1. Humans, Senses, and Proteins

As a species, we are deeply visual. The dazzling lights of our cities as seen from space, the patchwork of farmlands from an airplane, or the gaze of a loved one all captivate us. For many, sight equates to comprehension.

The genetics and evolution behind our vision are intriguing. Recent discoveries shed light on the origins of opsins, proteins crucial for vision, which can be traced back to serendipitous findings.

In April 2020, researchers revealed that certain light-sensitive proteins known as rhodopsins, typically located in the eyes, were also present in the mouthparts of the fruit fly, Drosophila melanogaster.

While it's not uncommon to find proteins in atypical locations within an organism, these particular rhodopsins are primarily recognized for their role in vision. Interestingly, in Drosophila, these proteins also served as taste sensors, enabling the flies to detect minute quantities of bitter (toxic) substances.

The striking implication is that opsins might have originally evolved from chemical sensors, suggesting a fascinating link between taste and vision.

2. Evolution of Eyes

Our eyes, which resemble sophisticated cameras, comprise a cornea to capture light, an iris to control its intensity, a lens for focusing, and a retina that transforms light into neural signals. For a more detailed exploration of eyes, I have discussed them extensively in previous articles.

Vertebrates, including mammals, birds, reptiles, amphibians, and fish, share a common eye design, stemming from ancestors that lived hundreds of millions of years ago. These ancestors possessed a central nervous system and camera-like eyes, features that have been inherited across various species.

However, evolution is complex; while vertebrate eyes may have evolved from a common ancestor, similar traits can also arise independently in different species, a phenomenon known as convergent evolution. An example is found in certain algae that have developed camera-like eyes independently from ours.

Interestingly, even among species with vastly different eye structures, such as insects with compound eyes, the fundamental genetics for eye development remain consistent. Rhodopsins are a common feature across these diverse taxa.

Tracing back to our common ancestors, we find rudimentary eye structures in early organisms like worms and mollusks, which utilized primitive rhodopsins for light detection. This underscores the idea that certain proteins resembling rhodopsins evolved independently across various life forms, highlighting the antiquity of light-sensing proteins.

3. The Role of Flies in Our Understanding

The discovery of rhodopsins in the mouthparts of Drosophila raises questions about the significance of flies in our biological understanding. They serve as a convenient model organism due to our shared evolutionary lineage, as noted by Darwin:

“…all the organic beings which have ever lived on this earth may be descended from some one primordial form” — C. Darwin

This evolutionary connection allows scientists to study simpler organisms, such as yeast and the roundworm Caenorhabditis elegans, to gain insights into human biology.

Research from 2008 and 2016 revealed that C. elegans possesses a UV light-sensing protein, LITE-1, which has connections to taste-sensing proteins, including GUR-3. Both belong to a category known as receptors.

Drosophila's rhodopsin, which serves a dual purpose in sensing light and taste, and C. elegans’ UV receptor highlight an intricate relationship between the two senses. Researchers have postulated that rhodopsins may have evolved from chemoreceptors, while parallel studies demonstrated the close relationship between taste and UV receptors.

4. Understanding Receptors

From the earliest single-celled organisms, the need to perceive environmental stimuli was crucial for survival. Cells are encased in membranes that protect vital components while allowing communication with the outside world.

Receptors are integral to this process. They detect signals—light, chemicals, or other stimuli—and transmit them into the cell. For instance, your eyes are receptors that sense light and communicate with your brain via the optic nerve.

The G-protein coupled receptors (GPCRs) form a significant class of these receptors. They, including rhodopsin, share a distinctive structure characterized by seven membrane-spanning segments. The intricate design of these proteins is crucial for their functionality.

In rhodopsin, a molecule known as 11-cis retinal, derived from vitamin A, is essential for light detection. When it absorbs a photon, it undergoes a structural change, triggering a series of reactions within the cell.

This alteration in the retinal molecule prompts a corresponding change in the rhodopsin protein, initiating a signaling cascade within the cell—a fundamental aspect of GPCR functionality.

5. Taste Receptors: An Evolutionary Perspective

Taste receptors, stemming from primitive chemical sensors, have evolved to detect essential compounds in the environment. These receptors, like those for taste in humans and other animals, also belong to the GPCR family.

The connection between taste receptors and light-sensing proteins highlights the evolutionary versatility of receptors. For instance, researchers demonstrated that a minor alteration to the GUR-3 receptor in worms could convert it into a UV sensor, showcasing the adaptability of these proteins.

Modern technology facilitates the exploration of biological relationships. For example, I utilized the DELTA-BLAST program to compare amino acid sequences of rhodopsins from various organisms with proteins in Schizosaccharomyces pombe, a yeast model. The results consistently pointed to a specific GPCR, Map3, which detects pheromones essential for yeast mating.

Map3 functions as a chemical sensor that detects mating pheromones, paralleling the function of taste receptors. This connection suggests that human and animal rhodopsin GPCRs share a lineage with yeast proteins, indicating an evolutionary relationship.

The interrelation between taste receptors like GUR-3 and UV receptors like LITE-1 further emphasizes the adaptability of proteins. It implies that minor modifications in protein structure could lead to the evolution of new functions, such as transforming a taste receptor into a light sensor.

In essence, chemical and taste receptors likely represent one of the earliest sensor types, evolving to detect various stimuli as life progressed into light-permeable environments.