The Plant Journal (2005) 41, 755–766 doi: 10.1111/j.1365-313X.2005.02336.x Floral MADS box genes and homeotic gender dimorphism in Thalictrum dioicum (Ranunculaceae) – a new model for the study of dioecy Vero´ nica S. Di Stilio1,*,†, Elena M. Kramer1 and David A. Baum1,2 1Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA, and 2Department of Botany, University of Wisconsin, Madison, WI 53706, USA Received 23 September 2004; revised 8 December 2004; accepted 14 December 2004. *For correspondence (fax þ206 616 2011; e-mail
[email protected]). †Present address: Department of Biology, University of Washington, Seattle, WA 98195, USA. Summary In most dioecious angiosperm species, flowers are initially perfect but abort either stamens or carpels during their development, indicating that sex determination occurs after floral organ identity has been established. Dioecious members of the genus Thalictrum (meadow-rue), however, produce flowers that lack aborted organs. Examination of early flower development of T. dioicum confirms that flowers are male or female from inception, raising the possibility that genetic mechanisms working at or above the level of organ identity promote sex determination through a homeotic-like mechanism. In order to investigate this possibility, we identified homologs of the organ identity genes PISTILLATA (PI), APETALA3 (AP3) and AGAMOUS (AG) from T. dioicum and the hermaphroditic species T. thalictroides. A combination of early and late duplication events was uncovered in these gene lineages and expression analyses indicate that these events are generally associated with divergence in gene regulation. In light of these findings, we discuss the potential of T.