The systems biology of Chlamydomonas phototaxis as determined by electric current and ciliary beating

Collin Capano, Jureepan Saranak, and Kenneth Foster

Physics Department Syracuse University, Syracuse NY, 12344-1130 USA

Chlamydomonas phototaxis is an excellent model for studies of sensory-signal processing networks. We have temporally correlated the net electric current of a population of cells with light stimuli and compared that with the responses of ciliary beating of individual cells. These stimulus-response functions enable us to locate and identify intracellular processing modules that make up the signal processing network. For wild type which swims toward light, and the agg1 mutant which swims away from light, there is a fast (< 1 ms delay) and slow (~10 ms delay) electrical response. The fast response shows a peak in gain at about 6 Hz and shows the signal to be twice differentiated and four times integrated. The delay of the slow process implies a diffusion step; possibly the running of a signal to and from an internal site. Differences in the ciliary beating account for the altered phototaxis of the two strains. However, the initial fast responses of wild type and agg1 appear similar and how the differences in the secondary slow response might be causing the wild type and agg1 to swim in opposite directions is not known. Dividing the stimulus-ciliary response functions previously obtained with our new stimulus-electrical response functions we get the response functions of the individual cilia. As seen with our measures of beating frequency and stroke velocity, they have unique delays, phasing and signal filtering. Continuation of these studies using rapid high-resolution images would yield the individual response functions of the eight or nine controlling variables of each cilium. The physiological function of the biological modules that control each cilium could be identified and understood in terms of their role in phototaxis.

e-mail address of presenting author: kwfoster@syr.edu