Frogs have a ‘third eye’ for direct light-sensing

Living beings have evolved over millennia to respond to varying amounts of sunlight, affecting everything from sleep-wake cycles to seasonal changes. The proteins responsible for these responses, called opsins, are still an underexplored area of science. 

Now, a study led by York University has revealed that frogs have retained a surprising number and diversity of these light-sensing proteins over evolutionary time. 

Nonvisual opsin genes in frogs

“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,” said senior author Ryan Schott, an assistant professor of biology at York. 

“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.”

Light-sensing third eye in frogs

Nonvisual opsins are present 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 responds to light by suppressing or secreting hormones. 

This process is indirect, but frogs still possess a directly light-sensing “third eye” that other animals 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,” Schott explained. “Most of these opsins are also still expressed in the eye, playing a large role in light detection functions not directly related to vision.”

Genetic data from frog eyes

Frogs provide an excellent opportunity to study these proteins under diverse ecological conditions. 

To investigate this diversity, the researchers combined genetic data from the eyes of 81 frog species with publicly available genomes and multi-tissue transcriptome data from 21 additional species. This sampling covered a wide range of frogs with different ecological adaptations.

“Frogs are cool because different species can live in the water, on land, in trees, or even underground,” said lead author Jack Boyette, a doctoral student at Pennsylvania State University. “All these different habitats have very distinct light environments, which has implications for the evolution and function of sensory systems.”

Drastically different light environments 

While many animal groups, including mammals and snakes, have lost opsin genes through evolution, frogs have retained all 18 ancestral vertebrate nonvisual opsins. This light-sensing retention may be due to the complex life histories of frogs. 

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

The researchers also identified genetic differences in opsins between groups with different ecologies, life histories, and body types, suggesting that frog nonvisual opsins have adapted to specific lifestyles or environments. This finding aligns with Schott’s previous research on visual opsins in frogs’ eyes. 

Unknown functions of opsin genes 

Thus, this study offers initial insights into how opsin genes, whose functions are currently unknown, might operate in frogs. The experts have already 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 need,” said Schott.

“It’s a really exciting step towards a better understanding of these seasonal patterns and how frogs and other animals use light in different ways to regulate their biological functions.”

Specialized vision in frogs

Frogs have a unique and specialized vision that allows them to thrive in their environments. 

Their eyes are typically positioned on the sides of their heads, giving them a wide field of view and enabling them to see almost 360 degrees around them. This positioning is crucial for detecting predators and prey. 

Retinal ganglion cells

Frog eyes have a horizontal stripe of retinal ganglion cells, which is highly sensitive to movement across the water’s surface. This adaptation helps them detect the movement of insects and other prey. 

Night vision

Frogs also have excellent night vision, as many species are nocturnal. They possess a high number of rod cells in their retinas, which are more sensitive to low light levels than cone cells.

Color vision

Additionally, frogs can see in color, which is uncommon among many nocturnal animals. 

They have three types of cone cells that detect different wavelengths of light, allowing them to see a range of colors. This capability is essential for recognizing potential mates and food sources. 

Underwater vision

Interestingly, frog vision is adapted to their semi-aquatic lifestyle. When underwater, frogs can adjust their focus by moving the lens in their eyes, similar to how humans focus their vision, allowing them to see clearly both in water and on land. 

This multifaceted visual system is a key factor in their ability to survive and thrive in various environments.

The study is published in the journal Molecular Biology and Evolution.

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