Coral Vincent

An epigenetic mutation responsible for natural variation in floral symmetry

Although there have been many molecular studies of morphological mutants generated in the laboratory, it is unclear how these are related to mutants in natural populations, where the constraints of natural selection and breeding structure are quite different. Here we characterize a naturally occurring mutant of Linaria vulgaris, originally described more than 250 years ago by Linnaeus, in which the fundamental symmetry of the flower is changed from bilateral to radial. We show that the mutant carries a defect in Lcyc, a homologue of the cycloidea gene which controls dorsoventral asymmetry in Antirrhinum. The Lcyc gene is extensively methylated and transcriptionally silent in the mutant. This modification is heritable and co-segregates with the mutant phenotype. Occasionally the mutant reverts phenotypically during somatic development, correlating with demethylation of Lcyc and restoration of gene expression. It is surprising that the first natural morphological mutant to be characterized should trace to methylation, given the rarity of this mutational mechanism in the laboratory. This indicates that epigenetic mutations may play a more significant role in evolution than has hitherto been suspected.

Control of Organ Asymmetry in Flowers of Antirrhinum

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.

Control of flower development and phyllotaxy by meristem identity genes in antirrhinum.

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.

Cell lineage patterns and homeotic gene activity during Antirrhinum flower development

BACKGROUND Homeotic genes controlling the identity of flower organs have been characterized in several plant species. To determine whether cells expressing these genes are specified to follow particular developmental fates, we have studied the pattern of cell lineages in developing flowers of Antirrhinum. Each flower has four whorls of organs, and progenitor cells of these can be marked at particular stages of development using a temperature-sensitive transposon. This allows the cell lineages in the flower to be followed, as well as giving information about rates of cell division. RESULTS We show here that, prior to the emergence of organ primordia, cells in the floral meristem have not been allocated organ identities. After this time, lineage restrictions arise between whorls, correlating with the onset of expression of genes that control organ identity. A further lineage restriction appears slightly later on, between the dorsal and ventral surfaces of the petal. Our results further suggest that the rates of cell division fluctuate during key stages of meristern development, perhaps as a consequence of meristem-identity gene expression. CONCLUSIONS The patterns of lineage restriction and organ-identity gene expression in early floral meristems are consistent with some cells being allocated specific identities at about this stage of development. Plant cells cannot move relative to each other, so lineage restrictions in plants may reflect particular orientations and/or rates of growth at boundary regions.