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Abstract Detail



Anatomy and Morphology

Williams, Joseph H. [1].

Ancient angiosperm pollen biology: conflicts between pollinator rewards, dispersal biology and germination timing?

Angiosperms have markedly different pollen ecology than other seed plants, but little is known about the functional biology of pollen of extant ancient angiosperm lineages. Such knowledge might help us to reconstruct early features of angiosperm reproduction not available from fossils. Early-divergent angiosperm species, within Amborellales, Nymphaeales and Austrobaileyales (ANA grade), and Chloranthales/Eumagnoliids, have stigmas that support larger pollen loads with faster pollen germination times than in almost all extant gymnosperms. Germination speed depends on the rate of hydration on the stigma and the speed at which the pollen tube synthesis apparatus can be mobilized, which are afected by the water content and type of energy reserves of mature pollen. Here we report on the hydration status and the energy storage mechanisms of mature pollen from 21 species of early-divergent angiosperms from field collections and the greenhouse. For hydration status, volumes of oven-dried (D), fresh (F) and fully hydrated (H) pollen were compared to construct a hydration index: HI = (F-D)/(H-D), where higher values indicate higher water content. Various stains were used on fresh and glycol-methacrylate embedded pollen to detect starches, dissolved polysaccharides, DNA and lipids in the cytoplasm of immature and mature pollen and lipids in the exine. All species had hydration indices of < 30%, except for Brasenia schreberi (66%), Hedyosmum brasiliense (64%) and Illicium parviflorum (44%). Mature pollen of Amborella trichopoda, some Nymphaeales, all Austrobaileyales and a few magnoliids/chloranthoids did not stain for starch but were positive for dissolved cytoplasmic polysaccharides. Storage oils were present in the pollen cytoplasm of A. trichopoda, Trithuria bibracteata, most Nymphaeales, and several magnoliids/chloranthoids. Most pollen had lipids in the exine and was sticky, including partially (A. trichopoda) or fully (B. schreberi and H. brasiliensis) wind-pollinated species. Pollen dispersal states in these deeply-divergent lineages were diverse, but almost all show signs of insect-pollination, such as pollen clumping, exine-embedded lipids, and oils as a common storage reserve (a rich reward for pollen-eating insects). Pollen is a known reward in all insect-pollinated ANA grade angiosperms. Pollen with high inferred water content is metabolically active and short-lived, and was only found in two wind-pollinated species, where successful dispersal is rapid. These facts suggest wind-pollination is multiply derived within ancient angiosperm lineages. We conclude that early angiosperms dispersed somewhat dormant, dehydrated, bicellular pollen with a lifespan matching the longer presentation and dispersal schedules of insect-pollinated flowers.


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1 - University of Tennessee, Ecology and Evolutionary Biology, Knoxville, TN, 37996, USA

Keywords:
Amborella
Dormancy
Early-divergent angiosperm
Life history
Pollen development
dispersal
Trithuria
Wind-pollination.

Presentation Type: Oral Paper
Number: 20012
Abstract ID:306
Candidate for Awards:None


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