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A fundamental issue in biology is the nature of evolutionary transitions fromunicellular to multicellular organisms. Volvocine algae are models for thistransition, as they span from the unicellular biflagellate Chlamydomonas tomulticellular species of Volvox with up to 50,000 Chlamydomonas-like cells onthe surface of a spherical extracellular matrix. The mechanism of phototaxis inthese species is of particular interest since they lack a nervous system andintercellular connections; steering is a consequence of the response ofindividual cells to light. Studies of Volvox and Gonium, a 16-cell organismwith a plate-like structure, have shown that the flagellar response to changingillumination of the cellular photosensor is adaptive, with a recovery timetuned to the rotation period of the colony around its primary axis. Here,combining high-resolution studies of the flagellar photoresponse with 3Dtracking of freely-swimming cells, we show that such tuning also underliesphototaxis of Chlamydomonas. A mathematical model is developed based on therotations around an axis perpendicular to the flagellar beat plane that occurthrough the adaptive response to oscillating light levels as the organismspins. Exploiting a separation of time scales between the flagellarphotoresponse and phototurning, we develop an equation of motion thataccurately describes the observed photoalignment. In showing that the adaptivetime scale is tuned to the organisms' rotational period across three orders ofmagnitude in cell number, our results suggest a unified picture of phototaxisin green algae in which the asymmetry in torques that produce phototurns arisefrom the individual flagella of Chlamydomonas, the flagellated edges of Goniumand the flagellated hemispheres of Volvox.

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