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. In this talk I will describe experimental and theoretical work on the mechanism of phototaxis in three members of the Volvocine  algae, spanning 3 orders of magnitude in cell number. We find that there is an evolutionarily conserved dynamics, with a tuning between  the time scales for an adaptive flagellar response to changing light levels illuminating the photoreceptors and the spinning frequencies of the  microorganisms. The implication of these results for the evolution of multicellularity are discussed.