Nuclear and Organelle DNA Replication during Spore Germination in Bryophytes and Equisetum

In contrast to ferns, the spore nuclei of the moss, Funaria hygrometrica, are in the G₂ phase. They do not incorporate the thymidine analogue BUdr before the first mitosis. When spores germinate in the presence of aphidicolin, an inhibitor of the (nuclear) DNA polymerase alpha, they give rise to two-celled, negatively phototropic protonemata, consisting of relatively long and thin cells. Determinations of nuclear DNA content further confirm the presence of G₂-nuclei in Funaria spores. Spores of Pogonatum urnigerum and of Equisetum hyemale likewise contain G₂-nuclei, as shown by BUdr incorporation or experiments with aphidicolin, respectively. In germinating Funaria spores, the organelle DNA is synthesized very early and independent of nuclear DNA. A gradient in replication of organelle DNA is found only in the long tip cells which are formed in the presence of aphidicolin.