Evolutionary Developmental Biology (Evo-Devo)
Hofer, Katherine , Ruonala, Raili , Albert, Victor .
The double corolla phenotype present in two-thirds of species in the Hawaiian lobelioid genus Clermontia involves ectopic expression of PISTILLATA B-function MADS box gene homologs.
The Hawaiian endemic genus Clermontia includes 22 species, 15 of which - the double corolla species - are characterized by a whorl of organs that appear to be true petals occupying what is normally the sepal whorl. The presence of homeotic petaloid organs in some other plant groups has been correlated with ectopic expression of B-function MADS box genes, but similar core eudicot examples of apparent groundplan divergence remain unstudied. B-function genes, which are not normally expressed in the sepal whorl, are required for determination and maintenance of petal identity. Here, we investigate the potential role of altered B-function gene expression contributing to the morphological diversity of this island genus. Our micromorphological analysis of perianth organs in the double corolla species Clermontia parviflora, using scanning electron microscopy, revealed conical epidermal cells on the adaxial surfaces of first and second whorl petaloid organs, strongly suggesting a homeotic conversion in the former. Our phylogenetic data indicates a probable single and geologically recent origin of the double corolla trait within the genus, with numerous potential reversals to the standard sepal-petal format. Quantitative PCR (qPCR) analysis of homologs of the B-function genes PISTILLATA (PI), APETALA3 (AP3), and TOMATO MADS 6 (TM6) shows ectopic expression of two PI paralogs in the first whorl of C. parviflora; no such homeotic expression was observed for the other two genes, nor for several other MADS box genes. In the standard sepal-petal species C. arborescens, ectopic expression of PI homologs was not observed. The up- versus down regulation of PI homologs was precisely restricted to the perianth and stamen whorls, excluding a simple overexpression phenotype. In situ hybridization analysis of C. parviflora material similarly showed first and second whorl PI homolog expression in developing flower buds. Our data strongly indicate that a drastic and heritable phenotypic change, at the level of floral groundplan, can originate from a homeotic mutation that is likely regulatory, being under precise spatiotemporal control as opposed to having pleiotropic characteristics. The uniqueness of this trait among core eudicots could be linked to increased ecological viability in an unstable island environment, a chance event which need not have posed any immediate adaptive benefit. We argue that the evolutionarily young morphological radiation of Clermontia may form a model system for general understanding of mechanisms of larger-scale angiosperm diversification in past, similarly unstable environments, in which small regulatory changes may have been responsible for modern-day groundplan differences.
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1 - University at Buffalo, Biological Sciences, 109 Cooke Hall, Buffalo, NY, 14260, USA
MADS box gene.
Presentation Type: Oral Paper:Papers for Topics
Location: Delaware C/Hyatt
Date: Tuesday, July 10th, 2012
Time: 9:15 AM