Allium cepa L. cultivars from four continents compared by flow cytometry show nuclear DNA constancy

In 1965 Van't Hof estimated the nuclear DNA amount of an unidentified Alliu m cepa L. cultivar as 2C = 33.55 pg (Experimental Cell Research 39: 8-58). This value has been adopted by common usage as the main calibration standar d 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 h eterochromatic segments, so intraspecific variation in C-value, possibly co rrelated with environmental factors as seen in Zea mays L., might be expect ed. We used laser how cytometry to compare nuclear DNA amounts in roots of six A. cepa cultivars used as calibration standards or from different envir onments. Tissues from one cultivar. or similar volumes of tissue from two c ultivars, were run and the Variance between nuclei in 2C peaks compared. On ly one shoulderless 2C peak was seen for all pairs of co-chopped cultivars. Thus, no large differences in C-value between cultivars from different env ironments were found. Moreover. comparing cultivars run singly or as pairs shelved no evidence for increased variation in 2C peaks in the latter, and hence of critical differences in DNA amounts between 'Ailsa Craig' and anot her cultivar. Such variation was insufficient to make their use as alternat ive 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 o f constancy in DNA C-value found seems remarkable. (C) 2000 Annals of Botan y Company.