Molecular evolution of the teosinte branched gene among maize and related grasses

Several authors have proposed that changes in a small number of regulatory genes may be sufficient for the evolution of novel morphologies. Recent ana lyses have indicated that teosinte branched1 (tb1), a putative bHLH transcr iption factor, played such a role during the morphological evolution of mai ze from its wild ancestor, teosinte. To address whether or not tb1 played a similar role during the evolution of the Andropogoneae, the tribe to which maize belongs, and to examine the rate and pattern of tb1 evolution within this tribe, we analyzed tb1-like sequences from 23 members of the Andropog oneae and five other grasses. Our analysis revealed that the TB1 protein ev olves slowly within three conserved domains but rapidly outside these domai ns. The nonconserved regions of the gene are characterized by both a high n onsynonymous substitution rate and frequent indels. The ratio of nonsynonym ous substitutions per nonsynonymous site (d(N)) to synonymous substitutions per synonymous site (d(S)) was not significantly greater than 1.0. providi ng no evidence for positive selection. However, the d(N)/d(S) ratio varied significantly among lineages and was high compared with those of other plan t nuclear genes. Variation in the d(N)/d(S) ratio among the Andropogoneae c ould be explained by unequal levels of purifying selection among lineages. Consistent with this interpretation, the rate of nonsynonymous substitution differed along several lineages, while the synonymous substitution rate di d not differ significantly. Finally, using tb1, we examined phylogenetic re lationships within the Andropogoneae. The phylogeny suggests that the tribe underwent a rapid radiation during its early history and that the monoecio us Andropogoneae are polyphyletic.