Genetics of Floral Development An Evo-Devo Approach

Transcription

1 Genetics of Floral Development An Evo-Devo Approach Biology 317 Kelsey Galimba Why are flowering plants so diverse?

2 Why are flowering plants so diverse? Evolution of Development (Evo-Devo) The Logic! Morphology is the product of development! Development results from genetic regulatory programs.! Evolution of diversity is directly related to the evolution of genetic regulatory programs.! A limited number of genetic pathways have been used and re-used to build different body plans.

3 Using the Evo-Devo Approach to study floral diversity Developmental processes that occur in the flowers different taxa. Infer evolutionary events that led to the differences or similarities. Zahn et al. (2005) Floral morphology can be described with different characteristics Phyllotaxy Symmetry Organ Fusion Whorl Number Organ Elaboration Organ Identity

4 Floral Symmetry Bilateral Symmetry has evolved many times Carl Linneaus Cubas 2004 Linaria vulgaris Linaria vulgaris peloric mutant L. vulgaris peloric mutations caused by methylation of CYCLOIDEA

5 Floral Symmetry controlled by CYCLOIDEA(CYC) and DICHOTOMA(DICH) Antirrhinum majus WT Peloric CYC expression in developing flowers Luo et al Mohavea confertiflora - Changes in symmetry with an evolutionary effect Cubas 2004 Hileman et al. 2003

6 M. confertiflora mimics a radial species. Mohavea confertiflora Mentzelia involucrata CYC affects symmetry in Helianthus Cubas 2004

7 Changes in CYC expression cause Van Gogh's sunflowers Chapman et al Summary Bilateral symmetry has evolved many time independently. CYC homologs appear to have been recruited as symmetry genes in all cases of bilateral symmetry within the core eudicots.

8 Floral Organ Identity Sepal 1 st Whorl Petal 2 nd Whorl Stamen 3 rd Whorl Carpel 4 th Whorl Different phases of shoot growth are regulated by environmental and endogenous signals Developmental Biology, Seventh Edition, Figure 20.38

9 Floral Organ Identity genes are MADS Box genes = Transcription Factors HOX Genes MADS-Box genes in flowers are important for correct organ identity. HOX genes in animals are important for correct body-plan formation. MIKC-type MADS-Box genes MADS I K C Highly conserved 60 amino acid domain involved in DNA binding and protein dimerization. Intervening domain, plays role in dimerization specificity. Alpha-helical K-box, involved in protein dimerization. Generally divergent domain with small highly conserved motifs, recently implicated as mediator of ternary protein complex formation.

10 MIKC-type MADS-Box genes are involved in many aspects of plant development Smaczniak et al. (2012) ABC Model of Flower Development Arabidopsis thaliana Antirrhinum majus! 'A' genes control the sepals 'A' and 'B' genes in combination control the petals 'B' and 'C' genes in combination control the stamens 'C' genes control the carpels

11 Homeotic Mutants used to study HOX genes in Drosophila WT HOX Genes antennapedia Carroll et al. (2005) Floral homeotic mutants How the ABCE Model was determined Krizek & Fletcher Nature Reviews Genetics (2005). 6(9),

12 Sliding Boundary model is one modification to the ABC Model Kramer et al. (2003) Explaining diversity with the ABC model Transference of genetic identity from one structure to another B A C SE PE ST CA B A C PE PE ST CA

13 Thalictrum thalictroides is a good model system for studying Floral Evo-Devo Pivotal phylogenetic position - Ranunculales High success with gene silencing techniques (VIGS) Many horticultural mutants available Numerous duplications of Floral Organ Identity Genes Soltis et al. Am J Bot 2009 T. thalictroides Horticultural Mutants Wildtype Shoaff s Double Betty Blake Green Dragon Double White

