[15] Comparative Studies of Genome Content

This chapter presents comparative studies of genome content. Genome content, or genome size, refers to the amount of DNA contained in the genome of an organism. A genome is defined as the basic (monoploid) chromosome set of an organism, consisting of a species-specific number of linkage groups. Therefore, when determining the genome size of an organism, only the DNA contained in the chromosomes (nuclear DNA) is considered. The nuclear DNA content of a cell is usually expressed in terms of the C value of the cell. The letter C is used to distinguish terminology used for genome studies from the terms x and n, reserved for chromosome number. One must be cautious when comparing DNA amounts in various species. Comparisons are made at either the C level or genome level. This becomes increasingly important when dealing with polyploid species. The two ways genome content may be reported are in absolute amounts and in arbitrary units. The genome size of experimental lines may also be expressed relative to a standard. The standard might be a taxonomically well-defined plant of constant genome size. The genome size of experimental lines may also be expressed relative to a standard. The standard might be a taxonomically well-defined plant of constant genome size.