JOHANNES FISHER, KIRSTEN M. Department of Integrative Biology and University and Jepson Herbaria, University of California, Berkeley, CA 94720. - The heteroblastic series in leucobryoid mosses.
An emphasis is being placed on integrating development into systematic
and evolutionary research programs. Plants provide an excellent
opportunity for ontogenetic studies, as they are modular organisms
which record a temporally-ordered array of leaves arrested in
successive stages of development, the heteroblastic series, that
arises from a single initial cell (as opposed to a meristematic region
of cells in seed plants). In acrocarpous mosses, a new branch module
is initiated sympodially following the production of archegonia, so
the heteroblastic series is reiterated along each new branch,
providing an investigator with numerous replicates. Earlier studies
have indicated possible parallels between the heteroblastic series and
phylogenetic transformations. Within the haplolepideous mosses, a
very distinct leaf architecture known as the 'leucobyroid' morphology
has arisen at least five times. Leaves of mosses with this unique
architecture are dominated by at least two strata of large thin-walled
dead cells (hyalocysts), with one or more layers of green cells
(chlorocysts) intercalated between them. Traditional interpretation
of the tissue homologies for these leaves, derived from observations
of Leucobryum and related taxa, contends that the leucobryoid
morphology arose via a gradual expansion of the costa and simultaneous
reduction of the lamina. Here I examine the heteroblastic leaf series
for two taxa which represent independent derivations of the
leucobryoid morphology, Leucobryum sanctum and Arthrocormus shimperi ,
and those of their respective sister taxa,Campylopus and Syrrhopodon .
When placed in a phylogenetic framework, the heteroblasty of these
taxa suggests that different developmental modifications have been
involved in the origins of the leucobryoid leaf. In a comparative
context, heteroblasty provides evidence for transformational
homologies which may not be apparent at the level of the mature
plants.
Key words: , Calymperaceae, heteroblastic series, homology