| Summary: | An unknown Bidens was recently discovered on Starbuck Island, a small, remote coral atoll in the South
Pacific and one of the least visited islands in the world. I determined the identity and evolutionary origin
of this Bidens using both morphological analyses and phylogenetic relationships in Bidens (Asteraceae;
Heliantheae) and Coreopsis (Asteraceae; Heliantheae) using variation in nuclear rDNA internal
transcribed spacer (ITS) sequences. The ITS analyses include all species of Bidens sequenced to date,
including five from this study and sequences from Ganders et al. (2000) and a selection of Coreopsis
sequences from Kim et al. (1999). Using morphological and molecular data 1 tested the following
hypotheses: (1) Starbuck Island Bidens is an endemic species that evolved from a second colonization of
Pacific Islands that has not led to further speciation or adaptive radiation, independent of the colonization
that radiated in the Hawaiian Islands and French Polynesia; (2) Starbuck Island Bidens is B. aurea or B.
serrulata; (3) Starbuck Island Bidens recently evolved from either B. aurea or B. serrulata. Based on ITS
sequences, B. aurea and B. serrulata are rejected as being the same as or even closely related to Starbuck
Island Bidens. Surprisingly, Bidens cynapiifolia, a weedy species that occurs from Mexico to Brazil and
is widely introduced elsewhere, appears as a very close sister species to Starbuck Island Bidens.
However, the two taxa are distinct morphologically. Starbuck Island Bidens is described as a new
species, endemic to Starbuck Island. Also revealed in the ITS phylogeny are new sectional and generic
relationships. Two species of Cosmos (Asteraceae; Heliantheae), originally included as outgroup species,
appear nested within Coreopsis and are most closely related to section Pseudoagarista from Mexico.
Bidens mitis (section Meduseae) is nested within section Bidens, indicating that there is no support for
the recognition of section Meduseae. Bidens section Psilocarpaea remains polyphyletic, now composed
of four distinct clades. Both Bidens laevis (section Bidens) and Bidens squarrosa (section Greenmania)
cluster with the other species in their respective sections. My data indicate that section Greenmania is
monophyletic and section Bidens is monophyletic if section Meduseae is included in it. It is now known
that one of the South Pacific Bidens species, Starbuck Island Bidens, is not closely related to the
Hawaiian and Marquesan species of Bidens. To fully understand the evolutionary relationships among
indigenous Pacific island Bidens a complete ITS phylogeny should be constructed that includes the
remaining South Pacific Bidens species not yet sequenced.
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