A study of genome sizes and repetitive and nonrepetitive nucleotide fractions in some reptiles, selachians, and amphibians showed that not one of these organisms possesses the percentage of single-copy DNA presumed to be typical of animals (70%). The data on the samples investigated, when compared with those existing in literature on other vertebrates, indicate certain general characteristics of the genome of this important animal group, which in this work are interpreted in terms of their adaptive functions. Each of the main systematic categories of the subphylum (classes or orders) has peculiar features as to the genome size and percentage of unique or repetitive DNA fractions. Among anamniotes (i.e., teleosts, selachians, and amphibians) the percentages of the single-copy DNA are lower than among amniotes (i.e., reptiles, birds, and mammals) and inversely proportional to genome sizes, which does not occur in amniotes. Also on the basis of some karyologic evidence, the hypothesis is put forth that such differences may reflect the adoption, in the course of evolution of the amniotes, of new genetic strategies which have deeply influenced the genomic molecular structure.
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