CYCLOIDEA-like genes in the evolution of floral asymmetry in Leguminosae
Bilaterally symmetric - or zygomorphic flowers - have evolved several times and are associated with pollinator specialisation. The different types of petals form in response to gradient of the gene CYCLOIDEA (CYC), which is strongly expressed at the top or dorsal side of the flower bud, and not expressed on the bottom or ventral side. Such a gradient is seen in both bilaterally symmetric and radialy symmetric flowers. The different types of flowers differ in how they respond to the gradient.
The ancestors of the peas are thought to have had radial flowers - their petals would not have changed shape in response to the CYC gradient. This changed when bilateral flowers evolved in this group. In today's pea family petals expressing high levels of CYC genes develop as standards, those expressing inermediate levels develop as wing petals and those that do not express CYC genes develop as keel petals.
The genus Cadia is an oddity in the pea family. It is a group of shrubs found in East Africa, Madagascar and Arabia which have radially symmetrical flowers, all five petals are identical. This appears to be a reversal to the ancestral state. Helene Citerne, a University of Edinburgh PhD student studying at RBGE cloned two CYC-like genes from Lupin and Cadia and showed that in Lupin, as in other flowers, they are expressed in a gradient across the developing flower. In Cadia, however, one of the CYC-like genes is expressed throughout the flower. In effect each of Cadia's five petals is a standard petal. The radial flowers of Cadia are not a reversion to an ancestral type but an evolutionary novelty. This illustrates how interpretations based on morphology can be mistaken and that evolution can take unexpected routes.
Citerne HL, Pennington RT, Cronk QC. (2006) An apparent reversal in floral symmetry in the legume Cadia is a homeotic transformation. Proc Natl Acad Sci U S A. 103:12017-20.
Ree RH, Citerne HL, Lavin M, Cronk QC. (2004) Heterogeneous selection on LEGCYC paralogs in relation to flower morphology and the phylogeny of Lupinus (Leguminosae). Mol Biol Evol. 21:321-31.
Citerne HL, Luo D, Pennington RT, Coen E, Cronk QC. (2003) A phylogenomic investigation of CYCLOIDEA-like TCP genes in the Leguminosae. Plant Physiol. 131:1042-53.
CYCLOIDEA-like genes in the evolution of floral diversity in Gesneriaceae
Most species in the Gesneriaceae possess zygomorphic flowers, probably in response to pollinator adaptations. We isolated Gcyc, the Gesneriaceae homologue of CYCLOIDEA (a gene involved in the expression of floral symmetry in Antirrhinum), from a range of taxa with zygomorphic and actinomorphic flowers. The results indicated that the gene is still active in those species that possess actinomorphic flowers, as no frame shifts or premature stop codons were detected. Thus, Gcyc appears to be down-regulated, possibly due to changes in promoter regions, or the gene is recruited for alternative processes. Analyses in a phylogenetic context revealed that the loss of asymmetry has arisen independently on several occasions across the Gesneriaceae.
Helene Citerne - MSc student 1998/9
Citerne, HL, Möller M, Cronk QCB. (2000). Diversity of cycloidea-like genes in Gesneriaceae in relation to floral symmetry. Annals of Botany 86(1): 167-176.
Möller, M., Clokie, M., Cubas, P. & Cronk, Q.C.B. (1999). Integrating molecular phylogenies and developmental genetics: a Gesneriaceae case study. In: Molecular systematics and plant evolution. Edited by P. M. Hollingsworth, R. M. Bateman and R. J. Gornall. 375-402. London: Taylor & Francis.