Genome Size Estimation in Medicago (Fabaceae): Independent Loss and Gain?

Genome size in flowering plants has been correlated with various aspects of life history, rarity and geographic distribution. To consider these hypotheses in the genus Medicago we obtained genome size estimates for 49 species (over half of the species in the genus) representing all major lineages using standard methods of flow cytometry performed on a FACSCalibur flow cytometer. Glycine max ‘Polanka’ with an estimated genome size of 2.5 pg was used as the primary standard, and  Medicago truncatula Jemalong, with an estimated genome size of 1.15 pg, was used for species with a genome size close to that of Glycine. Genome size ranged from just under 1.0 to 3.8 pg among diploid species with an average genome size of 1.79 (s.d. = 0.66). Five polyploid species were sampled, interestingly all have smaller genome sizes than that of the largest two diploid species, M. carstiensis (3.8 pg) and M. suffruticosa (3.5 pg). Genome sizes were mapped onto a phylogeny based on DNA sequence data from the same accessions to examine patterns of genome size distribution. Genome size has a non-random distribution, with some clades being characterized by smaller genome sizes than others. Seven species in early diverging lineages have an average genome size of 2.31 pg (s.d. = 0.64). Two clades of annual species are characterized by an average genome size smaller than their sister group: one clade that includes M. polymorpha has an average genome size of 1.16 pg (s.d. = 0.06) and another clade that includes M. truncatula, has an average genome size of 1.19 pg (s.d. = 0.11), supporting the hypothesis of independent losses of genetic material in these two clades.