Leslie, Andrew .
Exploring the Role of Pollen Flotation in the Reproductive Biology of Ancient Gymnosperms.
Sacci, or inflated air bladders, are a prominent feature of the pollen grains of many extinct gymnosperm taxa and two families of living conifers. In modern plants, sacci have been argued to aid in the aerial dispersal of pollen grains and also in pollen capture through the flotation of buoyant saccate pollen grains into inverted or downwards-facing ovules. However, the effectiveness of sacci in both these processes has been questioned: spherical bladders do a poor job of increasing surface area for wind dispersal and results have been conflicting from experimental flotation studies. Here, evidence from laboratory experiments using model ovules and artificial pollination experiments in living pine species demonstrate that flotation is an effective pollination mechanism in conifers. In experimental ovules receiving both saccate and non-saccate pollen, only saccate pollen readily floats through the pollination drop secreted by the ovule. Saccate pollen is therefore concentrated inside ovules relative to nonbuoyant non-saccate grains, which are not effectively transported. Analyses of multivariate morphological data and traditional morphometric data from in situ pollen grains produced by extinct conifer families and several pteridosperm groups reveal that the sacci of many pteridosperms is morphologically and anatomically similar to that of modern conifers, although early conifer pollen is distinct until relatively late in conifer history. This suggests that a wide variety of Paleozoic and Mesozoic gymnosperms groups possessed buoyant pollen grains that could have functioned in a flotation-based pollination mechanism similar to that of modern conifers. Although this mechanism is dependent on ovule orientation and can therefore be difficult to assess in fossil plants, many features of pteridosperm and conifer ovulate structures are consistent with its operation. The potential prevalence of this mechanism provides an alternative explanation to air dispersal for the widespread occurrence of saccate pollen and the presence of inverted or downwards-facing ovules in gymnosperms.
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1 - University of Chicago, Department of Geophysical Sciences, 5734 S. Ellis Avenue, Chicago, Illinois, 60637, USA
Presentation Type: Oral Paper:Papers for Sections
Location: 555B/Convention Center
Date: Monday, August 2nd, 2010
Time: 9:15 AM