Self Fertilization
Overview of Vocabulary
Alleles
One of two or more versions of a gene. An individual inherits two alleles for each gene, one from each parent. If the two alleles are the same, the individual is homozygous for that gene. If the alleles are different, the individual is heterozygous.
Epidermal Cells
Skin cells or surface cells
Gametophyte
Gamete-producing haploid phase, producing the zygote from which the sporophyte arises. It is the dominant form in bryophytes
See more here
Dioecious
Species of flowers that have either staminate (male) or carpellate (female) flowers
Parental Sporophyte
Sporophyte which gave rise to the gamete (pollen grain or egg)
Self-incompatibility
The ability of a plant to reject it's own pollen, and sometimes the pollen of closely related individuals
Selfing
Process where plants fertilize themselves
Signal Transduction Pathway
A set of chemical reactions in a cell that occurs when a molecule, such as a hormone, attaches to a receptor on the cell membrane. The pathway is actually a cascade of biochemical reactions inside the cell that eventually reach the target molecule or reaction.
Sporophyte
The asexual and usually diploid phase, producing spores from which the gametophyte arises
See more here
Stigma
Top of female reproductive organs, platform for pollen grains. Often sticky.
Style
Long neck that attaches stigma to ovules
Ways to Prevent Self-Fertilization
1. Being dioecious
2. Being anatomically structured to make it difficult
3. Having male and female parts mature at different times
4. Being gametophytic self-incompatibility
5. Being sporophytic self-incompatibility
Dioecious
Plants that have either male of female parts cannot fertilize
themselves. Plants that have both are called Monoecious.
These are all examples of dioecious:
Plants can have their floral organs placed in such a way that it would be unlikely to fertilize itself.
These species will produce two types of flowers, one with the stigma above the anthers and one with anthers above the stigma.
When a pollinator lands on the Thrum flower, it gets covered in pollen and then flies to a Pin flower and fertilizes it
Anatomy
Gametophytic Incompatibility
Genes for self-incompatibility are called S-genes
Steps in rejection:
Level 1
1. Pollen has one allele
2. Pollen gets destroyed if allele matches one allele in from the flower's alleles (called a self-type)
Level 2
1. Pollen lands on stigma and drops pollen tube down style
2. Enzymes in style will destroy pollen tube if the allele is a self type
Level 3
1. Pollen grain will have one allele from parental sporophyte
2. Pollen grain drops tube down style
3. RNA-hydrolyzing enzymes destroy tube if allele is a self type
4. These enzymes destroy the RNA of pollen tube
Level 4
1. An S1 pollen grain from an S1S2 parental sporophyte will fail to fertilize an S1S2 flower, but will fertilize an S2S3 flower. An S2 pollen grain will fertilize neither.
Video of successful fertilization:
Sporophytic Incompatibility
Genes for self-incompatibility are called S-genes
Steps in rejection:
Level 1
Fertilization is blocked by products in the tissues of the parental sporophyte, as opposed to enzymes
Level 2
1. Parent sporophyte has two alleles
2. Pollen grain has one of two alleles from parent sporophyte
3. In sporophytic incompatibility, fertilization will not occur if either one of the pollen grain's parent sporophyte alleles are present in the flower.
Example: the S2 pollen, which was produced by a S1S2 parent, cannot germinate on an S1S3 stigma (image to right)
Level 3
1. Parental sporophyte leaves gene products on pollen wall that are indicative of its alleles
2. Epidermal cells on the stigma read these gene products
3. If even one of those gene products matches one of the flower's alleles, the pollen grain will get rejected
4. The pollen grain gets rejected by a signal transduction pathway which prevents germination of pollen grain
5. This occurs on the stigma of the flower (from epithelial cells)
Plant Breeders and Self-Incompatibility
Many crops to self-fertilize, which end up producing very little variety. Plant breeders use hybridizing techniques, which combine the best traits from different varieties to create variety in the plants.
To get hybrid seeds they must prevent self-fertilization, they do so by:
1. Removing anthers
2. Developing sterile male plants