Feulgen-DNA Cytophotometry for Estimating C Values

The genome size of an organism, commonly known as its C value, is defined as the content of DNA (measured by weight or numbers of basepairs) in a single copy of the sequence of DNA found within the cells of an organism (i.e., the amount of DNA in a haploid chromosome set, where 1n=1C) (1 ). A single sperm of Drosophila melanogaster contains 0.18 pg DNA, which is the C value for this species (2 –4 ). By convention, the DNA content of a diploid somatic cell of this species can then be expressed as 2n=8=2C=0.36 pg DNA for the male diploid genotype of AAXY (3 ). And, 2C=0.40 pg DNA for a diploid Drosophila female somatic cell with the genotype of AAXX because the X chromosome contains a bit more DNA than the Y chromosome in this species (see Table 1 ). These considerations follow from the “DNA Constancy Hypothesis” formulated in the 1950s (5 –8 ) that the DNA content of nuclei of a eukaryotic species is essentially constant among the individuals within a given species and constitutes the repository of genetic information for that taxon. For example, genome sizes, expressed as C values, are listed in Table 1 for 10 different species of Drosophila .