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Comparative pollen morphology and ultrastructure of the Callitrichaceae
R L Cooper1, J M Osborn, C T Philbrick
1Division of Science, Truman State University, Kirksville, Missouri 63501 USA; and.
American Journal of Botany
|February 17, 2000
Summary
Pollen exine reduction in Callitriche species correlates with aquatic adaptations and unique pollination strategies. This study reveals evolutionary loss of pollen exine in some species, linked to chromosome number changes.
Area of Science:
- Plant reproductive biology
- Palynology
- Evolutionary botany
Background:
- The Callitrichaceae family, genus Callitriche, is unique for its aquatic species and dual pollination syndromes (aerial and hydrophilous).
- Understanding pollen morphology variations is crucial for correlating with diverse growth habits and pollination strategies within this genus.
Purpose of the Study:
- To describe pollen morphology and ultrastructure for 13 Callitriche species representing terrestrial, amphibious, and submersed growth forms.
- To correlate palynological data with pollination biologies, growth habits, and phylogenetic interpretations of Callitriche.
Main Methods:
- Pollen samples from 13 Callitriche species were analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
- Species included terrestrial, amphibious (with and without internal geitonogamy), and obligately submersed growth forms.
Main Results:
- All Callitriche pollen examined was spheroidal, small, intectate, and lacked distinct apertures, with variations in exine thickness and surface ornamentation.
- Terrestrial species and C. marginata exhibited well-developed, thick exines. Amphibious species generally had thinner exines, with thicker basal layers in those exhibiting internal geitonogamy.
- Significant exine reduction or complete loss was observed in some amphibious species (C. hamulata, C. brutia, C. lusitanica) and the obligately submersed species (C. truncata).
Conclusions:
- Pollen exine reduction or loss has evolved at least twice within Callitriche, associated with adaptation to aquatic environments and pollination syndromes.
- These palynological findings support phylogenetic interpretations and highlight the evolutionary plasticity of pollen structure in response to ecological pressures.