Quantitative and qualitative DNA variations in Anabaena azollae Strasb. living in Azolla filiculoides lam.

Heterocysts and vegetative cells of the filamentous nitrogen-fixing Anabaena azollae isolated from the apex to the basal leaf cavities of Azolla filiculoides were examined by epifluorescent microscope after fluorochrome staining. Acridine orange (AO), DAPI, and chromomycin fluorochromes were used in order to evidence total DNA content and respectively, A + T and G + C bases. Measurements of fluorescence intensities were made on photographic prints by the automatic image analysis system Quantimet 970. Heterocysts contained higher amounts of DNA than did vegetative cells, and their content strongly increased in the basal leaf cavities. The heterocyst DAPI brightness was quite uniform, whereas in vegetative cells DAPI brightness increased from the apex to the basal groups. In vegetative cells from the apex to the median group, the percentage of DAPI brightness was 60-85% with respect to AO brightness, whereas in heterocysts of the same groups DAPI brightness was 40-50% with respect to AO brightness. In the basal group, brightness due to DAPI staining was comparable with those of previous group both in heterocysts and in vegetative cells, whereas chromomycin brightness increased strongly in heterocysts. These data show that heterocyst changes its DNA content and composition in the basal leaf cavities, suggesting that its lifetime is not completely over.