Paper List

期刊: ArXiv Preprint
发布日期: 2026-03-11
BiophysicsCell Biology

Theory of Cell Body Lensing and Phototaxis Sign Reversal in “Eyeless” Mutants of Chlamydomonas

Department of Applied Mathematics and Theoretical Physics, Centre for Mathematical Sciences, University of Cambridge, United Kingdom

Sumit Kumar Birwa, Ming Yang, Adriana I. Pesci, Raymond E. Goldstein
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IN SHORT: This paper solves the core puzzle of how eyeless mutants of Chlamydomonas exhibit reversed phototaxis by quantitatively modeling the competition between direct and cell-body-lensed light signals.

核心创新

  • Methodology Develops a complete geometric optics model for off-axis lensing in spherical cells, incorporating caustic formation and deriving the angular dependence of light intensity boost (e.g., η≈1.5 for n=1.1).
  • Biology Integrates the lensing model into an established adaptive phototaxis framework, revealing that sign reversal stems from the flagellar response dominance to the signal with the higher time derivative (the shorter, rapidly-varying lensed pulse).
  • Theory Predicts bistability in phototactic direction choice for eyeless mutants, dependent on initial cell orientation, a testable hypothesis for single-cell tracking experiments.

主要结论

  • The spherical cell body (n_c≈1.47) acts as a lens, creating an internal caustic and boosting light intensity on the photoreceptor from behind by up to ~1.5x for a relative refractive index n=1.1.
  • Phototaxis sign reversal in eyeless mutants results from the flagellar photoresponse being dominated by the shorter, stronger, rapidly-varying lensed signal (higher dI/dt) over the longer, direct signal during each rotational period.
  • The model predicts initial orientation-dependent bistability in phototactic direction for mutants, with most orientations leading to negative phototaxis (sign reversal), while a subset maintains positive phototaxis.
研究空白： While the lensing effect in Chlamydomonas was known, a quantitative, mechanistic theory linking the complex internal light patterns (caustics) to the observed behavioral sign reversal in eyeless mutants was lacking.

摘要: 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.