Nuclear crystals, lampbrush-chromosome-like structures, and perinuclear cytoskeletal elements associated with nuclear fragmentation in characean internodal cells

This article gives a survey of nucleus-associated structures and inclusions in a diverse range of characean algae including Chara braunii Gm., Chara corallina Klein ex Willd, em. R.D.W., Nitella cristata A.Br., em. R.D.W., Nitella flexilis (L.) Ag., Nitella furcata (Roxb. ex Bruz.) Ag. era. R.D.W., Nitella hyalina (DC.) Ag., Nitella pseudoflabellata A.Br., em. R.D.W., Nitella pseudoflabellata var. imperialis T.F.A., Nitella translucens var. axillaris (A.Br.) R.D.W. and Nitellopsis obtusa (Desv. in Lois.) J. Gr. Lampbrush-chromosome-like structures were found in nuclei of Nitella flexilis and Nitellopsis obtusa and seem to be involved in the distribution of genetic material during nuclear fragmentation. Intranuclear tubular crystals of unknown protein composition were present in all species, especially in young, elongating cells, and could be important for establishing the main axis of the nuclei. Spindle-shaped protein crystals that originate in the nucleus and are released into the cytoplasm upon nuclear degeneration were observed in branchlet internodal cells of one population of Nitella flexilis. Perinuclear microtubules were present in all species, but perinuclear actin fibrils were hitherto only found in most Nitella species and in Nitellopsis obtusa. None of these nucleus-associated structures seems to be responsible for the formation of constrictions leading to nuclear fragmentation. These constrictions were perpendicular to the main axis of the nucleus and symmetrical in the Nitella species but asymmetric in C. braunii, C. corallina, and in Nitellopsis obtusa. Statistical analysis of nuclear size, number and constriction sites indicate that fragmentation is a nonsynchronous process independent of the light-dark cycle.