive information on the other end. That didn't happen, the researchers found. To their surprise, no adaptive change was needed.
The fly brain regularly overproduces neurons that die off if they fail to find a connection. The brain of the butterfly-fly, they found, knew precisely how to handle the new photoreceptor by using its extra "standby" neurons and put them in play.
"When we gave those spare neurons something to connect to, they survived and wired up correctly — immediately — with no further genetic change," said Perry. "In other words, the brain was ready before the eye asked. This is a rare, concrete case of evolution making use of neurons that were otherwise going to die."
The new study's findings provide a glimpse of how insects, with a flexible brain framework, were able to accommodate new inputs to evolutionarily adapt to an emerging need in their environment: an elevated ability to visualize flowering plants.
The team also found a hawkmoth that appears to be partway through the same transition: the lower half of its eye is butterfly-like, with two of these cells per unit, while the upper half is fly-like, with one. That is roughly what you would expect if the change began in one region of the eye and spread.
The research team is now exploring further unanswered questions, such as the difficult-to-study issue of whether the butterfly-fly can in fact see more vivid color with the additional photoreceptor, as butterflies do.
