LIVSHULTZ, TATYANA. L.H. Bailey Hortorium, Department of Plant Biology, 228 Plant Science Building, Cornell University, Ithaca, NY 14853-5908 USA. - Comparative morphology and development of staminal coronas in Dischidia (Asclepiadoideae, Apocynaceae).
Coronas, outgrowths of the corolla and/or stamens, are prominent
structures in the flowers of many Apocynaceae sensu lato. Their
extreme positional and morphological variability has made primary
homology assessment of mature structures very difficult both across
the family as a whole and within more recently diverged groups. The
genus Dischidia R. Br., ca. 80 species, is characterized by
membranous, dorsiventrally flattened staminal corona segments, each
consisting of a stalk and a bifid blade with recurved arms, but there
is such diversity in the genus that homology of the full range of
mature structures cannot be assessed with confidence. Development of
staminal coronas in 12 species of Dischidia, encompassing a
large fraction of the morphological diversity in the genus, was
studied with SEM. Staminal coronas occur alone or in combination with
two additional structures, here termed dorsal and basal staminal
appendages. Development of the typical staminal corona segment can be
divided into five modules: 1) primordium initiation, 2) primordium
differentiation, 3) arm primordium formation, 4) stalk elongation, and
5) blade differentiation. Two distinct types of primordia, here termed
plate and bilobed primordia, develop during module 2; however their
subsequent development results in some very similar mature
morphologies. Different blade morphologies are generated by
differential growth and post-genital fusion during module 5.
Additional diversity results from non-execution of one or more of the
modules, although a primordium is always initiated. Nineteen
developmental and morphological characters were identified, allowing
detailed comparison of the full range of coronas in the genus.
Cladistic analysis of these characters in combination with other
morphological characters and sequences of the second intron of
LEAFY indicates that the bilobed primordium is an unreversed
synapomorphy, while elimination of later modules has evolved
independently at least twice.
Key words: Dischidia , Apocynaceae, corona, development, flower, phylogeny