STRUCTURE AND PHYSIOLOGY OF THE HAPTONEMA IN CHRYSOCHROMULINA (PRYMNESIOPHYCEAE) .2. MECHANISMS OF HAPTONEMATAL COILING AND THE REGENERATION PROCESS

The axoneme in the free part of the haptonema in Chrysochromulina acan tha Leadbeater & Manton and C. simplex Estep et al. consists of seven single microtubules, except in the extreme distal swelling where, in C . simplex, there are only three microtubules. In the extended haptonem a, the microtubules are arranged in a ring though they are not evenly spaced, the gap between two of the microtubules being larger than that between any other neighboring pairs. In the coiled haptonema, rearran gement of the microtubules occurs so that the ring becomes distorted a nd the microtubules form two superposed arcs. A sliding microtubule me chanism is considered as a means by which haptonematal movement might be affected, and this is discussed in relation to the fine structure o f both embedded material and negatively stained demembranated cells. W e show that haptonematal coiling is dependent on the presence of calci um ions and that an external concentration of between 10(-6) and 10(-7 ) M Ca2+ is the threshold below which the frequency of coiling on cell death is reduced. The results of experiments using ethylene bis-(oxye thylenenitrilo)-tetracetic acid (EGTA) and lanthanum ions to control e xtracellular and intracellular Ca2+ concentrations are discussed in te rms of both external free calcium and intracellular pools. We also sho w that haptonematal regeneration following excision begins with a shor t lag phase. This is followed by a period of rapid growth, decreasing after approximately 4 h. Full haptonematal regrowth is not achieved un til after 12-15 h. The rate of haptonematal regeneration is strongly a ffected when the flagella are regenerating simultaneously. These obser vations are interpreted in terms of competition for intracellular prec ursors.