MBE Advance Access published online on May 30, 2003
Molecular Biology and Evolution, doi:10.1093/molbev/msg123
Molecular Biology and Evolution © Society for Molecular Biology and Evolution 2003; all rights reserved
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1 Division of Biological Sciences, University of Missouri, Columbia, Missouri 65211
* To whom correspondence should be addressed. E-mail: mathewss{at}missouri.edu.
Flowering plant diversity now far exceeds the combined diversity of all other plant groups. Recently identified extant remnants of the earliest-diverging lines suggest that the first angiosperms may have lived in shady, disturbed, and moist understory habitats, and that the aquatic habit also arose early. This would have required the capacity to begin life in dimly lit environments. If so, evolution in light-sensing mechanisms may have been crucial to their success. The photoreceptor phytochrome A is unique among angiosperm phytochromes in its capacity to serve a transient role under conditions where an extremely high sensitivity is required. We present evidence of altered functional constraints between phytochrome A (PHYA) and its paralog, PHYC. Tests for selection suggest that an elevation in nonsynonymous rates resulted from an episode of selection along the branch leading to all angiosperm PHYA sequences. Most nucleotide sites (95%) are selectively constrained, and the ratio of nonsynonymous to synonymous substitutions on branches within the PHYA clade does not differ from the ratio on the branches in the PHYC clade. Thus, positive selection at a handful of sites, rather than relaxation of selective constraints, apparently has played a major role in the evolution of the photosensory domain of phytochrome A. The episode of selection occurred very early in the history of flowering plants, suggesting that innovation in phyA may have given the first angiosperms some adaptive advantage. Key Words:
phytochrome A, positive selection, adaptive evolution, early angiosperms
© 2003 Society for Molecular Biology and Evolution
Original Articles
Adaptive Evolution in the Photosensory Domain of Phytochrome A in Early Angiosperms
2 Department of Ecology and Evolutionary Biology, Yale University, New Haven, CT 06520
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