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Theory of cell body lensing and phototaxis sign reversal in “eyeless” mutants of Chlamydomonas

Sumit Kumar Birwa*, Ming Yang†, Adriana I. Pesci‡, and Raymond E. Goldstein§

  • *Contact author: skb61@cam.ac.uk
  • †Contact author: my365@cam.ac.uk
  • ‡Contact author: A.I.Pesci@damtp.cam.ac.uk
  • §Contact author: R.E.Goldstein@damtp.cam.ac.uk
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Phys. Rev. E 114, 014408 – Published 21 July, 2026

DOI: https://doi.org/10.1103/fw4b-pg8q

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, 024401 (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.

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Outline

Information

Phys. Rev. E 114, 014408– Published 21 July, 2026
  • Received 11 March 2026
  • Accepted 8 July 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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