[Functional specialization of the nuclei in tip-growing coenocytic algae Vaucheria sessilis].

The availability of numerous nuclei in coenocytic thalli creates a problem of their activity and division cycle coordination. An apparent random distribution of nuclei in tip-growing cells is regulated by some principles associated with physiological compartmentalization in the absence of true septae. In vegetative thallome of Vaucheria sessilis the nuclei are distributed equally in proximal parts, to be accumulated in a high density cluster in the apical zone. The cluster appears in the course of germination in the late phase as a result of nuclear migration in a forming branch. The treatment with (methyl-3H)-thymidine and (5-3H)-uridine revealed the functional specialization of nuclei. The most active accumulation of thymidine was observed at early phases of germination in the nuclei of aplanospore. Proportionally with the growth of the new branch, the nuclei migrate into it, and the most active DNA replication zone moves to the filament base. At the late phases of morphogenesis, when the apical cluster of nuclei is available, thymidine accumulation is observed only in the proximal parts of a vegetative branch, rather far from the apical zone. The most active uridine accumulation was registered in the base of a forming filament at early morphogenesis, and in aplanospore at late phases. The apical cluster of nuclei is characterized by a low transcription level and the absence of replication. The obtained results have demonstrated that in the course of morphogenesis the equipotent nuclei acquire some specialization depending on their position in the thallome. The nuclei migrating via the cytoplasm toward the basal part of a filament are able to divide and actively transcript their own genome. The nuclei in the aplanospore at early phases of germination may predominantly realize the division cycle. The nuclei in the apical cluster, with a special organization of the cytoskeleton net, lose their ability to divide and migrate, and lack the ability to transcript a proper genome. The space delimination of the growth zone and a zone of division, as well as a functional specialization of nuclei can be considered as some basic characters of the apical growth.