To understand how genes control floral asymmetry, we have isolated and analyzed the role of the RADIALIS (RAD) gene in Antirrhinum. We show that the RAD gene encodes a small MYB-like protein that is specifically expressed in the dorsal region of developing flowers. RAD has a single MYB-like domain that is closely related to one of the two MYB-like domains of DIV, a protein that has an antagonistic effect to RAD on floral development. Interactions between RAD and other genes indicate that floral asymmetry depends on the interplay between two pairs of transcription factors. First, a pair of TCP proteins is expressed in dorsal regions of the floral meristem, leading to the activation of RAD in the dorsal domain. The RAD MYB-like protein then antagonizes the related DIV MYB-like protein, preventing DIV activity in dorsal regions. In addition to its role in dorsal regions, RAD acts nonautonomously on lateral regions either directly, through RAD protein movement, or indirectly, through a signaling molecule.F loral asymmetry is thought to have evolved many times independently as a specialized mechanism for pollinator interaction (1-3). In a few cases, most notably in Antirrhinum majus, the molecular genetic basis of floral asymmetry has begun to be understood. Four key genes have been shown to control dorsoventral asymmetry in Antirrhinum: CYCLOIDEA (CYC), DICHOTOMA (DICH), RADIALIS (RAD), and DIVARICATA (DIV) (4-8). Two of these genes, CYC and DICH, promote dorsal identity and encode proteins belonging to the TCP family of transcription factors. DIV promotes ventral identity and encodes a protein belonging to the MYB family of transcription factors, carrying two MYB-like domains. However, the mechanism by which CYC, DICH, and DIV interact remains unclear. To address this question, we have isolated and characterized RAD, the fourth member of this group of genes, and explored how it acts in combination with the other genes to establish floral asymmetry.Flowers of wild-type Antirrhinum are zygomorphic, having a single plane of symmetry (bilateral symmetry), in contrast to actinomorphic flowers, which have multiple planes of symmetry (radial symmetry) (3). The zygomorphy of Antirrhinum flowers reflects morphological distinctions between the upper (dorsal) and lower (lateral and ventral) organs of whorls two and three. In whorl two, each flower has two dorsal petals, two lateral petals, and one ventral petal, whereas whorl three comprises a single arrested dorsal stamen (staminode), two lateral stamens, and two ventral stamens (Fig. 1 a and b).The CYC and DICH genes are required for dorsoventral asymmetry in Antirrhinum and are expressed from an early stage in the dorsal domain of the floral meristem (5, 7, 9). At later stages, CYC expression persists throughout most of the dorsal domain, whereas DICH becomes restricted to the most dorsal half of the dorsal domain. Inactivation of both CYC and DICH results in peloric (radially symmetrical) flowers, in which all petals have ventral identity (Fig. 1h). In cyc or dich single ...

Vascular plants evolved in the Middle to Late Silurian period, about 420 million years ago. The fossil record indicates that these primitive plants had branched stems with sporangia but no leaves. Leaf-like lateral outgrowths subsequently evolved on at least two independent occasions. In extant plants, these events are represented by microphyllous leaves in lycophytes (clubmosses, spikemosses and quillworts) and megaphyllous leaves in euphyllophytes (ferns, gymnosperms and angiosperms). Our current understanding of how leaves develop is restricted to processes that operate during megaphyll formation. Because microphylls and megaphylls evolved independently, different mechanisms might be required for leaf formation. Here we show that this is not so. Gene expression data from a microphyllous lycophyte, phylogenetic analyses, and a cross-species complementation experiment all show that a common developmental mechanism can underpin both microphyll and megaphyll formation. We propose that this mechanism might have operated originally in the context of primitive plant apices to facilitate bifurcation. Recruitment of this pathway to form leaves occurred independently and in parallel in different plant lineages.

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