Nuclear DNA changes within Helianthus annuus L.: origin and control mechanism.

Previous results suggested that the amount of nuclear DNA varies in one and the same progeny of Helianthus annuus, depending on the head portion in which seeds have developed. Accordingly, cytophotometric determinations were carried out in a selfed line, after Feulgen-staining, to obtain information on the developmental stages at which DNA changes are produced and on the mechanism controlling the variation. It was found that the Feulgen absorption values of mitotic prophases in immature anthers and pistils and of meiotic prophases I are the same in any flower of the head. In contrast, the Feulgen/DNA contents of early prophases in heart-shaped embryos differ significantly, increasing from those developing at the centre of the head to those developing at its periphery, and remaining unchanged in each embryo during further development and seed germination. Variations in the number of chromosomes do not account for the differences observed in nuclear DNA contents in which sequences comprised in heterochromatic nuclear regions are involved. The Feulgen absorption values of seedlings obtained from seeds developed in different portions of single heads increase or diminish starting from those found in the mother plant. This depends on whether these latter are relatively low or high and on the gradient of seed location in the head. The variation occurring within each single progeny covers the whole range existing within the line.