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Department of Plant Biology and Plant Biotechnology Center, Ohio State University, Columbus, Ohio 43210 (A.P.D., E.G.); Department of Zoology, University of Florida, Gainesville, Florida 32611 (E.L.B.); and Plant Genetics Research and Plant Science Units, United States Department of Agriculture-Agricultural Research Service, University of Missouri, Columbia, Missouri 65211 (M.D.M.)
R2R3 Myb genes are widely distributed in the higher plants and
comprise one of the largest known families of regulatory proteins.Here,
we provide an evolutionary framework that helps explainthe origin of the
plant-specific R2R3 Myb genes from widely distributed R1R2R3
Myb genes, through a series of well-established steps. To
understand the routes of sequence divergence that followed Myb
gene duplication, we supplemented the information available on
recently duplicated maize (Zea mays) R2R3 Myb genes (C1/Pl1and
P1/P2)
by cloning and characterizing
ZmMyb-IF35 and ZmMyb-IF25.These
two genes correspond to the recently expanded P-to-A groupof maize
R2R3
Myb genes. Although the origins of
C1/Pl1 and
ZmMyb-IF35/ZmMyb-IF25are
associated with the segmental allotetraploid origin of themaize genome,
other gene duplication events also shaped the P-to-Aclade. Our analyses
indicate that some recently duplicated Mybgene pairs display substantial
differences in the numbers of synonymoussubstitutions that have accumulated
in the conserved MYB domainand the divergent C-terminal regions. Thus,
differences in theaccumulation of substitutions during evolution can explain
inpart the rapid divergence of C-terminal regions for these proteinsin
some cases. Contrary to previous studies, we show that thedivergent C termini
of these R2R3 MYB proteins are subject topurifying selection. Our results
provide an in-depth analysisof the sequence divergence for some recently
duplicated R2R3 Mybgenes, yielding important information on general
patterns of evolutionfor this large family of plant regulatory genes.