STRUWE, LENA1*, JASON R. GRANT2, and KATHERINE R. GOULD3. 1Dept. of Ecology, Evolution, and Natural Resources, Rutgers University, Cook College, 237 Foran Hall, 59 Dudley Road, New Brunswick, NJ; 2Institut de botanique, Universite de Neuchatel, rue Emile-Argand 11, Case Postale 2, CH-2007 Neuchatel, Switzerland; 3Department of Biology, P.O. Box 4718, Austin Peay State University, Clarksville, TN. - Coronas and curved stamens: shape, size, and color in the floral evolution of tribe Helieae (Gentianaceae; Asteridae).
Neotropical flowers exhibit a large diversity in shape, size, and
color. To understand the evolution of the complexity of floral traits,
these characters need to be evaluated in the context and constraints
of detailed phylogenetic data. Hypotheses of time and place of the
origin of floral characters are only well-founded when supported by
detailed morphological and anatomical investigations as well as robust
phylogenetic data (morphological and/or molecular). For example, the
monophyletic tribe Helieae (Gentianaceae; 22 genera, ca. 190 spp.)
includes 0.5-15 cm long flowers that are pollinated by flies, bees,
hawk-moths, bats, or hummingbirds. An array of floral characters is
represented, but no previous work has attempted to present
evolutionary hypotheses for these. The flowers have the sympetalous
bauplan characteristic of hypogynous Asteridae, but are uncommon in
having zygomorphic stamens and styles. The anthers are usually
strongly bent after anthesis and positioned in or protruding from the
lower part of the corolla mouth. Such a placement of the anthers might
facilitate increased pollination by larger animals such as bats or
hummingbirds, which is also supported by our phylogenetic trees based
on ITS sequence data. Large, red-pink(-yellow) flowers pollinated by
hummingbirds is a derived trait, most likely derived from
bat-pollinated flowers. Symbolanthus has a ring-shaped corona
inside the corolla at the base of the stamens. New investigations show
that coronas are strongly variable in size and shape between different
species. Staminal gaps between the corolla tube and the staminal base
are found in Symbolanthus and close relatives and may have been
a precursor to the evolution of the corona. The unique valvate-contort
corolla aestivation of the presumably hawk-moth-pollinated
Aripuana is also discussed. A hypothesis for the development of
and homology among corolla parts in contort (stamens in corolla tube)
vs. valvate-contort (stamens in sinuses) corolla buds is presented.
Key words: Asteridae, floral anatomy, floral evolution, Gentianaceae, molecular systematics, pollination