# The TRP-channel painless mediates substrate stiffness sensing in the legs during Drosophila oviposition

## TL;DR

Female fruit flies use touch sensors in their legs to detect soft surfaces for laying eggs, and a protein called Painless helps them sense texture.

## Contribution

The study identifies a mechanosensory pathway involving the TrpA channel Painless that enables fruit flies to detect substrate stiffness during oviposition.

## Key findings

- Painless is expressed in mechanosensory neurons and is necessary for substrate stiffness sensing during egg-laying.
- Campaniform sensilla and mechanosensory bristles function as indirect stiffness sensors through cuticular deformation.
- Overexpression of Painless in sensory organs partially rescues oviposition preference for softer substrates in mutants.

## Abstract

The distinct textural properties of fruits in varying stages of ripening present unique ecological opportunities for several species of fruit flies, resulting, over evolutionary times, in specialized egg-laying behaviors. In this study we identified a TrpA channel-dependent mechanosensory pathway in the legs, through the gene painless, that modulates the discernment of softer patches for oviposition in gravid D. melanogaster females. We report that the stiffness-sensing role of tarsi is mediated through external sensory organs, namely ventral mechanosensory bristles and subsets of campaniform sensilla present primarily at the joints between tarsomeres. Our findings provide new evidence that campaniform sensilla function as indirect stiffness sensors of oviposition substrates, owing to their placement at joints that experience maximal cuticular distortion. We show that Painless is expressed in mechanosensory neurons innervating peripheral organs and is necessary for their functions in mediating oviposition substrate selection in gravid females. Furthermore, we observed that overexpression of painless in both campaniform sensilla and mechanosensory bristles partially rescues preference for the softer substrates in painless mutants, indicating that painless activity in these organs is necessary to mediate the preference. We propose that different interactions with a soft vs. a hard substrate (compression of the cuticle, distribution of contacts) results in differential mechanotransduction in painless-expressing neurons, determining oviposition preferences.

Female fruit flies use touch to evaluate the texture of potential egg-laying sites. Previous studies have shown that substrate stiffness strongly influences oviposition across Drosophila species; for example, Drosophila melanogaster females reliably lay eggs on softer substrates. However, the sensory pathways through which females gather texture information during oviposition remain unclear. Here, we show that mechanosensory inputs from specialized tactile sensors—mechanosensory bristles and campaniform sensilla—are required for D. melanogaster females to prefer softer substrates. The TrpA channel Painless, expressed in these organs, contributes to this preference: females with impaired painless display reduced bias toward softer substrates, whereas restoring painless in these sensory organs partially rescues the behavior. We propose that stiffer substrates more strongly deflect bristles and compress campaniform sensilla via cuticular deformation, enabling these structures to function as stiffness detectors during egg-laying.