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Phylogenetic comparative analysis of microsporogenesis in angiosperms with a focus on monocots¹

² Université Paris-Sud, Laboratoire Ecologie Systématique et Evolution, UMR8079 CNRS, AgroParisTech, 91405 Orsay cedex, France ³ Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey, TW9 3DS, UK ⁴ UMR Génétique Végétale, INRA, AgroParisTech, CNRS, Gif-sur-Yvette, 91190, France

ABSTRACT

This paper presents the first broad overview of three main features of microsporogenesis (male meiosis) in angiosperms: cytokinesis (cell division), intersporal wall formation, and tetrad form. A phylogenetic comparative approach was used to test for correlated evolution among these characters and to make hypotheses about evolutionary trends in microsporogenesis. The link between features of microsporogenesis and pollen aperture type was examined. We show that the pathway associated with successive cytokinesis (cytoplasm is partitioned after each meiotic division) is restricted to wall formation mediated by centrifugally developing cell plates, and tetragonal (or decussate, T-shaped, linear) tetrads. Conversely, much more flexibility is observed when cytokinesis is simultaneous (two meiotic divisions completed before cytoplasmic partitioning). We suggest that the ancestral type of microsporogenesis for angiosperms, and perhaps for all seed plants, associated simultaneous cytokinesis with centripetal wall formation, resulting in a large diversity in tetrad forms, ranging from regular tetrahedral to tetragonal tetrads, including rhomboidal tetrads. From this ancestral pathway, switches toward successive cytokinesis occurred among basal angiosperms and monocots, generally associated with a switch toward centrifugal intersporal wall formation, whereas eudicots evolved toward an almost exclusive production of regular tetrahedral tetrads. No straightforward link is found between the type of microsporogenesis and pollen aperture type.

Key Words: comparative analysis • correlated evolution • cytokinesis • intersporal wall formation • microsporogenesis • pollen aperture type • tetrad form

Received for publication 25 March 2008. Accepted for publication 10 August 2008.

FOOTNOTES

¹ The authors thank the various botanical gardens from which plant material was obtained: Jardin Botanique de la Ville de Paris (France), Conservatoire Botanique National de Brest (France), Parc Botanique de Launay (France), Royal Botanic Gardens Kew (UK), Kirstenbosch National Botanical Garden (South Africa), CBNB. The authors thank A. Forchioni for technical support and F. Jabbour for helpful discussions, the two reviewers for their helpful comments, and the IFR87 "La plante et son environnement" for financial support.

⁵ Author for correspondence (e-mail: sophie.nadot{at}u-psud.fr)