CHLOROPLAST-DNA RESTRICTION SITE ANALYSIS IN THE GENUS BROMUS (POACEAE)

Chloroplast DNA (cpDNA) restriction site variation was examined in 32 species, representing five subgenera, of Bromus (Poaceae). Thirty-seve n phylogenetically informative restriction sites were detected. Cladis tic analysis of the restriction site data produced a single most-parsi monious tree of 50 steps. The cladogram indicated two major clades wit hin the genus. One clade included B. trinii of subgenus Neobromus and species of subgenus Ceratochloa. The other was composed of subgenera F estucaria, Stenobromus, and Bromus. Within the second clade, species o f subgenus Festucaria appeared in three lineages. The second clade als o contained an assemblage of species belonging to subgenera Stenobromu s and Bromus in a separate lineage. There was very little resolution o f relationships in this assemblage since several species appeared indi vidually in separate lineages. The cpDNA phylogenetic hypothesis did n ot separate species of subgenera Stenobromus and Bromus into well-defi ned clades as circumscribed by morphology and cytogenetics. The cpDNA tree is in agreement with the phylogenetic scheme based on traditional data in that: 1) subgenera Neobromus and Ceratochloa were the first t o diverge, while Bromus and Stenobromus diverged later; 2) within the genus Bromus species with small chromosomes are ancestral; and 3) subg enera Bromus and Stenobromus probably originated from similar ancestor s as Festucaria. The tree based on cpDNA data does not support that: 1 ) subgenera Neobromus and Ceratochloa did not have a common origin; 2) subgenus Festucaria is monophyletic; and 3) subgenera Stenobromus and Bromus are distinct entities. The mean nucleotide sequence divergence values between pairs of subgenera ranged from p = 0.0 to 0.9. These v alues suggest that cpDNA evolution in Bromus is slow.