The ultrastructure of Ulothrix mucosa. I. Mitosis and cytokinesis

Abstract

For the most part, the fine structure of the cytokinetic apparatus of Ulothrix mucosa Thuret has been found to possess distinctive ulvophycean features including initiation of the cleavage furrow when the nucleus is still in interphase, replication of the lateral centrioles at prophase and their changed position near the polar fenestrae of the broad metaphase spindle pole, the semiclosed condition of the nuclear envelope throughout most mitotic stages, and the rapid completion of the cleavage furrow following restoration of the interphase condition in daughter nuclei. However, outstanding characteristics reminiscent of the typical chlorophycean pattern are also found, including the frequent presence of a microtubular system, which appears to be most distinct in the final developmental stage of the cleavage furrow, and the quick descent of the sets of centrioles along the telophase nuclei until they are close to the cleavage plane opposite each other, which is particularly reminiscent of a chlorococcalean behaviour. Comparison of the fine structure of the cytokinetic apparatuses within Ulothrix reveals variation in the pattern of deposition of cell wall material onto the intervening new septum, and in the biogenesis of the ingrowing plasmalemma. It is stated that comparative cytological analyses which show cell variability at the generic level (as in Ulothrix) continue to be useful and are needed before recommendations may be made as to the validity of the modern green algal classifications in use today.