Adaptive phototaxis of Chlamydomonas and the evolutionary transition to multicellularity in Volvocine green algae

TL;DR

This study reveals that adaptive phototaxis in green algae, from unicellular Chlamydomonas to multicellular Volvox, is governed by a unified mechanism involving flagellar response tuning to rotational periods.

Contribution

The paper demonstrates that the adaptive phototactic response is tuned to the organism's rotation period across different species, unifying the understanding of phototaxis in Volvocine algae.

Findings

Adaptive time scale matches rotational period in Chlamydomonas, Gonium, and Volvox.

A mathematical model accurately predicts phototactic behavior.

Phototaxis mechanism is conserved across species despite structural differences.

Abstract

A fundamental issue in biology is the nature of evolutionary transitions from unicellular to multicellular organisms. Volvocine algae are models for this transition, as they span from the unicellular biflagellate Chlamydomonas to multicellular species of Volvox with up to 50,000 Chlamydomonas-like cells on the surface of a spherical extracellular matrix. The mechanism of phototaxis in these species is of particular interest since they lack a nervous system and intercellular connections; steering is a consequence of the response of individual cells to light. Studies of Volvox and Gonium, a 16-cell organism with a plate-like structure, have shown that the flagellar response to changing illumination of the cellular photosensor is adaptive, with a recovery time tuned to the rotation period of the colony around its primary axis. Here, combining high-resolution studies of the flagellar photoresponse with 3D tracking of freely-swimming cells, we show that such tuning also underlies phototaxis of Chlamydomonas. A mathematical model is developed based on the rotations around an axis perpendicular to the flagellar beat plane that occur through the adaptive response to oscillating light levels as the organism spins. Exploiting a separation of time scales between the flagellar photoresponse and phototurning, we develop an equation of motion that accurately describes the observed photoalignment. In showing that the adaptive time scale is tuned to the organisms' rotational period across three orders of magnitude in cell number, our results suggest a unified picture of phototaxis in green algae in which the asymmetry in torques that produce phototurns arise from the individual flagella of Chlamydomonas, the flagellated edges of Gonium and the flagellated hemispheres of Volvox.