Studies on DNA sequences in the Osmundaceae

Abstract

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.

Formation of nucleosomes on positively supercoiled DNA.

“…How could numbers of octamers in excess of 15 be accommodated? One possibility is that they might bind in 'piggyback' fashion to the outside of the normal nucleosomes (Voordouw and Eisenberg, 1978;Stein, 1979;Eisenberg and Felsenfeld, 1981), thus avoiding the introduction of more stress without losing their compact structure. Although we cannot rule out other subtle conformational changes in these octamers, they are obviously not disrupted in any dramatic way that would alter the response to the chemical probes we have used.…”

Subcellular localization of viroids in highly purified nuclei from tomato leaf tissue.

“…Microscopic studies show that the average diameter of the tomato nuclei is 4-5 /tm from which an intranuclear concentration of viroids of -10-I M can be calculated. The dissociation constants of various protein-nucleic acid complexes such as the ones between nucleic acids and polymerases (e.g., de Haseth et al, 1978) or DNA and histones (e.g., Stein, 1979) would guarantee complex formation in this or even lower concentration ranges. Therefore, it is not unreasonable that viroids exist in protein-nucleic acid complexes.…”

Single copy DNA homologies in Atriplex. II. Hybrid thermal stabilities and molecular phylogeny

“…Similar TE ratios were obtained for interspecific hybrid duplexes between single copy tracers and total DNA of several Osmunda species (Stein et a!., 1979, and unpublished data). It is generally assumed that a 1° TE in 120 mM NaPB corresponds to about 1 per cent mismatched base pairs (e.g., Britten et al, 1974;Angerer eta!., 1976;Stein et al, 1979). If we accept this assumption, it is reasonable to suppose that the same zT in 75 mM NaPB would correspond to about 2 per cent mispairing.…”

“…Thus there will be sequence divergence among the members of repetitive families within ony one genome, obscuring any difference between different genomes. We have argued previously (Stein et al, 1979) that interspecific differences in repeated DNA hybrids must arise from events other than random base substitution (e.g., amplification or reamplification) and thus cannot be expected to reflect divergence time.…”