Stabilization of the polarity axis in the zygotes of some Fucaceae

Abstract

The existence of a period of latent but stable polarity, i.e. a period in which the polarity axis has been irreversibly established but no morphological asymmetry can be detected, was studied in germinating populations of zygotes of Pelvetia fastigiata. We found that the time course of the loss of sensitivity to a single polarity-axis-determining light stimulus coincided with the time course of germination (rhizoid outgrowth), up to the germination of about one-third the population, showing that the cells remained responsive to the light stimulus until or almost until the appearance of visible asymmetry. In the later part of the germination period, some of the zygotes may have lost their light sensitivity somewhat before rhizoid outgrowth, suggesting that at least some of the ungerminated cells may at this time possess, for a brief period, a latent but stable polarity axis. The loss of responsiveness with time to an antagonistic, second light stimulus followed the same time course as the loss of sensitivity to a single light stimulus. There was no suggestion of the existence of a latent but stable polarity axis in any members of the population in this experiment. An analysis of data of Jaffe (1968) on polarity-axis determination and germination in P. fastigiata following a single light stimulus yields essentially the same conclusions as our own single-light-stimulus experiments. In contrast, analysis of data of Whitaker and Lowrance (1936) on Fucus furcatus indicated that in these zygotes latent but stable determination of the polarity axes had taken place 3–4 hours before germination. A similar situation emerges from the analysis of another experiment of Jaffe (personal communication) with P. fastigiata zygotes in which the loss of sensitivity to an orienting light stimulus appeared accelerated and germination may have been delayed in comparison with his 1968 data. We conclude, therefore, that populations of Fucaceae zygotes may vary with regard to the existence of a latent but stable polarity axis. However, when existence of such a latent, stable polarity axis can be inferred, its duration usually is brief, and it seems to be in most cases limited to a small fraction of the individuals of the total population at any particular time. In order to infer rigorously the existence of latent but stable polarity axes in populations of germinating zygotes and similar cells, it is essential to obtain the time courses for axis stabilization and for the development of visible asymmetry simultaneously.