Molecular evolution and phylogeny of the atpB-rbcL spacer of chloroplast DNA in the true mosses

The nucleotide variation of a noncoding region between the atpB andrbcL gen es of the chloroplast genome was used to estimate the phylogeny of 11 speci es of true mosses (subclass Bryidae). The A+T rich (82.6%) spacer sequence is conserved with 48% of bases showing no variation between the ingroup and outgroup. Rooted at liverworts, Marchantia and Bazzania, the monophyly of true mosses was supported cladistically and statistically. A nonparametric Wilcoxon Signed-Ranks test Ts statistic for testing the taxonomic congruenc e showed no significant differences between gene trees and organism trees a s well as between parsimony trees and neighbor-joining trees. The reconstru cted phylogeny based on the atpB-rbcL spacer sequences indicated the validi ty of the division of acrocarpous and pleurocarpous mosses. The size of the chloroplast spacer in mosses fits into an evolutionary trend of increasing spacer length from liverworts through ferns to seed plants. According to t he relative rate tests, the hypothesis of a molecular clock was supported i n all species except for Thuidium, which evolved relatively fast. The evolu tionary rate of the chloroplast DNA spacer in mosses was estimated to be (1 .12 +/- 0.019) x 10(-10) nucleotides per site per year, which is close to t he nonsynonymous substitution rates of the rbcL gene in the vascular plants . The constrained molecular evolution (total nucleotide substitutions, K ap proximate to 0.0248) of the chloroplast DNA spacer is consistent with the s low evolution in morphological traits of mosses. Based on the calibrated ev olutionary rate, the time of the divergence of true mosses was estimated to have been as early as 220 million years ago.