Evolution of C‐class MADS‐box genes across angiosperms
MADS-box genes make up a large family of transcription factors that are prevalent in plants and involved in diverse physiological and developmental processes, including flower development. MADS-box genes are critical to the ABC model of floral development, which describes the combinatorial interaction of A, B, and C-class genes and their roles in the initiation and development of floral whorls. Genetic studies have indicated that C-class genes are important to the development of stamens and carpels. Carpels are a synapomorphy of angiosperms, and represent a conserved morphological structure across this large and diverse plant lineage. Functional and expression studies of C-class genes in model angiosperms have suggested that the role of C-class genes in specifying stamen and carpel development is conserved across angiosperms. However, like other clades of MADS-box genes, C-class genes have experienced a complex gene duplication history during angiosperm evolution. According to theoretical models of duplicate gene evolution, duplicate genes provide an opportunity for genes to evolve with less selective constraint, potentially leading to novel gene functions. My research aims to expand the understanding of the complex duplication history of C-class genes, and characterize how C-class genes have diversified during angiosperm evolution. During the course of my research, I have isolated C-class gene sequences from diverse non-model angiosperms and used them in a phylogenetic analysis, expression, and characterization study. In my phylogenetic study, I incorporated the C-class sequences obtained from non-model species into a large data set of all known C- and D-class genes in order to better resolve previously reported duplications of C-class genes. Second, I used semi-quantitative RT-PCR to study presence or absence of C-class gene expression in leaves and floral parts of angiosperm species from several diverse lineages. The results of this study showed that in four distantly related angiosperm lineages, C-class genes maintained expression patterns similar to those characterized in stamens and carpels of higher eudicots, but also show evidence of expressional diversification across floral tissues. Third, I performed transgenic experiments using Arabidopsis as a heterologous system. In this study, duplicate C-class genes from the higher eudicot Glycine max, the grass Pharus viridens, and a single C-class gene from the monocot Joinvillea ascendens were constitutively overexpressed in Arabidopsis using the Cauliflower Mosaic Virus (CaMV) 35S promoter. This resulted in both previously documented and novel ectopic stamen and carpel, as well as petal and ovule phenotypes, and suggest that divergence in the coding sequences of C-class genes may be linked to subtle functional changes in these genes in both pre- and post-duplication angiosperm taxa. Interested in the larger context of the evolution and radiation of the MADS-box gene family, I also isolated three novel MADS-box gene sequences from the early tracheophyte Equisetum arvense, and included them in a phylogenetic analysis of MADS-box genes. In summary, my dissertation research has focused upon the impact of gene duplications on C-class MADS-box gene conservation and diversification in angiosperms, and initiated an investigation of MADS-box gene diversification during early land plant evolution.
Zanis, Purdue University.
Plant biology|Evolution and Development
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