Dorsoventral asymmetry in flowers is thought to have evolved many times from a radially symmetrical ancestral condition. The first gene controlling floral asymmetry, cycloidea in Antirrhinum, has been isolated. The cycloidea gene is expressed at a very early stage in dorsal regions of floral meristems, where it affects growth rate and primordium initiation. Expression continues through to later stages in dorsal primordia to affect the asymmetry, size and cell types of petals and stamens.

Flowering plants exhibit two types of inflorescence architecture: determinate and indeterminate. The centroradialis mutation causes the normally indeterminate inflorescence of Antirrhinum to terminate in a flower. We show that centroradialis is expressed in the inflorescence apex a few days after floral induction, and interacts with the floral-meristem-identity gene floricaula to regulate flower position and morphology. The protein CEN is similar to animal proteins that associate with lipids and GTP-binding proteins. We propose a model for how different inflorescence structures may arise through the action and evolution of centroradialis.

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 ...

Understanding evolutionary change requires phenotypic differences between organisms to be placed in a genetic context. However, there are few cases where it has been possible to define an appropriate genotypic space for a range of species. Here we address this problem by defining a genetically controlled space that captures variation in shape and size between closely related species of Antirrhinum. The axes of the space are based on an allometric model of leaves from an F 2 of an interspecific cross between Antirrhinum majus and Antirrhinum charidemi. Three principal components were found to capture most of the genetic variation in shape and size, allowing a three-dimensional allometric space to be defined. The contribution of individual genetic loci was determined from QTL analysis, allowing each locus to be represented as a vector in the allometric space. Leaf shapes and sizes of 18 different Antirrhinum taxa, encompassing a broad range of leaf morphologies, could be accurately represented as clouds within the space. Most taxa overlapped with, or were near to, at least one other species in the space, so that together they defined a largely interconnected domain of viable forms. It is likely that the pattern of evolution within this domain reflects a combination of directional selection and evolutionary tradeoffs within a high dimensional space.leaf ͉ morphometry ͉ QTL analysis ͉ shape variation ͉ species

Organ asymmetry is thought to have evolved many times independently in plants. In Antirrhinum, asymmetry of the flower and its component organs requires cyc and dich gene activity. We show that, like cyc, the dich gene encodes a product belonging to the TCP family of DNA-binding proteins that is first expressed in the dorsal domain of early floral meristems. However, whereas cyc continues to be expressed throughout dorsal regions, expression of dich eventually becomes restricted to the most dorsal half of each dorsal petal. This correlates with the effects of dich mutations and ectopic cyc expression on petal shape, providing an indication that plant organ asymmetry can reflect subdomains of gene activity. Taken together, the results indicate that plant organ asymmetry can arise through a series of steps during which early asymmetry in the developing meristem is progressively built upon.

SignificancePopulations often show “islands of divergence” in the genome. Analysis of divergence between subspecies of Antirrhinum that differ in flower color patterns shows that sharp peaks in relative divergence occur at two causal loci. The island is shaped by a combination of gene flow and multiple selective sweeps, showing how divergence and barriers between populations can arise and be maintained.

Snapdragon (Antirrhinum majus L.), a member of the Plantaginaceae family, is an important model for plant genetics and molecular studies on plant growth and development, transposon biology and self-incompatibility. Here we report a near-complete genome assembly of A. majus cultivar JI7 (A. majus cv.JI7) comprising 510 Megabases (Mb) of genomic sequence and containing 37,714 annotated protein-coding genes. Scaffolds covering 97.12% of the assembled genome were anchored on eight chromosomes. Comparative and evolutionary analyses revealed that a whole-genome duplication event occurred in the Plantaginaceae around 46–49 million years ago (Ma). We also uncovered the genetic architectures associated with complex traits such as flower asymmetry and self-incompatibility, identifying a unique duplication of TCP family genes dated to around 46–49 Ma and reconstructing a near-complete ψS-locus of roughly 2 Mb. The genome sequence obtained in this study not only provides a representative genome sequenced from the Plantaginaceae but also brings the popular plant model system of Antirrhinum into the genomic age.

The flower meristem identity genes floricaula (flo) and squamosa (squa) promote a change in phyllotaxy from spiral to whorled in Antirrhinum. To determine how this might be achieved, we have performed a combination of morphological, genetic, and expression analyses. Comparison of the phenotypes and RNA expression patterns of single and double mutants with the wild type showed that flo and squa act together to promote flower development but that flo is epistatic to squa with respect to early effects on phyllotaxy. We propose that a common process underlies the phyllotaxy of wildtype, flo, and squa meristem development but that the relative timing of primordium initiation or growth is altered. This process depends on two separable events: setting aside zones for potential primordium initiation and partitioning these zones into discrete primordia. Failure of the second event can lead to the formation of continuous double spirals, which are occasionally seen in flo mutants.