Unexpected diversity of light-sensing proteins goes beyond vision in frogs

June 20 marks the first day of summer in the Northern hemisphere, the longest day of the year. Living beings have evolved over many millennia to react to varying amounts of sunlight exposure, governing everything from sleep-wake cycles, seasonal changes and more, but the proteins responsible for responding to different light environments for non-visual purposes are an under explored area of science.  New research, led by a York University Faculty of Science professor and a former York researcher, found that frogs have maintained a shocking number, and diversity, of these light-sensing proteins, called opsins, over evolutionary time.

“We, and other animals, have many different types of nonvisual opsins and they can be present in different parts of the body including the eyes, brain, and skin. Right now, the days are getting longer as we approach summer and nonvisual opsins are involved in how our bodies respond to those differences,” says York Assistant Professor Ryan Schott in the Department of Biology & Centre for Vision Research. “We found that frogs, despite being a largely nocturnal group, actually maintain more of these nonvisual opsin genes than any other group that is ancestrally nocturnal.”

Nonvisual opsins are found throughout the animal kingdom. In humans and other mammals, information about lighting conditions enters through the eye and is sent to the pineal gland, which will respond to light by suppressing or secreting hormones. This is an indirect process, but frogs still have a directly light sensing “third eye” that others in the animal kingdom lost long ago.

“There are several nonvisual opsins present in that organ in the top of the head, and that is going to help them regulate their day and night cycles,” says Schott. “Something interesting we found though was that most of these opsins are also still expressed in the eye, so the eye is still having a large role to play in light detection functions that aren't directly related to vision.”

Frogs, the researchers said, provide an opportunity to study the proteins under diverse ecological conditions. To investigate this diversity in frogs, the researchers combined genetic data from transcriptomes — the genetic sequences of all genes expressed in an organ — from the eyes of 81 frog species with publicly available genomes and multi-tissue transcriptome data from 21 additional species. These 102 species provided a broad sampling of frogs with different ecological adaptations.

“Frogs are cool because different species can live in the water, on land, in trees, or even underground,” says former Schott and Bell lab researcher Jack Boyette, lead author on the paper and current doctoral student at Penn State. “This gets further complicated by things like activity period — a lot of frog species are active at night, but some are active during the daytime. As you can imagine, all these different habitats have very distinct light environments, which has implications for the evolution and the function of sensory systems.”

The researchers say several groups, including mammals and snakes, have lost many opsin genes through the course of evolution, which might be explained by going through an evolutionary period where they lived nocturnally and the ability to sense light was not as important.  

Frogs are also an ancestrally nocturnal group, so the researchers expected to find reduced nonvisual opsin diversity in frogs. Remarkably, the frog genomes assessed in this study contained all 18 ancestral vertebrate nonvisual opsins. This surprising finding may result from complex life histories. 

“Within the lifetime of a single animal, many frog species transition between drastically different light environments,” Boyette said. “Even though a lot of adult frogs are nocturnal, that's not necessarily true of the larval tadpoles.”  

Additionally, the researchers identified genetic differences in opsins between groups with differing ecologies, life histories, and body types. This could potentially indicate that frog nonvisual opsins have adapted to specific lifestyles or environments, similar to findings in Schott’s last study which looked at the visual opsins in frogs’ eyes.

Other members of the research team include Rayna C Bell, California Academy of Sciences and National Museum of Natural History, Smithsonian Institution; Matthew K Fujita and Kate N Thomas, University of Texas at Arlington; Jeffrey W Stretcher and David J Gower, Natural History Museum of London.

The findings were published today in the journal Molecular Biology and Evolution.

Schott says this study has given first hints about how opsin genes whose functions are currently unknown might operate in frogs and they’ve identified a candidate gene that may be involved in regulating seasonal breeding in frogs.

“We still need a better understanding of the specific functions of each type of nonvisual opsin and how those functions have evolved and adapted in different animals, like the frogs in our study, to meet their specific needs,” says Schott. “It's a really exciting step towards a better understanding these seasonal patterns and how frogs and other animals use light in different ways to regulate their biological functions.”

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Media Contacts: Emina Gamulin, York University Media Relations and External Communications, 437-217-6362, egamulin@yorku.ca