Temperature sensitivity and temperature response across development in the Drosophila larva

Abstract

The surrounding thermal environment is highly important for the survival and fitness of animals, and as a result most exhibit behavioral and neural responses to temperature changes. We study signals generated by thermosensory neurons in Drosophila larvae and also the physical and sensory effects of temperature variation on locomotion and navigation. In particular we characterize how sensory neuronal and behavioral responses to temperature variation both change across the development of the larva. Looking at a wide range of non-nociceptive isotropic thermal environments, we characterize the dependence of speed, turning rate, and other behavioral components on temperature, distinguishing the physical effects of temperature from behavior changes based on sensory processing. We also characterize the strategies larvae use to modulate individual behavioral components to produce directed navigation along thermal gradients, and how these strategies change during physical development. Simulations based on modified random walks show where thermotaxis in each developmental stage fits into the larger context of possible navigation strategies. We also investigate cool sensing neurons in the larva's dorsal organ ganglion, characterizing neural response to sine-wave modulation of temperature while performing single-cell-resolution 3D imaging. We determine the sensitivity of these neurons, which produce signals in response to extremely small temperature changes. Combining thermotaxis results with neurophysiology data, we observe, across development, sensitivity to temperature change as low as a few thousandths of a °C per second, or a few hundredths of a °C in absolute temperature change.

Drosophilalarvae demonstrate associative learning and memory in response to thermal conditioning

“…Larva are capable of sensing extremely small changes in temperature [47, 48, 51, 60], but exhibit randomly directed exploratory motion [61] within a temperature range of approximately 22 − 28 ◦ C, even in the presence of a spatial gradient [49]. In our experiments, naive control larvae, moved between agar plates but without any temperature-odor pairing, indeed show only very small (and statistically insignificant) net navigation on a gradient centered at 23.5 ◦ C, as seen in Fig.…”

“…Comparing to naive control larvae, with an essentially constant turn rate for all warming and cooling levels, we conclude that the conditioned larvae not only move towards warmer temperature, but are in fact actively attracted to the conditioned stimulus. In other research studying fly larva navigation without conditioning, both May 5, 2024 12/21 positive and negative thermotaxis are characterized as inherently avoidant behaviors, where larvae increase turning rates when experiencing cooling below the neutral range (22 − 28 • C) or warming above the neutral range [47,48,50]. In the latter two articles, the equivalent of Fig.…”

“…As noted above, the efficiency of movement along a gradient (the x -direction or r -direction) is quantified as a navigation index ( NI x or NI r ), a dimensionless value with a possible range between −1 and +1 (+1, for example, would be the navigation index if an entire population of larvae moved directly up the gradient without ever deviating). For reference, strong linear chemical concentration gradients [56] yield NI near +0.2, and strong thermal gradients [47, 48] yield NI near +0.3, and both behaviors are robust in crawling larvae. The 20-to-27 ◦ C gradients we use in this paper are strong (0.32 ◦ C/cm for the linear gradient, 0.50 ◦ C/cm for the radial gradient), but over a temperature range that does not normally elicit strong crawling in either direction ( NI near zero, see [49]).…”

“…Temperature is of vital importance to nearly every animal, especially small, slow ectotherms like the Drosophila larva. Two previously established aspects of thermal behavioral response in larvae are especially important for developing associative memory protocols: (1) Larvae are extremely sensitive to changes in temperature (both warming and cooling) and can thus robustly navigate thermal environments [47,48]; but (2) there is a range of temperatures where larvae do not normally exhibit thermotaxis [49], so temperatures within that range can be freely used as the conditioned stimulus (CS).…”

“…Larvae sense changes in the temperature of their environment using three cool-sensing neurons located on each side of their heads [47] as well as two warm-sensing neurons of opposite valence [50], both housed in the dorsal organ ganglion. The three cool-sensing cells are extremely sensitive to very small changes in temperature [47, 48], responding to changes as small as a few thousandths of a ◦ C per second. Three sub-types of ionotropic receptors expressed by the cool-sensing cells, Ir21a, Ir25a, and Ir93a, are all necessary for the temperature sensitivity of these neurons [51, 52].…”