Define dioecy:
Having male and female sex organs on separate plants
How common is dioecy?
4% of angiosperms, 10-20% of trees and shrubs, 1-5% of herbs. Often associated with wind pollination
What is the origin of dioecy?
Multiple origins Enforces outcrossing Recessive X carries male sterility, dominant Y = female sterility - only requires 2 mutations Would still expect 50:50 sex ratio.
How is dioecy distribution geographically?
Higher frequency in tropical floras and oceanic islands.
- In tropical forests chances of inbreeding are high due to low species density –> inbreeding depression.
What are the benefits of dioecy?
Enforces outcrossing - prevents inbreeding depression
Prevents pollen clogging which can be an issue with self-incompatibility
Better resource allocation
Why are dioecy and resource allocation linked?
In tropics females produce large seeds (fleshy) due to large amounts of competition. Therefore requires lots of resources / input.
When is self-compatibility an advantage?
Colonising species use self-compatibility
Allows establishment after long distance dispersal
- Remote islands tend to be small and lack pollinators
How comes dioecy is common on oceanic islands?
Adaptation to enforce outbreeding following colonisation and establishment as self compatible.
- More prevalent in endemic species
- Frequency higher in older islands
Why is dioecy adaptively significant?
Many vacant niches exist - opportunity for speciation and diversification.
- This requires high genetic variability which is not going to happen with inbreeding
- Outcrossing mutations favoured - gyanceous (male sterile)
- Female sterility arises later
It is a lot easier than becoming self-incompatible.
How are dioecous species spread?
Fleshy seeds can be eaten by birds - contains male and female seeds.
- If self incompatible then fecundity is low due to being siblings (chances of successful offspring is decreased).
When is dioecy adaptive?
When founding populations are small and contain closely related individuals.
