DUVALL, MELVIN R.* and AUTUMN J. BRICKER. Plant Molecular Biology Center and Department of Biological Sciences, Northern Illinois University, DeKalb, IL 60115-2861. - Nuclear-cytoplasmic incongruence among monocots and related paleoherb dicots.
Molecular evidence for the monophyly of the monocots is consistent
across several chloroplast and mitochondrial loci. Most of these
studies also find the monogeneric Acoraceae J. G. Agardh (Acoranae
Reveal) to be the sister lineage of all other extant monocots in what
has been called the Acoranan Hypothesis. Phylogenetic studies of
nuclear 18S loci, conducted broadly across the angiosperms, find the
monocots to be paraphyletic because Acorus calamus is
anomalously found in a grade of basal dicots, either at the base of a
clade of selected Piperales, or flanked by representative Nymphaeaceae
and Annonaceae, depending on the sampling. This nuclear/cytoplasmic
incongruence was investigated herein. Further sampling among
paleoherbs failed to resolve the monocots as monophyletic in 18S gene
trees. The 18S locus was then sequenced from a second species, A.
gramineus, in two different laboratories from independent sources
of plant material and found to be over 99% identical to the published
18S sequence of A. calamus. 18S loci were then cloned from
A. calamus to explore the possibility that an underepresented
18S paralogue existed in the Acoraceae genome that, when found and
sequenced, would cluster with monocots. Sequences of 20, 18S clones
were compared to published sequences and again found to be nearly
identical. 18S sequences of 32 paleoherb species were then extensively
reanalyzed using Bayesian Inference. Analyses were performed both
including and excluding short ambiguous regions corresponding to helix
termini. Several substitution models, rates of among-site variation,
and different base frequencies were selected in consecutive analyses.
Although Acoraceae varied somewhat in phylogenetic position, it was
most frequently found at the base of Chloranthaceae (clade credibility
values: 49-57%). Monocots were never recovered as monophyletic. The
inability to demonstrate the monophyly of a group with so many
synapomorphies utilizing 18S sequences clearly reflects a singular
history for this locus among paleoherbs.