DNA base composition of Allium genomes with different chromosome numbers.

The present report examines whether the presence of an additional chromosome can be detected as modifying the nuclear DNA amount and base composition of the cell, determined here by flow cytometry of interphasic nuclei, using four monosomic additions (chromosomes 3C, 4C, 7C and 8C transmitted from Allium cepa to Allium fistulosum L.). A. cepa differs significantly from A. fistulosum in genome size (2C DNA = 33.2 pg in A. cepa and 23.3 pg in A. fistulosum) as well as in GC content (38.7% and 39.8%, respectively). The presence of an extra chromosome of A. cepa obviously increases the nuclear DNA amount above the A. fistulosum value but also alters the apparent mean GC content. By comparing the monosomic additions and the parental background, the DNA amount and base composition of each of the four single chromosomes were calculated to quantify the GC content per chromosome and therefore to deduce their initial contribution to the A. cepa genome. Taken individually, some chromosomes are atypical in terms of GC content: the single chromosome 3C is AT-rich, having only about only 25% GC. However, the three other chromosomes examined are typical of the A. cepa genome in base composition. Indeed, this biological panel gives access to chromosomal features via a cytometric assay of nuclei. It should facilitate quantification of GC-rich repetitive sequences forming heterochromatic domains located mainly at the telomeres in the monocotyledonous A. cepa genome.