In 1965 Van't Hof estimated the nuclear DNA amount of an unidentified Allium cepa L. cultivar as 2C = 33.55 pg ( Experimental Cell Research39: 8–58). This value has been adopted by common usage as the main calibration standard for angiosperm DNA C-value estimations. However, different cultivars have been used while assuming species DNA C-value constancy. Surprisingly this assumption has never been tested. A. cepa is an outbreeder with telomeric heterochromatic segments, so intraspecific variation in C-value, possibly correlated with environmental factors as seen in Zea mays L., might be expected. We used laser flow cytometry to compare nuclear DNA amounts in roots of six A. cepa cultivars used as calibration standards or from different environments. Tissues from one cultivar, or similar volumes of tissue from two cultivars, were run and the variance between nuclei in 2C peaks compared. Only one shoulderless 2C peak was seen for all pairs of co-chopped cultivars. Thus, no large differences in C-value between cultivars from different environments were found. Moreover, comparing cultivars run singly or as pairs showed no evidence for increased variation in 2C peaks in the latter, and hence of critical differences in DNA amounts between ‘Ailsa Craig’ and another cultivar. Such variation was insufficient to make their use as alternative calibration standards, or the practice of imputing Van't Hof's original C-value estimate to them, unacceptable for most practical purposes. Given the mechanisms known which can generate genome size variation, the degree of constancy in DNA C-value found seems remarkable. Copyright 2000 Annals of Botany Company

Received: 27 August 1999 ; Returned for revision: 23 September 1999 . Accepted: 11 November 1999


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