Temperature is a key environmental variable shaping organisms' biology. What enables or constrains species to survive at different temperatures is still not fully understood. Biologists tackle this issue from various points of view. Some investigate why animals prefer and inhabit environments with specific temperatures, while others advance our knowledge of how temperature affects intricate cellular processes. Bridging the gap between cellular level and whole animal behaviour is one of the current challenges in biology.
One approach used recently by a group of Japanese researchers at Kyoto University was to screen for mutant flies that have unusual temperature preferences. The larvae of one mutant, which they called the atsugari(atu) mutant, preferred and selected a temperature of 18°C when exposed to a temperature gradient, while unmutated larvae preferred 22°C. The team went on to identify how the mutant larvae differed from the unmutated insects and how the changes affected the insects' physiology.
The team found that the cold loving mutant larvae had suffered a mutation in the gene encoding the protein dystroglycan (DmDG), resulting in a significant reduction in the gene's expression levels to 15% of that of the unmutated flies.
The next step was to find the functional link between the gene mutation and the mutated larvae's low temperature preference behaviour. It turns out that the dystroglycan protein affects cellular membranes and lowering the gene's expression level causes an increase in the cell's calcium permeability. Alterations in the gene's expression caused an increase in the intracellular calcium level in the cold loving mutant larvae. As intracellular calcium levels are an important regulator of cellular energy metabolism through activation of the mitochondrial enzyme pyruvate dehydrogenase (which contributes to ATP production), the team measured the enzyme's activity level. They found that the enzyme had higher activity in the cold loving atumutant larvae, which could explain the higher cellular ATP levels. Also, the metabolic rate of the mutated larvae was almost double that of the wild-type. Drawing these observations together, the team suggest that the mutant larvae's increase in ATP levels and metabolic rates may alter the larvae's thermoregulatory behaviour allowing them to occupy colder locations.
This study shows the close link between energy metabolism and temperature preference in ectothermic animals. The specific gene targeted, DmDG,may or may not be a relevant target in nature but the consequence of variation in energetics on the temperature preference behaviour is clearly shown in this study.