Phylogenetic analysis of DNA C-values provides evidence for a small ancestral genome size in flowering plants

DNA C-value is a highly variable aspect of plant biodiversity whose origin and significance has often attracted general interest. Evaluation of the phylogenetic component of genome size variation is essential for a full explanation of its evolutionary significance but was previously prevented by insufficient data and lack of phylogenetic consensus. However, the recent development for the angiosperms of a DNA C-values database for 2802 species and a robust phylogenetic tree based on a three-gene DNA sequence matrix and 252 non-molecular characters allows meaningful new investigations of genome size in a phylogenetic context. Superimposing data from the former onto the latter shows that whereas all 15 higher order groups for which data are available contain species with small C-values, very large C-values occur in only two distantly related groups. At the lower taxonomic levels within these two groups similar trends were detected, with very large C-values restricted to species in the more derived families. The most parsimonious explanation for these observations is that ancestral angiosperms almost certainly had small genomes, and the possession of very large genomes represents a derived condition that has arisen independently at least twice. In contrast, gymnosperms (sister group to the angiosperms) are characterized by larger C-values than angiosperms. Thus within extant seed plants the possession of a small genome is a character unique to the angiosperms that was not only present in the ancestral species but has also been retained in most living taxa. (C) 1998 Annals of Botany Company.