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Homeotic gene expression and floral evolution in the early-divergent angiosperm family Annonaceae

amy930's picture

amy930 - Posted on 26 February 2013

Project Description: 

The Annonaceae is one of the most successful family within the order Magnoliales with large number of extant taxa and enormous number of species with diverse floral morphology. The Annonaceae is a member of the early divergent angiosperm which has a separate lineages with the eudicots. The floral morphologies that are shared by the basal angiosperms and the eudicots are of particular interest to us by giving evidences on how flowering plants evolve and the ecological significance of those characters throughout the evolutionary history of the angiosperms.
The purpose of this project is to look deeply into how floral organs evolve in the Annonaceae and the molecular mechanisms behind. Unlike most of its "relatives" with tepals, the Annonaceae has well differentiated rows of sepals and petals. Many of the Annonaceae species have two rows of three petals i.e. inner petal and outer petals. The two rows of petals might sometimes be morphologically very different from each other. Species of some particular genera possess inner or outer staminoids with ambiguous evolutionary origin.
The floral identify and development controlled by DNA and the molecular mechanisms can be briefly explained by the ABC models. The development of the floral parts basically depends on the expression of a group of MADS-box genes. The sepal is developed under the expression of the A class genes and at the same time it is affecting the development of the petals together with the expression of the B class genes. The combination of the B and C classes genes would influence on the development of the stamen and C class genes works on the carpel development. The ABC model is being very well studies in the model plant Arabidopsis. The expression and control of the floral development by the MADS-box genes varies in different lineages and the "fading model" is proposed to be the one that explains the floral development in the early divergent angiosperms.
In order to give a good coverage of data for the study of the genes related to the floral development and correlate with the evolutionary patterns of the floral organs, the project will be the fusion of transcriptomics studies, anatomical studies and phylogenetic studies. The application of the next generation sequencing techniques provides sufficiently large amount of data for the study of gene expression and regulation. Whole genome de novo assembling would be conducted using bioinformatic tools, i.e. Trinity, to identify all the MADS-box gene homologs, paralogs and splice variances. Detail anatomical studies using serial sectioning an differential staining also give strong evidence in the development and origin of each floral organs by the examination of the vascular bundle pattern. A minimum of 5 species form different lineages of the Annonaceae would be included in this study.

Research Project Details
Project Duration: 
09/2013 12/2017
Project Significance: 
The first objective of this study is to study the homeotic gene expression pattern among different Annonaceae genera and identification of putative homeotic gene homologues. The degrees of differentiation among inner and outer petals vary significantly among different Annonaceae genera. It is hypothesized that different classes of homeotic gene will have different expression patterns in genera with different perianth morphology. Different classes of homeotic gene will also be expressed in different stages of floral development. The degree of inner and outer petal differentiation can be classified as identical, slight variation and significant variation. Species with identical inner and outer petal is represented by a member of the genus Uvaria. Although flowers of Uvaria can be quite diverse in different geographical distribution, the inner and outer petal wholes of most Uvaria are relatively similar in morphology. Some of the Annonaceae genera, such as Anaxagorea, have inner and outer petals that differ slightly in size and shape: generally, the inner petals of Anaxagorea are slightly smaller than the outer petals. Goniothalamus is one of the genera with significant differences between inner and outer petals. The connivent inner petals of Goniothalamus are generally smaller than outer petals and clawed to form a cap over the stamens. Another focus would be on the homeotic gene disruption and floral organ identity of Dasymaschalon and Fitzalania species. Most Annonaceae genera have two whorls of three petals. However, some genera, such as Dasymaschalon, only possess a single whorl of petals which is commonly regarded as homologus with outer petals. Another species of Fitzalania possesses sepaloid outer petals in the second perianth whorl. According to the hypothesis of Saunders, some genera have undergone a shift of floral organ identity due to homeotic gene disruption. By studying the expression pattern of different classes of homeotic gene, the identity of the floral organs can be confirmed. Detailed anatomical study of the vascular bundle arrangement of Dasymaschalon will also provide strong evidence on its floral organ identity. The third major objective is focusing on the evolution of inner and outer staminodes in different lineages. This part of study will focus on the identity of inner and outer staminodes. Staminodes are sterile structures that fail to develop into stamen. Staminodes are generally classified into inner and outer staminodes according to their floral position. The development of staminodes is believed to be the evidence of the fading borders model homeotic gene expression pattern in basal angiosperms. It is hypothesized that a gene expression gradient has established across different floral organ. Anaxagorea is one of the two Annonaceae genera that process inner staminodes. The inner staminodes of Anaxagorea are intermediate between stamens and carpels although they are functionally different. There are more genera with outer staminodes in the Annonaceae, including some Asiatic species of Uvaria. The outer staminodes are morphologically intermediate between petals and fertile stamens. It is hypothesized that there were independent evolutionary origins of inner and outer staminodes in different lineages. By comparing the homeotic gene expression pattern between petals, outer staminodes, stamens, inner staminodes and carpels, the identity of varies floral organ can be confirmed. The study is based on a 4-year PhD project, co-supervised by Prof. R.M.K. Saunders and Dr. B.L. Lim (School of Biological Science, HKU) under the Plant Evolution and Adaptation Strategic Research Area. This research will also generate at least one journal article in an ISI-ranked journal prior to larger-scale funding by the RGC. Possible target journals include those specialising in molecular phylogenetics and/or evolutionary biology, such as Molecular Biology and Evolution (2011 impact factor, IF = 5.6), Evolution (IF = 5.1), Molecular Phylogenetics and Evolution (IF = 3.6) and BMC Evolutionary Biology (IF = 3.5), or alternatively more generalist botanical journals such as Annals of Botany (IF = 4.0) and American Journal of Botany (IF = 2.7).
Remarks: 
As large amount of data would be used obtain from the whole genome transcriptome sequencing, bioinformatic tools for De novo RNA assembling including Trinity, SOAPdenovo, TopHat, Bowtie will be used. Some phylogenetic analysis tools, such as MrBayes will also be used for later part of the study.