The present paper deals with a comparative study on the distribution of labelled DNA between the chromosomes of
Chrysanthemum lineare and those of
Ch. nipponicum, which have a large difference in chromosome size, in the number of DNA replicating points and in DNA content. A comparison of the distribution pattern of the labelled DNA was made in the chromosomes of the F
1 hybrid of
Ch. lineare×
Ch. nipponicum during successive three mitotic division after H
3-thymidine treatment.
Both the chromosomes of
Ch. lineare and those of
Ch. nipponicum had labelled DNA in both sister chromatids at the first mitotic metaphase, in one of sister chromatids at the second mitotic metaphase, and, at the third mitotic metaphase, only in one of the sister chromatids of either one of the sister chromosomes which were formed from the sister chromatids of chromosomes of the previous mitosis.
The distribution pattern of labelled DNA in dividing chromosomes indicates that the chromosomes of both species are characterized by the same semiconservative dividing process of their DNA at the level of the chromatid. It is presumed that the chromosomes of
Ch. lineare are composed of more replication units than the chromosomes of
Ch. nipponicum (cf. Tanaka 1966). A reduction in the number of DNA replication units might have occurred in the chromosomes of
Ch. nipponicum during the evolution of this species.
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