NOVEL TOUCH-INDUCED, CA2-DEPENDENT PHOBIC RESPONSE IN A FLAGELLATE GREEN-ALGA()

The biflagellate green alga Spermatozopsis similis exhibits a remarkab le avoidance reaction in addition to the photophobic or stop response characteristic of such algae. S. similis normally swims forward with i ts anteriorly attached flagella directed posteriorly and propagating s ine-like waves from base to tip. Upon contact with surfaces or other c ells, S. similis responds with rapid backward swimming, covering dista nces of up to 50 mu m in 140 to 220 msec. This reaction, which we term the mechanoshock response, also can be triggered by vigorous mechanic al stimulation, but not by physiological light intensities. It consist s of 3 phases: (1) a rapid acceleration phase with average duration of 31 msec; (2) a phase of about 66 msec with constant high speed (maxim al velocities of > 600 mu m.sec(-1)) or slow deceleration; and (3) a d eceleration phase of similar to 83 msec, followed by a stop or short p eriod of circling. The cells then resume forward swimming in a random direction. Prior to the mechanoshock response the flagella rapidly are brought together into a close parallel configuration extending anteri orly of the cell body. They then appear to propel the cell by undulato ry beating, while the cell describes a pronounced helical path. Small decreases in the extracellular Ca2+ concentration, as well as low conc entrations of Ba2+, strongly suppress the probability of this phobic r eaction. We conclude that this mechanoshock response involves large Ca 2+ influxes, probably mediated by mechanosensitive and/or stretch-acti vated ion-channel(s). (C) 1994 Wiley-Liss, Inc.