Phylogenetic relationships of Osmunda cinnamomea, O. claytoniana, and O. regalis were explored by means of DNA sequence comparisons. Hydroxyapatite thermal elution profiles of self-reassociated repetitive DNA fragments were very similar, indicating the absence of gross differences in the amount of recent amplification or addition of repetitive DNA in any of these three genomes. Interspecific DNA sequence comparisons showed, in contrast to our earlier interpretation, that repeated DNA sequences of O. claytoniana are nearly equally diverged from those of O. cinnamomea and O. regalis. Differences between repetitive sequences of the three species can be interpreted as reflecting amplification events which occurred subsequent to speciation. The data obtained suggest that the three Osmunda species most likely arose more or less simultaneously from a common ancestor. These findings were verified in experiments with tracer DNA preparations enriched for single copy sequences. On the basis of the hydridization data presented here and of the fossil record, the rate of single copy sequence divergence in the ferns is comparable to that in the primates, although slower than that observed in other animal taxa. From this first evaluation of rates of DNA evolution in plants it would seem that the rates for plants and animals are roughly comparable. The evidence suggests that species divergence is accompanied by further reiteration of preexisting repeat sequences. The rate of addition of repetitive sequences probably is slower in ferns than in angiosperms. This difference might be attributable to the much larger effective generation time in ferns.

Genome sizes and single copy complexity values have been estimated for eight Atriplex species and spinach by analysis of DNA reassociation kinetics. These values, together with measurements of interspecific hybridization carried out with purified single copy tracers, have been used to estimate the absolute amount of single copy DNA which is composed of homologous sequences in various species. The data show a large variation in cross reactivity for different species pairs which is best explained by postulating that these genomes were subject to extensive deletion during evolution of different lineages. At most, only about 5 x i0 nucleotide pairs of single copy DNA (about 10 times the amount inanE.coli genome) appear to be necessary to specify phenotypi features common to Atriplex species.