Abstract
Sensing environmental temperatures is essential for the survival of ectothermic organisms. In Drosophila, two of the most used methodologies to study temperature preferences (TP) and the genes involved in thermosensation are two-choice assays and temperature gradients. Whereas two-choice assays reveal a relative TP, temperature gradients can identify the absolute Tp. One drawback of gradients is that small ectothermic animals are susceptible to cold-trapping: a physiological inability to move at the cold area of the gradient. Often cold-trapping cannot be avoided, biasing the resulting TP to lower temperatures. Two mathematical models were previously developed to correct for cold-trapping. These models, however, focus on group behaviour which can lead to overestimation of cold-trapping due to group aggregation. Here we present a mathematical model that simulates the behaviour of individual Drosophila in temperature gradients. The model takes the spatial dimension and temperature difference of the gradient into account, as well as the rearing temperature of the flies. Furthermore, it allows the quantification of cold-trapping and reveals unbiased TP. Additionally, our model reveals that flies have a range of tolerable temperatures, and this measure is more informative about the behaviour than commonly used TP. Online simulation is hosted at http://igloo.uni-goettingen.de. The code can be accessed at https://github.com/zerotonin/igloo.
Highlights
In recent years, the thermoreceptive system of Drosophila has been the subject of intense study: Starting with the discovery of the role of the painless gene by[1], a number of receptor genes, signalling pathways and brain regions[14,15,16] have been shown to be involved in thermosensation and the processing of temperature information
An absolute temperature preferences (TP) can be determined in a temperature gradient: animals are introduced into a gradient ranging from cold to hot temperatures, and are allowed to move within the gradient in order to reside at the TP
When the flies traverse colder regions, they slow down and linger there, and can fall into a stasis-like state[22]. This velocity reduction will bias the identification of TP, as an observer will underestimate TP because flies will reside for longer periods in colder temperatures due to their reduced ability to leave those regions
Summary
The thermoreceptive system of Drosophila has been the subject of intense study: Starting with the discovery of the role of the painless gene by[1], a number of receptor genes (painless[2,3]; pyrexia[4]; dtrpA15,6, dtrp[5,6], dtrpL7; brv[8], Gr28bD9, Ir21a10, Ir25a10, Ir93a11), signalling pathways (cAMP-PKA-pathway[12]; phospholipase C pathway13) and brain regions[14,15,16] have been shown to be involved in thermosensation and the processing of temperature information. In spatial tasks (e.g. positioning within a temperature gradient) individual-based models surpass group models[29] This can be exemplified by swarm behaviour in many different species including humans (reviewed in30–32): Only a small number of informed animals are needed to steer a large group (e.g. fish swarms[33,34], reindeer herds[35], groups of humans[36]). In our case this would imply that only a few individuals have to align their TP to steer the whole group into their vicinity, which highlights the importance of testing individual preferences from an ethological point of view. IGLOOs most striking feature is its ability to predict if a mutant phenotype arises on the level of internal or external thermosensory systems
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