14 C-class and D-class Function in Thalictrum Floral Organ Identity Genes control development along the floral axis Sepal!!1 st!whorl! Petal!!2 nd!whorl! Stamen!!3 rd!whorl! Carpel!!4 th!whorl! ABC(D)E Model B! A! C! E! D! Ferrario et al. Curr Opin Plant Biol 2004

15 C-class function: conserved carpel and stamen identity across the angiosperms C-lineage D-lineage Arabidopsis C-class gene: AGAMOUS (AG) Kramer et al AGAMOUS Subfamily Kramer et al. Genetics 2004 Bowman et al. Plant Cell 1989 Riechmann & Meyerowitz. Molec Biol Cell 1997 D-class function: ovule identity is mostly redundant among C- and D-lineage genes C-lineage D-lineage Petunia D-class genes: FBP7 & FBP11 AGAMOUS Subfamily Kramer et al. Genetics 2004 Angenent et al. Plant Cell 1995

16 Thalictrum has two AG homologs with different expression patterns. C-class Genes ThtAGAMOUS1 (ThtAG1) ThtAGAMOUS2 (ThtAG2) No D-class Genes ThdAG1 carpels stamens ThdAG2 ovule carpel Di Stilio et al Kramer et al Thalictrum has two AG homologs with different expression patterns. Hypothesis After the duplication event that led to ThAG1 and ThAG2: ThAG1 kept the C-function role in stamen and carpel identity. ThAG2 sub-functionalized to become an ovule identity gene. Kramer et al. 2004

17 Forward Genetics Double White has a defect in ThAG1 T. thalictroides WT T. thalictroides Double White Reverse Genetics Down-regulating ThAG1 creates a double flower phenotype T. thalictroides WT Viral Induced Gene Silencing (VIGS) T. thalictroides Double White T. thalictroides TRV2ThAG1

18 Reverse Genetics Down-regulating ThAG1 creates a double flower phenotype T. thalictroides WT Conclusion: ThtAG1 does have the conserved C-function role in stamen and carpel identity. T. thalictroides Double White T. thalictroides TRV2ThAG1 Double Flowers the first recorded floral mutants T. thalictroides WT T. thalictroides Double White Theophrastus BC Arabidopsis WT Arabidopsis agamous mutant

19 VIGS of ThtAG2 does not result in a double-flower TRV1 LB! 2!x!35S! RdRp! MP! 16K! Rz! NOS t! RB! ThtPDS Bleaching TRV2 LB! MCS! 2!x!35S! CP! Rz! NOS t! RB! TRV2_ThtAG2_ThtPDS LB! 2!x!35S! CP! ThtAG2' ThtPDS' Rz! NOS t! RB! Untreated TRV2 Empty TRV2_ThtAG2_ThtPDS TRV2_ThtAG1 ThtAG2-silenced ovules have a carpel-like morphology and lack an embryo sac. oi ii es ii oi nu st-l ii oi WT Ovule ThtAG2 VIGS Ovule ThtAG2 VIGS Ovule sg Ovule Structure oi = outer integument ii = inner integument es = embryo sac nu = nucellus st-l = style-like Carpel Structure st = style sg = stigma ov = ovule ov! st WT Carpel

20 ThtAG2-silenced ovules have a carpel-like morphology and lack an embryo sac. oi ii es ii oi st-l ii nu oi WT Ovule ThtAG2 VIGS Ovule ThtAG2 VIGS Ovule sg Conclusion: ThtAG2 has sub-functionalized and is now an ovule identity gene. st ov! WT Carpel Summary Down-regulation of ThtAG1 causes a double-flower phenotype. The horticultural mutant Double White has a transposon disrupting ThtAG1. Down-regulation of ThtAG2 causes the loss of the embryo sac and the development of a carpel-like structure in place of the ovule. After the duplication that led to ThtAG1 and ThtAG2 The C-class gene ThtAG1 kept the conserved C-function role in stamen and carpel development. The C-class gene ThtAG2 has likely sub-functionalized to control ovule development a role traditionally assigned to the D-class.