Inhibition of cytoplasmic DNA synthesis may be an initial signal for the induction of sexual reproduction in Closterium ehrenbergii

Abstract

We investigated the effect that inhibitors of cytoplasmic DNA (cytDNA) and/or nuclear DNA (nDNA) synthesis have on the frequency of zygote formation in Closterium ehrenbergii and whether these inhibitors correlate with arrest of pyrenoid production and enlargement of pyrenoidal size accompanying pregametogenesis in the alga. CytDNA inhibitors, ethidium bromide (EB), acridine orange and novobiocin (NB), enhanced zygote formation in the presence of nitrate, and these chemicals correlated with both a decrease in pyrenoid production and an increase in pyrenoidal size. By contrast, a nDNA inhibitor, aphidicolin (APC), enhanced zygote formation in the presence and absence of nitrate, and correlated with only an increase in pyrenoidal size. Meanwhile, inhibitors of both nDNA and cytDNA synthesis, mitomycin C (MC), hydroxyurea (HU) and arabinosyl cytosine (Ara-C), enhanced zygote formation in the presence and/or absence of nitrate. In addition, MC and HU correlated with only an increase in pyrenoidal size and Ara-C only a decrease in pyrenoid production. This result seemed to indicate that MC and HU functioned as selective inhibitors of nDNA synthesis, and Ara-C as a selective inhibitor of cytDNA synthesis. It is suggested that differentiation into pregametes correlates with the inhibition of both nDNA and cytDNA synthesis, levels of which are probably low, the inhibition of cytDNA synthesis being the initial signal for pregametogenesis.