WHITLOCK, BARBARA A.1*, JUNGHO LEE1,2, OLENA DOMBROVSKA1,2, FABIANA BERNASCONI-QUADRONI2, and YIN-LONG QIU1,2. 1Biology Department, University of Massachusetts, Amherst MA 01003; 2Institute of Systematic Botany, University of Zurich, 8008 Switzerland. - Dating the age of angiosperms with DNA sequences of seven mitochondrial, plastid, and nuclear genes.
There has been remarkable progress recently in clarifying phylogenetic
relationships among basal angiosperm lineages using DNA sequences from
multiple sources. Two major questions, however, have not been
addressed using this massive amount of data: substitution rates of
these genes in all lineages of basal angiosperms and use of these data
to date major divergences within angiosperms. Here, we use a data set
of seven genes, from the chloroplast (atpB, rbcL), mitochondrial
(atp1,matR, and SSU and LSU rDNA), and nuclear genomes (18S rDNA), to
examine patterns of substitution rates of these genes and to estimate
the age of angiosperms. A global molecular clock can be rejected for
all genes, and all combinations of genes, under all models of
evolution explored. This is due in part to an apparent increase in
substitution rate in 3-4 lineages, including monocots, Piperales,
Ceratophyllum, and perhaps Ranunculales in the eudicots. This
increase in rate is found in all three plant genomes. Nevertheless, by
excluding these clades, it is possible to find local rate constancy
among the remaining lineages. Using this pruned phylogeny, we attempt
to estimate the age of angiosperms and divergence times of some basal
angiosperm clades. We compare these estimates to those derived from an
alternative approach of using models that allow for heterogeneous
rates of evolution.
Key words: angiosperms, evolutionary rates, molecular clocks