**3. Standardization**

Not unexpectedly, the issue applies as much to animal to plant or fungal genomes. To enable a comparison of data obtained by different laboratories, Tiersch et al. [9] calibrated a set of animal reference standards, choosing human male leukocytes (7 pg DNA/2C) as the primary reference; the 7 pg figure was based on estimates derived from Feulgen micro-densitometry [10]. The experiments derived a 2C value of 2.5 pg DNA for domestic chicken, which was close to the value given by [11]. The domestic chicken genome has been adopted since this time as the most widely used reference standard for the sizing of animal genomes [12]. In an effort to enable comparisons between animal and plant genomes, Doležel et al. [8] recommended a set of plant reference standards (Table 1), also calibrated with respect to the human male leukocyte genome, assuming the 7 pg value assigned by Tiersch et al. [9]. Over the past three decades, hundreds of genome size estimates have been published, based mainly on the 7 pg value. The question is how close to reality these estimates really are, which relates in the main to how accurate the 7 pg figure is. According to the arguments made by Doležel and Greilhuber [13], the value most probably over-estimates the true value by 5–10%.


**Table 1.** Plant DNA reference standards calibrated for the estimation of nuclear DNA amounts in absolute units [8].

\* Seeds of the reference standards can be obtained from the corresponding author free of charge at dolezel@ueb.cas.cz. Since the year 2000, seed samples were provided to 615 research projects worldwide. \*\* Estimated after considering 7 pg DNA/2C for human [9].
