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First published online November 10, 2009; doi:10.3732/ajb.0900079 American Journal of Botany 96: 2184-2196 (2009) © 2009 Botanical Society of America, Inc.

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Development of a complex floral trait: The pollinator-attracting petal spots of the beetle daisy, Gorteria diffusa (Asteraceae)¹

² Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EA, UK ³ Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey, TW9 3AB, UK ⁴ Botany and Zoology Department, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa ⁵ School of Biological and Conservation Sciences, University of Kwazulu-Natal, P. Bag X01 Scottsville, Pietermaritzburg 3209, South Africa ⁶ Imperial College London, Silwood Park Campus, Buckhurst Road, Ascot, Berks, SL5 7PY

ABSTRACT

Angiosperms possess a variety of complex floral traits that attract animal pollinators. Dark petal spots have evolved independently many times across the angiosperm phylogeny and have been shown to attract insect pollinators from several lineages. Here we present new data on the ontogeny and morphological complexity of the elaborate insect-mimicking petal spots of the South African daisy species, Gorteria diffusa (Asteraceae), commonly known as the beetle daisy, although it is fly-pollinated. Using light and scanning electron microscopy and histology, we identified three distinct specialized cell types of the petal epidermis that compose the petal spot. Sophisticated patterning of pigments, cuticular elaborations, and multicellular papillate trichomes make the G. diffusa petal spot a uniquely complex three-dimensional floral ornament. Examination of young inflorescence meristems revealed that G. diffusa ray florets develop (and probably also initiate) basipetally, in the opposite direction to the disc florets—a developmental phenomenon that has been found in some other daisies, but which contradicts conventional theories of daisy inflorescence architecture. Using these ontogenetic and morphological data, we have identified the mechanism by which G. diffusa patterns its insect-mimicking petal spots, and we propose a testable model for the genetic regulation of petal spot identity.

Key Words: Asteraceae • development • epidermal cell • floral evolution • Gorteria diffusa • micromorphology • petal spot • phyllotaxy • South Africa • trichome

Received for publication 10 March 2009. Accepted for publication 10 September 2009.

FOOTNOTES

¹ The authors thank M. Dorling, T. Eldridge, C. Prychid, and J. Westwood for technical help and M. Box, R. Bateman, P. Goldblatt, and J. Manning for insightful discussions. This work was funded by a BBSRC Doctoral Training Award (M.M.T.) and travel grants from the Linnean Society, Systematics Association, Bentham-Moxon Trust and Darwin College, Cambridge. Collecting permits were provided by the Northern Cape Nature Conservation Service and the Western Cape Nature Conservation Board to A.G.E.

⁷ Author for correspondence (e-mail: bjg26{at}cam.ac.uk)

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