Recent studies have shown that thermosensory signaling cascades may allow fruitflies to detect small temperature differences by amplifying them and allowing the animal to adapt to temperatures that are less than optimal but still permissive for survival.
Animals can directly differentiate between different temperatures in their environment using communication pathways between receptors inside their bodies.
However, some animals, including the fruitfly, have been found to sometimes use an indirect pathway, commonly referred to as a “signaling cascade,” that involves other factors such as G proteins and phospholipases to detect small differences in ambient temperature.
Receptors called G protein-coupled receptors (GPCRs) are potential factors for initiating such cascades that are involved in thermosensory functions because GPCRs also involve pathways that include G proteins and phospholipases.
Recently, a group of scientists led by Wei L. Shen from the Department of Biological Chemistry and Neuroscience at the Center for Sensory Biology at the the Hopkins School of Medicine conducted an investigation into whether GPCRs, and specifically a GPCR called rhodopsin, are involved in thermosensation by activating channels that cause fruitfly larvae to move preferentially towards their optimal temperature — 18 degrees Celsius.
Selecting which GPCR to study is a difficult task because there are up to 200 GPCRs encoded in flies, and there is no precedent for a GPCR that functions in thermosensation. However, the scientists chose a GPCR called rhodopsin, which functions exclusively in light sensation because other proteins were found to function in both thermosensation and light sensation.
“For decades, this well-known molecule — one of the most-studied sensory receptors — was thought to function exclusively in the eye as a light receptor, but now we have found that fly larvae and possibly other organisms use it to distinguish between slight temperature differences,” said Craig Montell, a professor of biological chemistry and a member of the Center for Sensory Biology in Hopkins’s Institute for Basic Biomedical Sciences, in a press release. “And it makes you wonder about what was the more ancient role for rhodopsin — was it used originally for light or temperature detection?”
The team released about 75 larvae missing the gene that codes for rhodopsin onto a plate that contained two temperature zones; half of the plate was kept at their optimal temperature while the other at a different temperature, ranging from 14 to 32 degrees Celsius.
After ten minutes, they counted the larvae in each section and discovered that the larvae lacking rhodopsin could not differentiate from alternative temperatures and their optimal temperature, showing that rhodopsin is required for thermosensation.
The team also noted that the larvae that were missing the rhodopsin gene were not able to distinguish between different alternate temperatures. This shows that rhodopsin is not part of the direct pathway for thermosensation, and only functions in a “quality of life” situation, where a fruitfly larvae has the option to choose between an alternate temperature environment and their optimal temperature environment.
Furthermore, due to this recent discovery of rhodopsin’s dual role, scientists may choose to pursue the question of whether the original role of rhodopsin was in light sensation or in thermosensation.
