WURDACK, KENNETH J.1* and JAMES W. HORN2. 1The Lewis B. and Dorothy Cullman Program for Molecular Systematics, The New York Botanical Garden, Bronx, NY 10458; 2Department of Biology, Duke University, Box 90338, Durham, NC 27708. - A reevaluation of the affinities of the Tepuianthaceae: molecular and morphological evidence for placement in the Malvales.
The Tepuianthaceae consists of the genus Tepuianthus,
containing 7 taxa that are narrow endemics within the Guayana
Highlands region of Venezuela and adjacent Colombia and Brazil.
Hypotheses of the systematic affinities of this family have suggested
relationships with the Celastrales (Cronquist) and Rutales (Takhtajan,
Thorne), though the Tepuianthaceae is unplaced in the APG system.
Parsimony analysis of sequence data from 18S rDNA, atpB, and
rbcL places Tepuianthus within the Malvales, sister to
the Thymelaeaceae, sensu lato (including Gonystylaceae).
The secondary phloem of Tepuianthus is characteristic of
Malvales in that it is tangentially stratified into fibrous and
nonfibrous layers, with radially dilated phloem rays. Potential
morphological synapomorphies with Thymelaeaceae include the absence of
stipules, unilacunar nodes, a single ovule per locule with
apical-axile placentation, ovules with a ventral raphe, and ovules
with the micropyle formed exclusively from the inner integument. We
propose to taxonomically accommodate Tepuianthus in a new
Thymelaeaceae subfamily. The unusual pollen of Tepuianthus may
represent an intermediate state between a tricolporate
type--ubiquitous in early-branching lineages of major clades within
Malvales--and the highly derived crotonoid exines found in
Thymelaeaceae. The presence of a well-formed, 5-merous corolla in
addition to a series of extrastaminal, glandular scales in
Tepuianthus leads to the reinterpretation of similar scales in
Gonystylus as not representing a vestigial corolla as
previously thought. The phylogenetic position of Tepuianthus is
biogeographically significant in that, like the malvalean dipterocarps
Pakaraimaea and Pseudomonotes, it represents an
early-branching, presumably relictual, Guayana Shield-centered lineage
of an otherwise mostly paleotropical clade.
Key words: Malvales, phylogeny, Tepiuanthus, Tepuianthaceae, Thymelaeaceae