MAGALLóN, SUSANA A. Section of Evolution and Ecology, University of California, Davis, CA 95616. - Extinct and extant Hamamelidoideae: phylogeny and character evolution.
The members of Hamamelidaceae have unambiguously been documented to
belong to the Saxifragoid clade, within the core eudicots. Although
the monophyly of the family has not been firmly established, the
monophyly of subfamily Hamamelidoideae is confirmed by several
independent phylogenetic studies. Insights about the early history of
Hamamelidoideae have been provided by the flowers of
Archamamelis (Santonian-Campanian, Sweden), and Allonia
(late Santonian, Georgia, U.S.A.). A third, recently discovered
flower, Androdecidua, from the late Santonian of Georgia,
provides further information about morphological diversity within
Hamamelidoideae. Androdecidua has two pentamerous androecial
whorls, in which the stamens of the outer whorl have bisporangiate
anthers, resulting from the absence of the adaxial pollen sac of each
theca. A phylogenetic analysis was conducted to obtain explicit
information about relationships within Hamamelidoideae, particularly
the affinity of fossil genera, and about character evolution. The
analysis included 38 taxa, comprising members of Hamamelidaceae,
representatives of the Saxifragoid clade, and outgroups form the basal
eudicot grade, and was based on 57 characters of floral form. The
strict consensus of 42 most parsimonious trees resolves
Archamamelis and Hamamelis as sister taxa, whereas
Androdecidua and Allonia are nested within a
monophyletic Loropetalinae. The obtained phylogenetic hypothesis
suggests that the strap-like, circinately coiled petals, which
characterize some genera of Hamamelidoideae, evolved twice; a
specialized stomium configuration that results in a distinctive mode
of anther dehiscence evolved once, but was subsequently lost, and
stamens with bithecal anthers evolved at least two times. The highly
nested placement of fossil taxa suggests that the main evolutionary
lineages within Hamamelidoideae had evolved by the late Santonian.
Considering that the oldest records for core eudicots are from
slightly older strata (Turonian), it appears that morphological and
taxic diversification proceeded quickly during the initial evolution
of Hamamelidoieae, and subsequently remained stable.
Key words: floral evolution, fossils, Hamamelidoideae, perianth, phylogeny, stamens