Nuclear DNA amounts in Acacia and Prosopis (Mimosaceae) and their evolutionary implications

The nuclear DNA amounts of 53 accessions representing 26 species of Acacia and Prosopis, were estimated with a flow cytometer. Accessions of the same level of ploidy in each of the three subgenera of Acacia and the genus Prosopis formed four classes. The 2C-values and ploidy levels of taxa within each class were strongly correlated (r = 0.968, 0.982, and 0.997 for the subgenera Acacia, Heterophyllum, and the genus Prosopis, respectively). Within species, genome size variations were mainly due to polyploidy. For subgenus Acacia, the mean 2C-value of the tetraploid accessions was 1.091 pg and for the octoploid 2.179 pg. The mean 2C-value of accessions of Aculeiferum (all are diploid) was 1.150 pg. The mean 2C-value of the diploid Helerophyllum was 1.583 and of the tetraploid 3.309 pg, whereas that of the diploid accessions of Prosopis was 0.846 pg and of the tetraploid 1.724 pg. The mean 2C-value of Acacia nubica (subgenus Acacia) was 1.680; a value closer to that of the tetraploid accessions of Prosopis. Accordingly, it is suggested that A. nubica, which also has 2n = 56, may better fit in Prosopis than in Acacia. The present results demonstrate considerable divergence in genome size among the three subgenera of Acacia and the genus Prosopis, and that speciation within subgenera may involve qualitative rather than quantitative changes in their nuclear DNA.