Cytomorphology of the ‘wound healing’ process in the green filamentous algae, Ulothrix zonata (F. Weber & Mohr) Kützing 1833
Keywords:filamentous algae, Ulothrix zonata, wound healing, repair, cell fusion
For the first time, we present a cytomorphological description of the self-healing (repair) process of damaged thalli sites, ‘wound healing’, in the green filamentous algae, Ulothrix zonata (F. Weber & Mohr) Kützing 1833. In the filaments of this species from Lake Baikal and the Angara (Baikal outflow), Zhilishche (Baikal inflow) and Ida (Angara inflow) rivers, light microscopy methods revealed dome-like and conical protrusions and elongations of transverse cell walls directed into adjacent dead (without protoplast) and defective (with deformed chloroplasts) cells. At the same time, there were mostly patterns where two cells formed protrusions directed into the same damaged filament site between them, i.e. towards each other. The growth of previously unconnected cells towards each other led to their convergence and adjacency. This had two important physiological consequences that ensured the restoration of the filament integrity. The first consequence was the formation of intercellular junctions. The second one was the fusion of the protoplasts and nuclei of the adjacent cells (cell fusion) with the formation of vegetative polyploid cells with increased size. During subsequent divisions of these cells, extended areas emerged with a two- to three-fold increase in the diameter of the algal filaments. It was also found that the process of ‘wound healing’ promoted the development of giant hypnospores. We showed that the H-shaped septa between cells of filamentous algae were not thickenings of the outer walls but the sheaths of the dead cells preserved after this reparation process. Analysis of the ‘wound healing’ patterns revealed that the Ulothrix cell nuclei did not migrate to the polarized regions of the cells but retained their central position, which testifies to their fixation in the protoplast. We observed sporadic cases of the development of lateral filaments in U. zonata were due to the self-repair of defective cells and their subsequent division during ‘wound healing’. A comparison of various cell deformations allowed us to determine the characteristic stages of the ‘wound healing’ process in U. zonata, which have some similarities and differences with those in marine red filamentous algae. Our study indicates that ‘wound healing’ is an evolutionarily developed and genetically programmed adaptation that may be widespread in the population of filamentous algae.
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