title: The Origin of Floral Organ Identity Quartets.
abstract: The origin of flowers has puzzled plant biologists ever since Darwin referred to their sudden appearance in the fossil record as an abominable mystery. Flowers are considered to be an assembly of protective, attractive, and reproductive male and female leaf-like organs. Their origin cannot be understood by a morphological comparison to gymnosperms, their closest relatives, which develop separate male or female cones. Despite these morphological differences, gymnosperms and angiosperms possess a similar genetic toolbox consisting of phylogenetically related MADS domain proteins. Using ancestral MADS domain protein reconstruction, we trace the evolution of organ identity quartets along the stem lineage of crown angiosperms. We provide evidence that current floral quartets specifying male organ identity, which consist of four types of subunits, evolved from ancestral complexes of two types of subunits through gene duplication and integration of SEPALLATA proteins just before the origin of flowering plants. Our results suggest that protein interaction changes underlying this compositional shift were the result of a gradual and reversible evolutionary trajectory. Modeling shows that such compositional changes may have facilitated the evolution of the perfect, bisexual flower.
authors.affiliation: Department of Biology, KU Leuven, B-3001 Leuven, Belgium.
authors.forename: Philip
authors.identifier: 0000-0003-1524-1897
authors.initials: P
authors.lastname: Ruelens
authors.affiliation: Department of Biology, KU Leuven, B-3001 Leuven, Belgium.
authors.forename: Zhicheng
authors.identifier: 0000-0002-8185-0140
authors.initials: Z
authors.lastname: Zhang
authors.affiliation: Laboratory of Molecular Biology, Wageningen University, 6708PB Wageningen, The Netherlands.
authors.forename: Hilda
authors.initials: H
authors.lastname: van Mourik
authors.affiliation: Department of Plant Systems Biology, VIB, 9052 Gent, Belgium.|Department of Plant Biotechnology and Bioinformatics, Ghent University, 9052 Gent, Belgium.
authors.forename: Steven
authors.identifier: 0000-0002-5341-136X
authors.initials: S
authors.lastname: Maere
authors.affiliation: Laboratory of Molecular Biology, Wageningen University, 6708PB Wageningen, The Netherlands.
authors.forename: Kerstin
authors.initials: K
authors.lastname: Kaufmann
authors.affiliation: Department of Biology, KU Leuven, B-3001 Leuven, Belgium koen.geuten@bio.kuleuven.be.
authors.forename: Koen
authors.identifier: 0000-0002-9692-4499
authors.initials: K
authors.lastname: Geuten
mesh_terms: D005075:Biological Evolution; D004843:Epistasis, Genetic; D019143:Evolution, Molecular; D035264:Flowers / Q000235:genetics* / Q000254:growth & development; D020440:Gene Duplication; D018745:Genome, Plant; D026161:MADS Domain Proteins / Q000737:chemistry / Q000235:genetics*; D008957:Models, Genetic; D010940:Plant Proteins / Q000737:chemistry / Q000235:genetics*; D010944:Plants / Q000235:genetics; D020798:Two-Hybrid System Techniques
chemical_list: D026161:MADS Domain Proteins; D010940:Plant Proteins