Gametophytic Self-Incompatibility vs. Sporophytic Self-Incompatibility

Attribute	Gametophytic Self-Incompatibility	Sporophytic Self-Incompatibility
Definition	Self-incompatibility mechanism where the recognition and rejection of self-pollen occurs at the gametophytic level.	Self-incompatibility mechanism where the recognition and rejection of self-pollen occurs at the sporophytic level.
Genetic Control	Controlled by the haploid genotype of the pollen.	Controlled by the diploid genotype of the sporophyte.
Expression	Expressed in the haploid pollen grains.	Expressed in the diploid tissues of the sporophyte.
Recognition	Pollen recognition is determined by the interaction between the pollen S-gene and the pistil S-RNase.	Pollen recognition is determined by the interaction between the sporophytic S-locus and the pollen S-locus.
Effect on Fertilization	Prevents self-fertilization by inhibiting pollen tube growth or causing pollen tube rupture.	Prevents self-fertilization by inhibiting pollen tube growth or causing pollen tube rupture.
Inheritance	Follows Mendelian inheritance patterns.	Follows Mendelian inheritance patterns.

Introduction

Self-incompatibility is a genetic mechanism found in many flowering plants that prevents self-fertilization, promoting outcrossing and genetic diversity. There are two main types of self-incompatibility: gametophytic self-incompatibility (GSI) and sporophytic self-incompatibility (SSI). While both mechanisms achieve the same goal, they differ in their genetic control, expression, and inheritance patterns.

Gametophytic Self-Incompatibility (GSI)

Gametophytic self-incompatibility is a mechanism where the incompatibility is determined by the genotype of the pollen grain. In GSI, the pollen carries the genetic information that determines whether it can successfully fertilize the ovule. The recognition and rejection of self-pollen occur at the gametophytic stage, specifically in the pollen tube.

In GSI, the S-locus, or the self-incompatibility locus, controls the recognition process. The S-locus is highly polymorphic, with multiple alleles that determine the specificity of the self-incompatibility response. Each allele at the S-locus encodes a specific S-protein, and the interaction between the S-protein in the pollen and the S-receptor in the pistil determines the compatibility. If the S-protein in the pollen matches the S-receptor in the pistil, the pollen tube is rejected, preventing self-fertilization.

GSI exhibits a sporadic expression pattern, where the self-incompatibility response can vary between different flowers on the same plant. This variation is due to the presence of multiple S-alleles within an individual plant. The inheritance of GSI is also complex, as it follows a Mendelian inheritance pattern with multiple alleles at the S-locus. This allows for a high level of genetic diversity within a population, as individuals with different S-alleles are more likely to cross-pollinate.

Sporophytic Self-Incompatibility (SSI)

Sporophytic self-incompatibility is a mechanism where the incompatibility is determined by the genotype of the parent plant. In SSI, the incompatibility is controlled by the diploid tissues of the parent plant, specifically the stigma. The recognition and rejection of self-pollen occur at the sporophytic stage, before the pollen reaches the gametophytic stage.

In SSI, the S-locus also plays a crucial role in determining the compatibility. However, unlike GSI, the S-locus in SSI is expressed in the diploid tissues of the parent plant, such as the stigma. The S-locus controls the production of a factor called the S-gene product, which determines the compatibility of the pollen. If the S-gene product matches the S-allele carried by the pollen, the pollen is rejected.

Unlike GSI, SSI exhibits a consistent expression pattern within an individual plant. All flowers on the same plant will have the same self-incompatibility response, as it is determined by the genotype of the parent plant. The inheritance of SSI is also simpler compared to GSI, as it follows a single-gene inheritance pattern. This means that the self-incompatibility response can be predicted based on the S-allele carried by the parent plant.

Comparison

While both GSI and SSI achieve the same goal of preventing self-fertilization, they differ in several key attributes:

• Genetic Control: GSI is controlled by the genotype of the pollen grain, while SSI is controlled by the genotype of the parent plant.
• Expression Pattern: GSI exhibits sporadic expression, varying between different flowers on the same plant, while SSI shows consistent expression within an individual plant.
• Inheritance: GSI follows a complex Mendelian inheritance pattern with multiple alleles at the S-locus, promoting genetic diversity, while SSI follows a simpler single-gene inheritance pattern.
• Stage of Recognition: GSI recognizes and rejects self-pollen at the gametophytic stage, specifically in the pollen tube, while SSI recognizes and rejects self-pollen at the sporophytic stage, before the pollen reaches the gametophytic stage.
• Control of Compatibility: GSI is determined by the interaction between the S-protein in the pollen and the S-receptor in the pistil, while SSI is determined by the interaction between the S-gene product in the parent plant and the S-allele carried by the pollen.

Conclusion

Gametophytic self-incompatibility (GSI) and sporophytic self-incompatibility (SSI) are two distinct mechanisms that prevent self-fertilization in flowering plants. GSI is controlled by the genotype of the pollen grain, exhibits sporadic expression, follows a complex inheritance pattern, and recognizes self-pollen at the gametophytic stage. On the other hand, SSI is controlled by the genotype of the parent plant, shows consistent expression, follows a simpler inheritance pattern, and recognizes self-pollen at the sporophytic stage. Understanding the differences between these two mechanisms is crucial for studying plant reproductive biology and the promotion of genetic diversity within plant populations.