Evolution of Flower

Development

Why are flowering plants so diverse?

Zahn et al. (2005)

Developmental processes that occur in the flowers

different taxa.

Infer evolutionary events that led to the differences

or similarities.

Using the Evo-Devo Approach to study floral diversity

Floral diversity is generated by different mature morphologies

Phyllotaxy

Whorl Number

Organ Fusion

Organ Identity

Organ Elaboration

Symmetry

Floral Symmetry

Reverse Genetics - Floral Symmetry controlled by CYCLOIDEA(CYC) and

DICHOTOMA(DICH) Antirrhinum majus

Luo et al. (1996)

CYC expression in developing flowers

WT Peloric

Linaria vulgaris

Linaria vulgaris peloric mutant

Bilateral Symmetry has evolved many times

Carl Linneaus 1707-1778

Forward genetics - L. vulgaris peloric mutations caused by methylation of CYCLOIDEA

Hileman et al. (2003)

Mohavea confertiflora - Changes in symmetry with an evolutionary effect

Cubas (2004)

Mentzelia involucrata Mohavea confertiflora

M. confertiflora mimics a radial species.

Floral Organ Identity Sepal – 1st Whorl

Petal – 2nd Whorl

Stamen – 3rd Whorl

Carpel – 4th Whorl

Floral Organ Identity genes are MADS Box genes = Transcription Factors

HOX Genes

HOX genes in animals are important for

correct body-plan formation.

MADS-Box genes in flowers are

important for correct organ identity.

“ABC”  Model of Flower Development  

'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  

Arabidopsis thaliana Antirrhinum majus

Homeotic Mutants – used to study HOX genes in Drosophila

Carroll et al. (2005).

HOX Genes

WT

antennapedia

Homeotic conversions of floral organs to study gene function in flower development

C-class gene in Arabidopsis = AGAMOUS

B

A C STA C A R

B B

SEP PE T

“Sliding Boundary” model is one modification to the ‘ABC’ Model 

Kramer et al. (2003)

PE T PE T SEP SEP

T. thalictroides Horticultural Mutants

Wildtype ‘Betty Blake’ ‘Shoaf’s Double’

‘Green Dragon’ ‘Double White’

Theophrastus 371-287 BC

Double Flowers – the first recorded floral mutants

Arabidopsis WT

Arabidopsis agamous mutant

T. thalictroides WT

T. thalictroides ‘Double White’

Thalictrum C-class genes

2 AGAMOUS (AG) orthologs: ThAG1 and ThAG2

Kramer et al. 2004

ThdAG2 carpellate flower

ThdAG1 carpellate flower

ThdAG1 staminate flower

Di Stilio et al. 2005

Forward Genetics – ‘Double White’ has a defect in ThAG1

T. thalictroides WT

T. thalictroides ‘Double White’

T. thalictroides WT

T. thalictroides TRV2ThAG1

Viral Induced Gene Silencing (VIGS)

Reverse Genetics – Down-regulating ThAG1 creates a ‘double flower’ phenotype

T. thalictroides ‘Double White’

Conclusions from the DW/ThAG1 story:

ThAG1 function is conserved. The double-flower phenotype in ‘Double White’ does result from a mutation in the AGAMOUS ortholog.

Evolutionary benefit?