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First published online October 23, 2009; doi:10.3732/ajb.0800431 American Journal of Botany 96: 1957-1966 (2009) © 2009 Botanical Society of America, Inc.

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Reproductive short-shoots of Ginkgo biloba: A quantitative analysis of the disposition of axillary structures¹

Department of Plant Sciences, One Shields Avenue, University of California, Davis, California 95616 USA

ABSTRACT

Ginkgo biloba, the only living representative in an otherwise extinct clade, is of pivotal importance to understanding seed plant phylogeny. Although G. biloba and its fossil relatives have been studied for over two centuries, there are both gaps and contradictions in the information available. We present data documenting the distributions of strobili and consider what an understanding of the disposition of strobili along short-shoots in Ginkgo adds to knowledge of the evolution of reproductive structures in seed plants in general. The megasporangiate strobili are found at and around the boundary between bracts and foliage leaves, while the expanse of microsporangiate strobili centers on the fifth bract back from that boundary. Quantitative analysis of the locations of the strobili along the short-shoot finds that increases in numbers of strobili are the result of recruitment of adjacent axils into morphogenetic activity. Gaps in the series of strobili are exceedingly rare. Further, while increased numbers of megasporangiate strobili arise from the symmetrical addition of axils into the fertile zone, increased numbers of microsporangiate strobili arise from a distinctly asymmetrical, basipetally biased, addition of axillary positions. This accurate morphological framework should orient molecular genetic studies that probe gymnosperm development itself or that consider gymnosperms as the proximate sources of gene expression redeployed in the origin of the angiosperm flower.

Key Words: evo-devo • Ginkgoaceae • gymnosperm • heteroblasty • leafy • megasporangiate • microsporangiate • needly • origin of the flower

Received for publication 22 December 2008. Accepted for publication 11 August 2009.

FOOTNOTES

¹ The authors thank Dr. Stefan Little, the proximal motivation of our work on Ginkgo, for securing short-shoots from the UC Berkeley Ginkgo. We thank Dr. W. E. Howard and Mrs. E. M. Gifford (Davis CA) for Ginkgo branches, Dr. Karl Niklas for comments on the manuscript and interest in this project, and an anonymous reviewer for the reference to W. K. H. Karstens’ work on Ginkgo. We acknowledge support from the Grady L. Webster Memorial Research Fund.

² Author for correspondence (e-mail: mxianson{at}pacbell.net)

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