OSBORN, JEFFREY M.1*, GAMAL EL-GHAZALY2, and RANESSA L. COOPER3. 1Division of Science, Truman State University, Kirksville, Missouri 63501-4221; 2Palynological Laboratory, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden; 3Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E9. - Development of the exineless pollen wall in Callitrichaceae and the evolution of underwater pollination.
The Callitrichaceae are a monogeneric family of aquatic angiosperms
comprising approximately 50 terrestrial, amphibious, and obligately
submersed species. Callitriche is unique in being the only
known genus with co-occurring aerial and underwater pollination
systems. Mature pollen structure is correlated with growth habit,
pollination biology, and phylogeny within the genus. In the present
study, development of exineless pollen in the obligately submersed
species Callitriche truncata was examined, with particular
emphasis on the tetrad stage. Pollen ontogeny occurred rapidly and
non-synchronously; tetrads, free microspores, and two-celled pollen
grains were identified within the same anthers. Formation of the
intine also occurred relatively early, during the tetrad stage.
Tetrads were surrounded by a structurally distinct envelope, and its
ultrastructure and histochemistry indicate that this callose-like
envelope is in a transitional state. Reduction or complete loss of the
exine has evolved at least twice in Callitrichaceae, and the new
ontogenetic data indicate that exine loss evolves more quickly than
the loss of callose. In addition, developmental information on
exineless pollen in C. truncata coupled with other
palynological data for the exine-bearing terrestrial and amphibious
growth forms provide support for the hypothesis that underwater
pollination has had a relatively recent origin in the family.
Key words: Aquatic plants, Callitrichaceae, development, evolution, pollen, pollination