The Phototactic Sensorimotor Mechanism of the Unicellular Alga Euglena Gracilis. An Application to the Evolutionary Study of Vision

Abstract

Our understanding of even basic functions of microtubules (MT) such as intracellular transport and ciliary beating, including their role in sensory cells and nerve cells, is still incomplete. For example, vertebrate photoreceptors contain a cilium whose possible dynamical function is still unknown. A new model of microtubular dynamics, developed by me together with P Zaborski and J Tuszynski [Insinna et al, 1996 BioSystems39(3) 187 – 226], is capable of accounting for most of the phenomena associated with cell motility. Additionally, it sheds new light not only on the phototactic behaviour of the Protozoan Euglena gracilis but also on some possible evolutionary steps leading from primitive sensory organs to more complex ones such as the photoreceptors of vertebrates. Euglena displays simple perceptive functions (phototaxis) based on a primitive photoreceptor. This unicellular organism offers a unique possibility to study the function of MT in a simple form of vision. The model is based on classical nonlinear physics and suggests a dynamical role for MT in vertebrate photoreceptors. I am convinced that a better understanding of photoreceptor dynamics requires knowledge of the evolutionary steps leading to their development.