BEILSTEIN, MARK* AND IHSAN AL-SHEHBAZ. Department of Biology, University of Missouri Saint Louis, Saint Louis, MO 63121 and Missouri Botanical Garden, Saint Louis, MO 63110. - Toward a Phylogeny of the Brassicaceae (Cruciferae).
The Brassicaceae (Cruciferae) is the largest family of the diverse
order including the mustard oil glycoside-producing plant families.
Morphological evidence (e.g., the presence of tetradynamous stamens,
cruciform corolla, radially symmetric flowers, and characteristic
fruit often with a false septum dividing it into two locules) and
recent molecular phylogenetic studies support the monophyly of
Brassicaceae. Nonetheless, relationships within the family remain
problematic. Previous classification schemes for the family relied
heavily on fruit characters to determine generic and tribal
boundaries. Published preliminary molecular data show that fruit
characters are highly homoplastic and that groups based on these
features often do not reflect phylogeny. As a result, current generic
circumscriptions in Brassicaceae make it difficult to identify
putatively monophyletic terminal clades appropriate for sampling at
the familial level. Recent molecular work at the generic and tribal
levels suggests that the traditionally circumscribed tribe,
Brassiceae, as well as a narrowly circumscribed Lepideae are
monophyletic. We suggest that trichome morphology may be useful in
delimiting monophyletic groups of genera in Brassicaceae. For
example, the genera Alyssum, Arabidopsis, Arabis, Draba, and
Lesquerella share branched trichomes and may form one putatively
monophyletic group, while Armoracia, Barbarea, Cardamine, Nasturtium
and Rorippa are either glabrous or have simple trichomes, forming
another putatively monophyletic group. By integrating current
molecular knowledge with detailed morphological investigations, we
present a series of hypotheses of evolution in the family, testable
using current molecular techniques. Furthermore, we discuss the
importance of trichome morphology in predicting relationships in
Brassicaceae.
Key words: Brassicaceae (Cruciferae), trichomes