Comparative analysis of the photophobic and phototaxis responses in Chlamydomonas reinhardtii

Abstract

Chlamydomonas reinhardtii is a motile, unicellular alga that exhibits two behavioral responses to light. One, phototaxis, is the orientation of the cells' swimming direction along the axis of a light beam. The second light-induced behavioral change is called the photophobic response and consists of a brief cessation in forward swimming after a temporal change in light intensity. In this thesis I have: (1) Developed a computerized motion analysis assay for the simultaneous measurement of photophobic and phototaxis responses in the same population of cells; (2) Applied this assay to the study of the chromophore(s)/photoreceptor(s) interactions by measuring the ability of various isomers and analogs of retinal to reconstitute both photobehaviors in retinal deficient mutants; and (3) Defined the relationship between the two responses by a model of the behavioral mechanism in which only one receptor is involved and the photophobic response provides the elemental course corrections causing phototaxis orientation. I tested and confirmed several strong predictions of the model, the most critical of which is that phototaxis alignment is achieved and maintained at each light intensity (I) in the cells' dynamic range by desensitizing the photophobic response to a point where the relative contrast ({dollar}\Delta{dollar}I/I) generated by the eyespot yields a constant photophobic signal, regardless of the signal expected from the absolute contrast ({dollar}\Delta{dollar}I) at the eyespot.