Unlike mammals, which regulate body temperature through metabolic heat production or sweating to cool off, insects must use behavioural adjustments to reach their desired body temperature. Behavioural thermoregulation, the ability to regulate body temperature to optimise growth and reproduction, is therefore a crucial facet of insect survival and evolutionary fitness. Much experimental work has shown that insects are capable of sensing and responding to environmental temperature variation on seasonal and daily timescales. Furthermore, some exciting progress has been made in unravelling the cellular basis of thermosensing with the recent discovery of temperature-gated ion channels. Generally, most scientists agree that behaviours maintaining optimal body temperatures of insects must have some genetic and cellular basis, but what these precise mechanisms are and where control centres are located are rather poorly understood. So how do behaviour and physiology interact to determine body temperature under a given set of conditions in insects?
Sung-Tae Hong and colleagues from the Korea Advanced Institute of Science and Technology explored the molecular and cellular components of behavioural thermoregulation using the model fly species Drosophila melanogaster. The team used a variety of methods to address this question. First, they undertook a large-scale screen of genetic mutants where the insects exhibited unusual thermoregulation behaviour to identify potential candidate genes. The results showed several genes involved in mushroom body formation and learning or memory formation, and also the cyclic AMP-dependent protein kinase A(cAMP-PKA) pathway, all of which are involved in the control of body temperature. From these results, the team hypothesized that mushroom bodies, a part of the fly's brain involved in cognition, and cAMP-PKA signalling might be vital components of thermoregulation in these flies.
The team also mapped brain regions involved in thermoregulation by knocking out discrete parts of the fly brain and found that loss of the mushroom body led to a loss of body temperature control, suggesting that the mushroom body is a critical component of normal thermoregulatory behaviour. Other experiments, in which the mushroom body was completely deactivated, resulted in extreme cold preference. By contrast, deactivation of mushroom body neurons resulted in no distinct high or low temperature preference and, hence, poor thermoregulation.
Second, the team closely examined the role of cAMP-PKA signalling in the behavioural control of body temperature. Mutant flies with defects in cAMP-PKA pathways were generally widely distributed across a temperature gradient,indicating a lack of thermoregulation. However, mutants with low cAMP levels tended not to avoid low temperatures, while mutants with above average cAMP levels were unable to avoid high temperatures properly. This and subsequent experiments verified that the flies' preferred body temperatures were correlated with cAMP levels and PKA activity. Moreover, these experiments suggested that the cAMP-PKA pathway aids temperature recognition and facilitates preferred body temperature control.
Next, the team asked whether cAMP-PKA signalling influences thermoregulation through only the mushroom body or whether other parts of a fly's body are also involved. To tackle this, they generated flies in which normal cAMP-PKA signalling occurs only in the mushroom body but not elsewhere. The researchers also undertook experiments that manipulated the locations of normal cAMP-PKA function. The results showed that cAMP-PKA signalling in the mushroom body, but not in other parts of the insect's body, are critical for maintaining normal thermoregulatory behaviour. In conclusion, Hong and co-workers have made significant advances in revealing the molecular and neural basis of how flies keep their cool through behavioural thermoregulation.