Norman
A. Douglas
A
nuclear phylogeny of Boerhavia (Nyctaginaceae) suggests that expansion
of North American deserts has facilitated multiple recent range
expansions from the Gulf of California region.
The Sonoran and Chihuahuan deserts only assumed their modern range and
composition after the end of the last glacial period. The expansion and
assembly of the modern desert flora has been documented by studies of
fossil packrat middens. However, relatively few phylogenetic studies
have centered on plants of this region; fewer have employed dense
population-level sampling. Here I report on a phylogenetic analysis of
the nuclear gene, nitrate reductase, across the genus Boerhavia.
Boerhavia is found across warmer areas of the world, but previous
analyses by the author have established that roughly sixteen species of
annual taxa in North America form a clade. Within each of two
subclades, there is evidence that some taxa are cohesive species; these
possess diagnostic morphological features and are sympatric with their
closest relatives. The remaining species in each subclade of annuals
are taxonomically ill-defined and do not form cohesive groups with
molecular data. In one subclade, Boerhavia xanti, which is endemic to
coastal Sonora and the Baja California Peninsula, is ancestral to the
cohesive Boerhavia wrightii, which is widespread from Baja California
to the southern Chihuahuan Desert. In another annual clade, samples of
3-4 poorly defined morphospecies are interspersed in two clades, with
no morphospecies forming a monophyletic group. In this case, samples
from the Gulf of California region are placed in both subclades, but
northern and eastern samples of the widespread species Boerhavia
intermedia are only found in one subclade. This group is also
paraphyletic with respect to three unique winged species which have
restricted distributions. These patterns suggest that, although some
species represent older divergences, areas adjoining the Gulf of
California are 1) refugial areas for several now-widespread Boerhavia,
and 2) a repository of genetic diversity in annual Boerhavia and the
source of two recent, complex radiations of species.
Department of Biology
Duke University
Durham, NC 27708
(919) 660-7301
(919) 949-9810
nad(at)duke.edu