Shifting Functional Requirements in the Evolution of Conifer Pollen Cone Morphology

Conifer pollen cones may be expected to exhibit less morphological diversity than ovulate organs because they perform a limited set of functions—such as protecting developing microsporangia and facilitating pollen dispersal—that have remained essentially unchanged during the evolution of the lineage. Qualitative evidence from the fossil record is consistent with this idea, and suggests that conifer pollen cones have had similar overall structures (an axis with helically or decussately arranged microsporophylls bearing microsporangia) for much of their history. However, multivariate analyses incorporating more detailed data from discrete characters of the microsporangium and microsporophyll, demonstrate that early conifers from the Upper Carboniferous and Permian periods are morphologically similar to some modern taxa (members of extant Araucariaceae) but occupy only a subset of the range of pollen cone morphologies exhibited by modern conifer species as a whole. 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 more efficient pollen dispersal. For example, the microsporophylls of early fossil conifer cones typically possess a long distal lamina, but this feature is lost in many modern taxa that produce small cones primarily encased in bud scales during growth that do not depend on imbricated distal laminae to protect developing pollen sacs. Additionally, similar microsporophyll morphologies lacking distal laminae 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.