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First published online July 10, 2009; doi:10.3732/ajb.0900062 American Journal of Botany 96: 1419-1429 (2009) © 2009 Botanical Society of America, Inc.

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MADS-box gene expression and implications for developmental origins of the grass spikelet¹

² Research 223, Department of Biology, University of Missouri–St. Louis, One University Boulevard, Saint Louis, Missouri 63121 USA ³ Department of Biological Sciences, California State University–Long Beach, 1250 Bellflower Boulevard, Long Beach, California 90840 USA

ABSTRACT

Basic questions regarding the origin and evolution of grass (Poaceae) inflorescence morphology remain unresolved, including the developmental genetic basis for evolution of the highly derived outer spikelet organs. To evaluate homologies between the outer sterile organs of grass spikelets and inflorescence structures of nongrass monocot flowers, we describe expression patterns of APETALA1/FRUITFULL-like (AP1/FUL) and LEAFY HULL STERILE-like (LHS1) MADS-box genes in an early-diverging grass (Streptochaeta angustifolia) and a nongrass outgroup (Joinvillea ascendens). AP1/FUL-like genes are expressed only in floral organs of J. ascendens, supporting the hypothesis that they mark the floral boundary in nongrass monocots, and JaLHS1/OsMADS5 is expressed in the inner and outer tepals, stamen filaments and pistil. In S. angustifolia, SaFUL2 is expressed in all 11 (or 12) bracts of the primary inflorescence branch, but not in the suppressed floral bract below the abscission zone. In contrast, SaLHS1 is only expressed in bracts 6–11 (or 12). Together, these data are consistent with the hypotheses that (1) bracts 1–5 of S. angustifolia primary inflorescence branches and glumes of grass spikelets are homologous and that (2) the outer tepals of immediate grass relatives, bracts 6–8 of S. angustifolia, and the lemma/palea are homologous, although other explanations are possible.

Key Words: glumes • grass spikelet • Joinvillea ascendens • monocot flower • Poaceae • Streptochaeta angustifolia

Received for publication 23 February 2009. Accepted for publication 21 April 2009.

FOOTNOTES

¹ The authors thank D. Lorence (Pacific Tropical Botanic Garden) for material of Joinvillea ascendens. This research was supported by National Science Foundation grants DEB-0716788 (S.M.) and DBI-0110189 (E.K.)

⁴ Author for correspondence (e-mail: jcpxt8{at}ku.edu); present address: 8009 Haworth Hall, Department of Ecology and Evolutionary Biology, University of Kansas, 1200 Sunnyside Avenue, Lawrence, KS 66045, USA

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