Natural and Induced Changes in Chromosome Size and Mass in Meristems

Abstract

PIERCE¹, in 1937, claimed to have effected large changes in chromosome size in meristematic root tip cells of Viola conspersa after treating seedlings with different culture solutions. For example, he reports that in seedlings grown for 2 months in a culture solution with a large content of phosphate the chromosomes were 300 per cent larger by volume than in seedlings grown in a solution without phosphate. We think that his results provide the only direct evidence to show that changes in chromosome size may be induced by external environmental influences. The results pose two questions of particular interest. First, did the induced changes in chromosome volume reflect changes in chromosome mass or merely represent changes in the organization, for example, the coiling, of the chromosome material ? Second, were the induced changes associated with quantitative changes in chromosomal DNA and possibly, therefore, in genetic information ? This question is especially pertinent because variation in chromosome size between different individuals is, normally, positively correlated with changes in the quantity of nuclear DNA^2,3. The following is a brief account of an experiment with seedlings of rye (Secale cereale, 2n = 14). The experiment completely confirms Pierce's results in showing that changes in chromosome size may readily be induced. The experiment also provides answers to the above questions in relation to chromosome mass and chromosomal DNA. In addition the experiment brings to light a variation in chromosome size, independent of the inducing treatments, that seems to be associated with the “ageing” of the seedlings.