Tomlinson, P. , Huggett, Brett A. .
Is saccate pollen ancestral in angiosperms? Either exaptation or adaptation?
Saccate pollen is widely distributed but also homoplasious in seed plants, including extant conifers, where it occurs in early divergent clades. Its function, in association with an inverted pollination drop on the ovule, provides a simple mechanism of pollen selection and rejection. This is seen most easily and convincingly in Podocarpaceae. Sacci are formed as an inflation of the exine to provide a porous alveolate texture. This reduces pollen density so that it floats upwards in water. In podocarps ovule inversion is occasioned by an epimatium, homologous with the ovuliferous cone scale of other conifers. An analogue is the anatropous ovule in angiosperms, where it is believed to be ancestral. This requires a second integument which functions to invert the ovule, but whose origin is unknown. An angiosperm ancestor could well have had saccate pollen, but modified in modern groups, as the tectate-columellate exine, also porous, in which the tapetal secretion, which the exine is now capable of retaining, has become the source of the cell recognition system in an interaction with a stigma, which is regarded as the hallmark of angiosperm success and dominance. A known gymnosperm model is the widely distributed Mesozoic pollen form Classopolis. This demonstrates , but only by analogy, a highly successful exaptation.
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1 - 54 Floret Circle, Hingham, MA, 02043, USA
2 - Harvard University, Organismic and Evolutionary Biology, 16 Divinity Ave, Biolabs 3107, Cambridge, MA, 02138, United States
Presentation Type: Poster:Posters for Sections
Location: Battelle South/Convention Center
Date: Monday, July 9th, 2012
Time: 5:30 PM