Fig. 1 Classical (A) and modified (B) ABC models. (A) In higher eudicotyledonous flowering plants, expression of an A-function gene specifies sepal formation in whorl 1. The combination of A- and B-function genes specifies the formation of petals in whorl 2, B- and C-function genes specify stamen formation in whorl 3, and expression of C-function genes alone determines the formation of carpels in whorl 4. (B) In contrast, many monocot plants, exemplified by the Liliaceae species, have two whorls of almost identical petaloid organs, called tepals. A modified ABC model was proposed by van Tunen et al. (1993) to explain the flower morphology of monocots such as tulip. In this model, class B genes are expressed in whorl 1 as well as in whorls 2 and 3; therefore, the organs of whorl 1 and whorl 2 have the same petaloid structure. Se; sepal, Pe; petal, St; stamen, Ca; carpel, Te; tepal. 5 The Differentiation of Perianth Morphologies in Monocotyledonous Plants Akira Kanno Graduate School of Life Sciences, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan Correspondence: kanno@ige.tohoku.ac.jp Keywords: ABC model, MADS-box ABSTRACT The morphological transition of the first whorl of flowers from petaloid tepals into sepals occurred frequently during the diversification of angiosperms. The Arabidopsis class B genes APETALA3 (AP3) and PISTILLATA (PI), and the Antirrhinum class B genes DEFICIENS (DEF) and GLOBOSA (GLO), are required for petal development in whorl 2, and their homologs have been isolated and characterized from various plants. A recent study of tulip flowers indicates that the morphology of petaloid tepals in whorls 1 and 2 is consistent with the expansion of class B gene expression. To clarify the evolutionary transition between sepals and petals in monocotyledonous plants, we isolated and characterized the class B genes from Agapanthus, Muscari and Tricyrtis, whose perianths consist of petaloid tepals in two whorls, and from Tradescantia, Commelina and Habenaria, whose perianths are differentiated into sepals and petals. Gene expression was examined by Northern hybridization and RT- PCR using dissected floral organs, or by in situ hybridization, and revealed the expansion of class B gene expression in whorl 1 in Agapanthus, Muscari and Tricyrtis, and a lack of DEF(AP3)-like gene expression in whorl 1 in Commelina, Tradescantia and Habenaria species. These results suggest that the DEF (AP3)-like gene expression pattern may be correlated with a morphological transition from petaloid tepals into sepals in the first whorl in monocot flowers. 1. INTRODUCTION Many studies have contributed to the great advances in our understanding of flower development. The key to this progress has been the availability of informative developmental mutants, primarily in the two model species Arabidopsis thaliana and Antirrhinum majus. Morphological and genetic analyses of floral homeotic mutants in these species have led to the development of several similar genetic models (Schwarz-Sommer et al. 1990, Coen and Meyerowitz 1991). The most inclusive of these models has been universally adopted and has become known as the ABC model of flower development (Coen and Meyerowitz 1991; Fig. 1A). This simple model predicts the existence of three genetic functions, A, B and C, which, alone or in combination, can specify the identity of the organs formed in each of the four whorls of the flower. Expression of the A function alone leads to the production of sepals, coexpression of the A+B or B+C functions leads to the formation of petals and stamens, respectively, and expression of the C function alone induces the formation of carpels (Coen and Meyerowitz 1991). A B ‘Classical ABC model’ ‘Modified ABC model’