KORALL, PETRA1* and WILSON A. TAYLOR2. 1Department of Botany, Stockholm University, SE-106 91 Stockholm, SWEDEN; 2Department of Biology, University of Wisconsin-Eau Claire, Eau Claire, WI, 54702-4004. - Integrating megaspore morphology and molecular phylogeny in Selaginella.
The lycophyte genus Selaginella has been the subject of several
taxonomic treatments in the past one hundred and fifty years. While
most have considered megaspore morphology to some extent, no one has
successfully reconciled this feature with a comprehensive phylogeny of
the group. Recent molecular phylogenies generated using rbcL,
ITS and 26S rDNA along with electron microscopy of the megaspores now
afford the opportunity to do so, at least in part. Granules of
sporopollenin on the innermost exospore surface are found in all
members of the monophyletic subgenus Tetragonostachys and its
sister taxon S. lepidophylla. Tetragonostachys includes
approx. fourty drought adapted species of worldwide distribution. So
far, this feature has been found in thirteen species, and no where
else within the genus. A highly ordered colloidal crystal-like
exospore structure is found only within an as yet unnamed, well
supported clade, which includes the monophyletic Articulatae.
All examined members of the Articulatae have this feature. The
species that possess this wall structure do not appear to form a
monophyletic group, but the number of reversals and/or parallelisms
involved is uncertain because of some weakly supported nodes. The
species of Selaginella now known to possess this unusual
structure throughout their exospore are: S. articulata, S. diffusa,
S. exaltata, S. galeottii, S. kraussiana, S. kunzeana, S. lingulata,
S. lyalii, S. marginata, S. myosurus, S. polymorpha, S. remotifolia,
S. sericea, S. silvestris, S. suavis, S. sulcata. S. pygmaea and
S. willdenovii show this pattern in places. A final emerging
correlation involves the possession of a coarse complete reticulum
with high muri by most members of this unnamed clade. The ability to
recognize megaspores with particular ultrastructural features of
phylogenetic significance without the use of the EM (i.e., by
correlation with surface features) would allow the incorporation of
dates from fossil material in the analysis.
Key words: megaspores, phylogeny, Selaginella