Evolution of unisexual flowers in grasses (Poaceae) and the putative sex-determination gene, TASSELSEED2 (TS2)

doi:10.1111/j.1469-8137.2006.01726.x

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Wiley InterScience

New Phytologist

Volume 170 Issue 4, Pages 885 - 899

Published Online: 25 Apr 2006

Published on behalf of the New Phytologist Trust

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Evolution of unisexual flowers in grasses (Poaceae) and the putative sex-determination gene, TASSELSEED2 (TS2)

Simon T. Malcomber and Elizabeth A. Kellogg

Research 223, Department of Biology, University of Missouri – St Louis, One University Boulevard, Saint Louis, MO 63121, USA

Author for correspondence: Elizabeth A. Kellogg Tel: +1 314 516 6217 Fax: +1 314 516 6233 Email: tkellogg@umsl.edu

KEYWORDS

evolution • gibberellin • grass • tasselseed • unisexual flower • Zea

New Phytologist (2006) 170: 885–899

ABSTRACT

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Unisexuality has evolved repeatedly in flowering plants, but its genetic control is not understood in most cases. In maize (Zea mays), unisexual flower development is regulated by a short-chain dehydrogenase/reductase protein, TASSELSEED2 (TS2), but its role in other grass lineages is unknown.
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TS2 was cloned and sequenced from a broad range of grasses and compared to available sequences from other flowering plants using phylogenetic analysis and tests for selection. Gene expression was investigated using reverse transcriptase–polymerase chain reaction (RT-PCR) and in situ hybridization.
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TS2 orthologs appear to be restricted to monocots. The TS2 protein sequence was found to be generally under purifying selection in bisexual and unisexual lineages alike. Only one site, in unisexual herbaceous bamboos, is potentially under positive selection. TS2 was expressed broadly in all sampled tissues of unisexual and bisexual grasses, and was also expressed in rice flowers in floral organs that do not abort.
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TS2 may have a more general developmental role in most grasses than programmed cell death of the developing gynoecium, but has been co-opted to this role within a subset of Poaceae, probably as a result of alterations in the activity or regulation of other genes in the gynoecial pathway.