Cell cycle duration in antheridial filaments ofChara spp. (Characeae) with different genome size and heterochromatin content

The relationship between nuclear 1 C DNA content and cell cycle progression throughout successive stages of antheridial filaments were studied among five taxa ofChara: two dioecious species (n = 14):C. aspera (7.2 pg DNA),C. tomentosa (7.4 pg DNA), and three monoecious species (n = 28):C. vulgaris (13.5 pg DNA),C. fragilis (19.3 pg DNA), andC. contraria (19.6 pg DNA). With the use of double³H-thymidine labelling and morphometry a number of characteristics common to all of the investigated species were determined within the proliferative periods preceding spermiogenesis. These include: (1) simplified type of the cell cycle (S + G₂ + M), due to complete lack of G₁ intervals, (2) constant duration of S phase, (3) progressive shortening of G₂ + M periods, and (4) gradual reduction of cell lengths at successive mitotic divisions. Nucleotypic dependence was found between genome size and several time parameters estimated for consecutive stages of antheridial filaments: the higher the DNA C-value, the longer the cell cycles, their component phases, the total duration of the proliferative period, as well as the lower the rate of growth of interphase cells. Differential Giemsa staining of late G₂ phase nuclei revealed that the higher content of C-heterochromatin is connected with prolonged cell cycle durations in species with similar DNA C-values.