Systematics Section / ASPT
Horn, James W. .
Evolution of Floral Symmetry and Foliar Organs in Hibbertia (Dilleniaceae).
HIBBERTIA, a monophyletic genus of c. 180 species centered in Australia, is remarkable among angiosperms in having both floral structure that varies in many characters of fundamental macroevolutionary significance, as well as leaves that range from being tiny and ericoid to distinctly laminar and over 1m in length. In the context of molecular phylogenetic analyses of both Dilleniaceae (four plastid loci) and Hibbertia (118 taxa; nr ITS and plastid rpl16 intron), the evolution of floral symmetry and leaf form are explored. Symmetry in Hibbertia flowers is perhaps unique within angiosperms as new observations demonstrate that the expression of monosymmetry may be either in the median (Pachynema clade) or transverse (Hemistemma clade) plane of the flower. Transverse floral monosymmetry has a minimum of three independent origins from an ancestrally polysymmetric state, and there are minimally four reversals back to a polysymmetric state. Evolutionary lability of floral symmetry in Hibbertia is potentially similar to that of asterid eudicots. Within Hibbertia, the “leafless” Pachynema clade (9 spp.) is an unexpectedly early-branching lineage. Its members have photosynthetic aerial shoot systems that have been interpreted as representing only the inflorescence region of the plant, relative to other Dilleniaceae. “True” foliage leaves are rarely produced in few species belonging to this clade, and resemble those of Dilleniaceae exclusive of Hibbertia in structure. The Pachynema clade is sister to the Hemistemma clade (c. 100 spp.), in which a majority of species possess needle-like, ericoid leaves. These leaves share several unique structural characteristics with the inflorescence bracts and bracteoles of members of the Pachynema clade (and not the “true” foliage leaves of this group), supporting a hypothesis that the highly-derived leaf organs of the Hemistemma clade are homeotic inflorescence bracts and bracteoles, and not homologous with foliage leaves of Dilleniaceae outside of Hibbertia.
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1 - Duke University, Department of Biology, 139 Biological Sciences Building, PO Box 90338, Durham, North Carolina, 27708, USA
comparative floral morphology
Presentation Type: Oral Paper:Papers for Sections
Date: Monday, July 31st, 2006
Time: 10:30 AM