Researching Eyes of the Butterflies

Sexual reproduction comes with a cost. Whether it be in the form of energy, time or survival, these factors are all something in which have to be consideration.

An example of this in nature can be found in butterflies. An international team of researchers have found compelling evidence that points to a species of South American butterflies that have acquired its unique vision as a result of unusual evolutionary conditions.

Adriana Briscoe, associate professor in the department of ecology and evolutionary biology, led a research team studying this species; specifically, the passion-vine, or Heliconius, butterflies. The team found that this species is fitted with ultra-violet yellow wing pigments and the ability to see those UV yellows. They believe that these vision and visual signal adaptations help Heliconius butterflies locate mates in an ecosystem rifed with mimicry.

Heliconius butterflies have piqued scientific interest for more than 150 years. This is partly because of their unique environmental pressures in which they live with thousands of closely-related look-a-likes that all share similar wing variations to avoid predators.

“Heliconius are highly species-rich, and have the additional pressure of living with mimicking relatives,” Briscoe, said.

Robert Reed, assistant professor in the department of ecology and evolutionary biology, commented on this phenomena.

“There are 40 species and over 300 different [wing] patterns of these related butterflies. There can be 20 different wing pattern variants in one species,”  Reed remarked.

Reed studies the wing patterns and development of Heliconius butterflies. He also assisted with a test in the group’s research.

Passion-vine butterflies receive their  name from the toxins they ingest from eating passion vines as caterpillars. Birds and other predators have learned to avoid these foul-tasting butterflies by recognizing wing characteristics of the most common passion-vine butterflies. Over the course of several  million years, many butterflies have strategically developed strikingly similar wing patterns to deter predators.

“Some butterflies I can only [tell apart] by looking at their DNA,” Reed commented as he showcased various Heliconius wing patterns.

It was both an evolutionary blessing and an accidental gene duplication that may have made the Heliconius butterfly able to distinguish its own kind from other species. Between 12 and 25 million years ago, a second copy of a gene expressing a range of UV visions, common in butterflies today, was introduced into their genetic makeup. An eventual mutation in that redundant gene introduced a new function: sight in a different and broader range of the UV spectrum.

“Most butterflies have three visual pigment genes — blue, green and UV. Heliconius have four visual pigment genes, two in the UV spectrum,” Briscoe said.

This broader range of UV vision in Heliconius butterflies developed around the same time as their unique ultra-violet yellow wing pigments.

Briscoe noted that their study provides compelling evidence that these traits developed together.”

Heliconius butterflies are likely an example of correlated evolution, where a visual signal and the ability to perceive that signal developed relatively closely in time. The aptitude to recognize the UV yellow wing pigments of  passion-vine butterflies may have aided the survival of the species by cutting down on the confusion and wasted energy spent on courting a mimicking butterfly of another species. This is critical in minimizing the cost-benefit ratio of sexual reproduction.

However, scientists cannot presume to say which trait was developed first in the species or even which appeared first in the genetic makeup of the Heliconius butterflies.

Marilous Sison-Mangus, a postdoctoral scholar in civil and environmental engineering, confirmed this.

“We have no idea which came first — it’s like a question of the chicken and the egg,” Sison-Mangus said.

Sison-Mangus collaborated with the team to produce the genotype of the Heliconius butterfly. Sison-Mangus first discovered the duplicate gene when working with a graduate student at UCI.

“It’s one of the few studies to connect wing color and vision [in these butterflies],” Sison-Mangus said.

If that is true, the attention the paper has received is no surprise. Even before its publication in the prestigious scientific journal “Proceedings of the National Academy of Sciences (PNAS),” the study received substantial interest. The Discovery Channel contacted Briscoe about discussing the work of her and her colleagues, Seth Bybee, Gary Bernard, Furong Yuan, Marilou Sison-Mangus, Robert Reed, Andrew Warren, Jorge Llorente-Bosquets and Chuan-Chin Chao.

Briscoe and colleagues hope to expand on the paper published in PNAS last Wednesday. With their roster of Heliconius butterflies with the duplicate gene expanded and statistical evidence that the evolution of this gene was a result of its benefit to the species, the group is already working on another paper.

Behavioral tests may help to substantiate their thesis that the UV yellow wing pigments of Heliconius butterflies heightened the species ability to locate and reproduce with appropriate mates.

“We want to leave no stone unturned. We want to make sure that this duplication applies to the whole of the Heliconius,” Briscoe said.