The relationship between nuclear DNA content and centromere volume in higher plants.

The total volume of centromeres per nucleus varies widely within Triticum aestivum cv. Chinese Spring (7-fold) and between 11 higher plant species (24-fold). Such variation is closely correlated with nuclear DNA content, nuclear volume and, to a lesser extent, the total volume of nucleoli per nucleus. Centromere volume reflects minor intraspecific developmental fluctuations in nuclear size independent of variation in nuclear DNA content, but variation in nuclear DNA plays the major role in determining centromere volume. Thus, in general a given total volume of centromeric material is apparently characteristic of an approximately constant nuclear volume and mass of nuclear DNA, but largely independent of chromosome number. The range of volume of single centromeres in 4 taxa corresponds with the ranges of their single chromosome lengths or chromosome DNA contents. The centromere is, therefore, not a unit structure of constant size and mass but a chromosome segment whose highly variable volume closely reflects the volume and mass of the chromosome to which it belongs. The correlation between centromere size and chromosome size and DNA content is potentially useful for identifying single centromeres in unsquashed interphase and dividing nuclei; thereby facilitating studies of the intranuclear disposition of chromosomes. The present results for centromeres provide probably the first example to indicate that variation in the total DNA content of small segments present on each chromosome sometimes varies directly in proportion to large interspecific variation in nuclear DNA C-value. The close correlation between centromere volume, and nuclear DNA content is probably nucleotypic in origin. The functional significance of the variation in centromere volume is unknown, as is the nature of the mechanism which determines that centromere volume closely reflects nuclear and chromosome size and mass.