Developmental and Structural Section
Leslie, Andrew .
Ontogeny and Shifting Functional Requirements in the Evolution of Conifer Pollen Cone Morphology.
Conifer pollen cones may be expected to show less morphological diversity than other reproductive organs because they perform a limited set of functions that have remained essentially unchanged over the evolution of the lineage, including protecting developing microsporangia and facilitating pollen dispersal. Qualitative evidence from the fossil record is consistent with this idea, and suggests that conifer pollen cones have had similar overall structures—consisting of an axis with helically arranged microsporophylls bearing microsporangia—for much of their history. However, multivariate analyses incorporating more detailed data from discrete characters of the microsporangium and microsporophyll, show that most early conifers from the Upper Carboniferous and Permian periods are morphologically similar to some modern taxa (Araucariaceae) but occupy only a subset of the range of pollen cone morphologies exhibited by modern conifer species. The expansion in morphospace occupation through time, and the increased morphological diversity in modern taxa, may be related more to the evolution of different strategies for pollen cone growth―and the need to protect developing cones―than to novel innovations in pollen cone morphology or pollen dispersal. For example, the microsporophylls of early fossil conifer cones typically possess a long distal lamina, but this feature is often lost in those modern taxa that produce small cones primarily encased in bud scales during growth which therefore do not depend on imbricated distal laminae to protect developing pollen sacs. Additionally, similar microsporophyll morphologies have independently evolved in living members of Pinaceae and Podocarpaceae with similar pollen cone growth patterns, further suggesting this may be an important driver in pollen cone morphological evolution. The results of this study may be applicable to evolutionary patterns in other groups as well, because pollen cones in many ancient and modern gymnosperms perform the same basic functions as those of conifers.
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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
Date: Tuesday, July 29th, 2008
Time: 8:15 AM