| Abstract: | Cyanophyte-like prokaryotes are widely presumed to be the progenitors of eukaryote plastids. A few rare protistan species bearing cyanophyte-like cyanelles may represent intermediate stages in the evolution of true organelles. Cyanophyte DNA disposition in the cell, so far as is known from electron microscopy, seems uniform within the group and distinctly different from the several known arrangements of DNA in plastids. Therefore a survey of representative cyanophytes and protistan cyanelles was undertaken to determine whether forms reminiscent of plastids could be found. DNA-specific fluorochromes were utilized, along with epifluorescent microscopy, to study the DNA arrangement in situ in whole cells. Only the endospore (baeocyte)-forming Cyanophyta contained more than one, centrally located DNA skein per cell, and then only for the period just preceding visible baeocyte formation. Such forms might, with modification, presage the “scattered nucleoid” DNA disposition found in plastids of several groups, including Rhodophytes, Cryptophytes, Chlorophytes and higher plants. The DNA arrangement in cyanelles of two protists, Cyanophora and Glaucocystis, appear different from each other and possibly related to, respectively, the cyanophytes Gloeobacter and Synechococcus. Cyanelles of the third protist, Glaucosphaera, like the cells of the unique prokaryote Prochloron, appear to have multiple sites of DNA, somewhat similar to those of the “scattered nucleoid” line of plastid evolution. No obvious precursor of the “ring nucleoid” or other types of plastid DNA conformation was found. |
