COMPARISON OF THE EVOLUTION OF RIBULOSE-1,5-BIPHOSPHATE CARBOXYLASE (RBCL) AND ATPB-RBCL NONCODING SPACER SEQUENCES IN A RECENT PLANT GROUP, THE TRIBE RUBIEAE (RUBIACEAE)

Plastid sequences of the atpB-rbcL spacer and rbcL gene itself were us ed to evaluate their respective potential in reconstructing the phylog eny of 15 taxa from the tribe Rubieae (Rubiaceae). From our previous a nalyses using the atpB-rbcL spacer, the 15 selected taxa represent mos t of the variability of the tribe. Since this group is considered to b e relatively recent (Upper Tertiary), it should allow the study of ear ly dynamics of nucleotide substitutions in recent divergences. The res ults show that the spacer and rbcL. inferred phylogenies are not total ly congruent; the spacer trees are more similar to interpretations of morphological data. A comparative analysis of the pattern of nucleotid e substitution of these two sequences in the Rubieae shows that (1) th e overall rate of substitution is similar in the spacer and in rbcL, a nd the rate of synonymous substitution in rbcL is much higher; (2) the level of homoplasy is higher in rbcL than in the spacer matrix which shows a higher phylogenetic structure; and (3) the pattern of transiti on and transversion substitutions is different in the two sequences, a nd is not linear in rbcL. As a result of these observations, we sugges t that (1) the spacer is evolving relatively slowly because of unsuspe cted, and phylogenetically important, selective constraints on its seq uence; and (2) in the rbcL sequence, many sites, free of constraint, a re changing at high rate, and some of these sites seem to have undergo ne multiple substitutions even in this recent tribe. This could explai n the high level of homoplasy found in Rubieae rbcL sequences.