| BENDICH and MCCARTHY (1970a) suggested that the lack of reciprocity of
DNA-DNA hybridization may be attributed to the different degree to which
individual families of nucleotide sequences have been diversified within
a genome. However, this explanation may be oversimplified. A certain portion
of eukaryotic DNA is usually represented by short nucleotide sequences repeated
many times, and because of this they readily reassociate (BRITTEN and KOHNE,
1968). It has been shown that in many species repeated sequences differ
considerably in their GC content from the rest of the DNA (e.g. YASMINEH
and YUNIS, 1971; MACGREGOR and KEZER, 1971; TRAVAGLINI et al., 1972;
GRAHAM and SKINNER, 1973). Evolutionary diversification of GC-rich and conservation
of GC-poor repeated sequences or vice versa, followed by unequal
reiteration of newly evolved sequences in different genomes, can easily
account for the lack of reciprocity in DNA-DNA hybridization as measured
by the relative binding of DNA's to each other as well as by the deltaTm.
The families of repeated sequences rich in GC will hybridize more readily
under stringent conditions, and also the thermal stability of GC-rich duplexes
will be higher than that of GC-poor duplexes. Since the Tm of
rye homologous DNA/DNA duplexes was higher than the Tm's of wheat
or Agropyron homologous DNA/DNA duplexes, it seems that there is
a larger portion of GC-rich repeated sequences in rye DNA than in wheat
or Agropyron DNA, although the GC content of native DNA's appeared
to be the same (Table 1). Conclusions The results presented in this paper and those reported by BENDICH and MCCARTHY indicate that rye DNA is as distantly related to Agropyron DNA as to wheat DNA, while the two latter DNA's appear more closely related. A comparison of the thermal stability of T. monococcum-Ae. squarrosa heterologous DNA/DNA duplexes with homologous T. monococcum DNA/DNA duplexes revealed that these DNA's differ slightly from each other (BENDICH and MCCARTHY, 1970b). The deltaTm of 1.0C is smaller than that found for wheat-Aygropron heterologous DNA/DNA duplexes (deltaTm of 2.2C). The data obtained from the present experiments and from those of BENDICH and MCCARTHY indicated that the DNA nucleotide sequence homology among Aegilops, Triticum, Agropyron, and Secale is in close agreement with the suggested phylogenetic relationships among these genera. The order of homology with wheat DNA nucleotide sequence is: Aegilops, Agropyron and Secale. The progressive decrease in homoeologous pairing of Aegilops, Agropyron and Secale chromosomes with wheat chromosomes is thus paralleled by the diversification of nucleotide sequences of their DNA's. However, although this parallelism is consistent with the hypothesis that the level of pairing between homoeologous chromosomes is determined by the similarity of nucleotide sequences in their DNA's, it does not provide direct evidence for it. This hypothesis must be tested by other means. |
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