Nature of the heterogeneity in mispairing of reannealed middle-repetitive fern DNA

Abstract

The genome of the homosporous fern Thelypteris normalis contains a large middle-repetitive component, essentially a single second-order kinetic class, which exhibits heterogeneity in the precision of pairing of the reformed duplexes upon remelting. There are two possible models to explain this observed sequence heterogeneity. Either different families of the middle-repetitive class exhibit different degrees of sequence divergence and T_m reduction (inter familial heterogeneity), or else all are equally diverged, and the broad melt is the sum of thousands of equally broad melts for all the families (intra familial heterogeneity). To differentiate between these two hypotheses, iodinated Thelypteris DNA, reannealed through middle-repetitive C₀t, was thermally fractionated on hydroxyapatite into low (65–75° C), medium (75–85° C), and high (85–95° C) thermal stability classes. When reannealed with excess cold DNA, each class remelted over its characteristic temperature range. C₀t curves of these thermal fractions reannealed with cold driver demonstrated that all were from the middle-repetitive class. It was shown that these results were not due to G + C differences nor to artifacts of the labeling technique. Therefore it was concluded that, although all families consisted of approximately the same number of repeats, the families ranged from those with virtually no sequence divergence to those barely able to reanneal at the criterion used, in accordance with the model of inter familial heterogeneity. Though this model may have wide applicability to middle-repetitive DNAs, a different pattern appears to prevail in Selaginella, a heterosporous pteridophyte. Some evolutionary implications are discussed.