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.