Overview
This lecture explains the ABC model of flowering, focusing on gene classes, gene combinations, and their effects on flower organ identity and mutations.
Structure of a Flower
- A typical flower has four main components: sepal, petal, stamen, and carpel.
- These components are arranged from outermost to innermost: sepal, petal, stamen, carpel ("sapestica" mnemonic).
ABC Model Gene Classes
- Three classes of genes regulate flower development: Class A, Class B, and Class C.
- Class A and Class C genes have antagonistic (opposing) relationships; if one is missing, the other takes over.
- Each gene class has specific gene examples:
- Class A: APETALA1 (AP1), APETALA2 (AP2)
- Class B: APETALA3 (AP3), PISTILLATA (PI)
- Class C: AGAMOUS (AG)
Combination of Gene Expression and Organ Identity
- Flower organs develop based on combinations of gene class expression across four "whorls" (concentric regions).
- Whorl 1: Class A genes alone → Sepal forms.
- Whorl 2: Class A + Class B genes → Petal forms.
- Whorl 3: Class B + Class C genes → Stamen forms.
- Whorl 4: Class C genes alone → Carpel forms.
Gene Mutations and Effects
- Class A Mutation: Class C replaces A, resulting in flowers with only carpels and stamens.
- Class C Mutation: Class A replaces C, resulting in flowers with only sepals and petals.
- Class B Mutation: Only Class A and C are present, leading to flowers with only sepals and carpels.
Key Terms & Definitions
- Whorl — A concentric ring in a flower where specific organs form.
- Class A gene — Gene group defining sepals and petals (e.g., AP1, AP2).
- Class B gene — Gene group defining petals and stamens (e.g., AP3, PI).
- Class C gene — Gene group defining stamens and carpels (e.g., AG).
- Antagonistic genes — Genes whose activities oppose each other (here, Class A and C).
Action Items / Next Steps
- Review the roles of AP1/AP2, AP3/PI, and AG genes in flower organ identity.
- Practice drawing and labeling the ABC model triangle and predicting outcomes of gene mutations.