Theory of Cell Body Lensing and Phototaxis Sign Reversal in "Eyeless" Mutants of $Chlamydomonas$

TL;DR

This paper develops a quantitative theory explaining how lensing by the cell body causes sign reversal in phototaxis of eyeless mutants of Chlamydomonas, highlighting the role of internal caustics and signal dynamics.

Contribution

It introduces a comprehensive model integrating cell body lensing effects into phototaxis behavior, explaining sign reversal and bistability in mutant algae.

Findings

Lensing causes reversal of phototaxis in eyeless mutants.

Internal caustics influence photoreceptor signals.

Model predicts bistability in phototactic responses.

Abstract

Phototaxis of many species of green algae relies upon directional sensitivity of their membrane-bound photoreceptors, which arises from the presence of a pigmented "eyespot" behind them that blocks light passing through the cell body from reaching the photoreceptor. A decade ago it was discovered that the spherical cell body of the alga $Chlamydomonas~reinhardtii$ acts as a lens to concentrate incoming light, and that in "eyeless" mutants of $Chlamydomonas$ the consequence of that focused light reaching the photoreceptor from behind is a reversal in the sign of phototaxis relative to the wild type behavior. We present a quantitative theory of this sign reversal by completing a recent simplified analysis of lensing [Yang, et al., Phys. Rev. E 113, 022401 (2026)] and incorporating it into an adaptive model for $Chlamydomonas$ phototaxis. This model shows that phototactic dynamics in the presence of lensing is subtle because of the existence of internal light caustics when the cellular index of refraction exceeds that of water. During each period of cellular rotation about its body-fixed axis, the photoreceptor receives two competing signals: a relatively long, slowly-varying signal from the direct illumination, and a stronger, shorter, rapidly-varying lensed signal. The reversal of the sign of phototaxis is then a consequence of the dominance of the flagellar photoresponse to the signal with the higher time derivative. These features lead to a quantitative understanding of phototaxis sign reversal, including bistability in the direction choice, a prediction that can be tested in single-cell tracking studies of mutant phototaxis.