Stylar polymorphism

Question 1

What is heterostyly, and why is it important?

Answer 1

A floral polymorphism where a species has two or more morphs with different reproductive organ arrangements.

• Promotes cross-pollination and prevents self-fertilization.
• Common in Primula, Amsinckia, and Turnera.

Question 2

What are the main types of heterostyly?

Answer 2

Distyly: Two morphs:
* L-morph (Pin): Long styles, short stamens.
* S-morph (Thrum): Short styles, long stamens.

Tristyly: Three morphs with different reproductive organ arrangements.

Other forms:
* Enantiostyly: Style bends left or right.
* Inversostyly: Reversed positioning of reproductive organs.

Question 3

How does heterostyly enforce outcrossing?

Answer 3

• Reciprocal herkogamy ensures that pollen is transferred between morphs.
• Intra-morph incompatibility prevents self-fertilization.
• Promotes efficient pollen transfer and increases genetic diversity.

Question 4

What is a supergene, and how does it control heterostyly?

Answer 4

A linked cluster of genes that determines floral morphs and prevents recombination.

• Regulates style length, anther height, and self-incompatibility.
• Prevents breakdown of heterostylous systems by maintaining genetic linkage.

Question 5

What are the key genes in the heterostyly supergene? (Primula)

Answer 5

• CYP734A50 on (G locus): Inactivates brassinosteroids, controls style length and is only present in the S-morph (Thrum).
• GLOBOSA2 (GLO2, A locus): MADS-box gene, controls anther height.
• Hemizygosity of the S-morph haplotype contributes to recombination suppression.

Hemizygosity occurs when a gene or chromosomal region is present in only one copy instead of the usual two, typically due to deletion, sex chromosome differentiation (e.g., Y chromosome in males), or suppressed recombination in supergenes.

Question 6

How did heterostyly evolve?

Answer 6

Stepwise duplications and neofunctionalization of genes contributed to its independent evolution.

Lloyd & Webb Model:
1) Style-shortening mutation → intermediate floral form.
2) Stamen-adjusting mutation → final heterostylous form.

Self-incompatibility evolves to reinforce outcrossing.

Question 7

Why does the heterostyly supergene always break down to long homostyles?

Answer 7

• Loss of CYP734A50 function removes both morphology and self incompatibility control.
• Mutation of GLO2 only affects morphology, so long homostyles persist.

Breakdown never results in short homostyles due to these genetic constraints.