SUBSTITUTION RATE COMPARISONS BETWEEN GRASSES AND PALMS - SYNONYMOUS RATE DIFFERENCES AT THE NUCLEAR GENE ADH PARALLEL RATE DIFFERENCES AT THE PLASTID GENE RBCL

A number of studies have noted that nucleotide substitution rates at t he chloroplast-encoded rbcL locus violate the molecular clock principl e. Substitution rate variation at this plastid gene is particularly pr onounced between palms and grasses; for example, a previous study esti mated that substitution rates in rbcL sequences are approximate to 5-f old faster in grasses than in palms. To determine whether a proportion ate change in substitution rates also occurs in plant nuclear genes, w e characterized nucleotide substitution rates in palm and grass sequen ces for the nuclear gene Adh. In this article, we report that palm seq uences evolve at a rate of 2.61 x 10(-9) substitution per synonymous s ite per year, a rate which is slower than most plant nuclear genes. Gr ass Adh sequences evolve approximate to 2.5-fold faster than palms at synonymous sites. Thus, synonymous rates in nuclear Adh genes show a m arked decrease in palms relative to grasses, paralleling the pattern f ound at the plastid rbcL locus. This shared pattern indicates that syn onymous rates are correlated between a nuclear and a plastid gene. Rem arkably, nonsynonymous rates do not show this correlation. Nonsynonymo us rates vary between two duplicated grass Adh loci, and nonsynonymous rates at the palm Adh locus are not markedly reduced relative to gras ses.