DODD, ATHENA* AND KEITH KAROLY. Biology Department, Reed College, Portland, OR 97202. - Separating the effects of gene flow and population history: a phylogeographic study of Delphinium nuttalliiusing nested clade analysis.
The distribution of genetic variation in a species depends on both
gene flow and population history. In this study, we demonstrate the
utility of nested clade analysis for identifying historical
factors—ignored by most traditional analyses—and provide one of the
first studies to apply this approach to plants. PCR-RFLP markers were
used to survey 912 bp of cpDNA in Delphinium nuttallii
(Ranunculaceae), a native of the Pacific Northwest. Twelve populations
located throughout the range of the species were examined, sampling
five individuals per population. Sixteen polymorphisms and nine
haplotypes were observed. A single most parsimonious network was
found, which contained only one homoplastic trait. AMOVA, a
non-historical analysis, suggested a regime of restricted gene flow to
explain the pattern of genetic variation that was observed (F ST = 0.87, P < 0.0001). A simple
historical approach, the exact nested contingency analysis, rejected
the null hypothesis of no geographical association for all but one
nesting group (P < 0.05 for these clades), but could not identify the
source of these associations. The full nested clade analysis detected
restricted gene flow or past fragmentation for all one-step clades,
and contiguous range expansion at high cladogram levels. These
inferences are supported by the biology of D. nuttallii which
has no special adaptations for seed dispersal, and by the geologic
history of the area. Post-Pleistocene glacial retreat would have
allowed northward expansion of D. nuttallii during the last
15,000 years, and the Missoula Floods of 13,500-15,000 years ago may
have permitted subsequent dispersal through the Columbia River Gorge
into the western portion of its current range. For D.
nuttallii, nested clade analysis detected and identified important
historical factors that could not be resolved by simpler analyses
employed in this study.
Key words: chloroplast DNA, Delphinium nuttallii, nested clade analysis, phylogeography