Principles of Plant Breeding

Homomorphic system

• In these, there is only one type of flower. The compatible and incompatible plants cannot be differentiated on the basis of flower morphology. There are two types of incompatibility systems in these species.
• a. Gametophytic incompatibility first reported by East &Mangelsdorf (1925) in Nicotiana sanderae. Gametophyte incompatibility is governedby multiple alleles of S gene pollen. As many as 21 multiple alleles have been found to control incompatibility. These are independent in their action and do not show any dominance relationship.
• Pollen and ovule carrying dissimilar S alleles can set seed if the style does not inhibit the tube growth. The tissue is diploid and if there is any S alleles are common in pollen, it arrests the tube growth. For example, styles of S1 genotype would not allow any pollen tube either S1 or alleles to penetrate whereas it will not inhibit S3 and S3 pollen tubes. Thus, there are three situations, fully incompatibility.E.g., S1S2 x S1S3 partially incompatible.E.g., S1S2 x S2S3 and fully compatable.E.g., S1S2 x S3S4.Gametophytic incompatability has been reported in Tomato, Pear, Peach, Tobacco, Lucerne, etc.