Pollination is the transfer of pollen from the male anther to the female stigma, resulting in the fertilisation of gametes

• When a pollen grain settles on the stigma, it develops a protrusion called a pollen tube that extends across the style and penetrates into the ovule
  

• The male sperm is transported within the cytoplasm of the pollen tube and then released into the ovule, whereupon it will fuse with an ova (fertilisation)
  

Monoecious plants have both male and female reproductive structures on their flowers and are capable of self-pollination

• These plants can either transfer pollen grains to the stigma of the same flower, or alternatively to a different flower of the same plant
  

• Self-pollination leads to inbreeding, which decreases genetic diversity and increases the proportion of deleterious alleles within a population
  

Dioecious plants possess only one type of reproductive structure (male or female) and must produce offspring via cross-pollination

• Because plants are immobile, cross-pollination requires either an environmental agent or biological vector to transfer the pollen 
  

• Terrestrial plants may rely on wind to disperse pollen, while aquatic plants may distribute pollen within the water

• Animals (such as insects and birds) can also transfer pollen, and flowers possess specific features (bright petals and nectar) to recruit these pollinators

Pollination Methods

Self Incompatibility

Monoecious plants may possess a variety of self incompatibility mechanisms to prevent inbreeding and increase genetic variation within a species

• Some plants may have different male and female flowers or possess different maturation times for pollen and stigma in order to promote cross-pollination
  

• Other plants may have heteromorphic (different shaped) structures to prevent self-pollination – such as long filaments but short styles
  

• Additionally, some plants may have specific genetic mechanisms that inhibit the fertilisation of an egg with a sperm
  

  • Flowers produce specific proteins that are present on both the pollen and the stigma
    

  • The stigma will reject any pollen that possess identical proteins – preventing self-fertilization
    

  • Only pollen with different proteins can fertilise the stigma – promoting cross-pollination between plants
    

Self Incompatibility Methods