INCOMPATIBILITY IN FLOWERING PLANTS
Authors: LEWIS D. 
Published: January 2008

1. Incompatibility is a physiological mechanism which enforces outbreeding. It is widespread throughout the families of flowering plants. There are two main types: (i) Heteromorphic. This is associated with differences in floral structure; distylic with two types of flower, thrum having a short style and high anthers, pin having a long style and low anthers; tristylic with three types of flowers, with long, mid and short styles. (2) Homomorphic, in which there are no floral differences.

2. Heteromorphic incompatibility is associated with six contrasting pairs of characters; these are normally inherited as a single unit. The genetic control is by one gene, S, with two alleles in distyly, and by two genes each with two alleles in tristyly. The incompatibility reaction of the pollen is sporophytically and not gametophytically determined. Short style is always dominant to long. Major modifying genes and polygenes which affect the expression of the S gene are present. In Primula hortensis abnormal plants exist in which the complex of characters has been broken down, presumably by abnormal crossing‐over between subunits of which the S gene must be composed. Both the morphological and the genetic determination restrict the number of different types, and hence of alleles.

3. Homomorphic incompatibility in many species is controlled by a single gene with a large number of alleles, Sj^Sg.Sn. Pollen is unable to grow in a style

which has the same allele as the pollen. Unlike heteromorphic incompatibility the pollen reaction is gametophytically determined and the two alleles in the style are independent. In some species, most of which are polyploids, the simple S type of control does not work without modifications.

4. In a new tetraploid incompatibility breaks down and frequently the tetraploids are fully self‐compatible. This breakdown is due to interactions between two different alleles in diploid pollen grains. With some alleles the interaction is competitive so that neither allele functions. This results in self‐compatibility. With other alleles, one may be dominant to the other, thus retaining the self‐incompatibility of the diploid plant.

5. Incompatibility is due in some species to the failure of pollen germination, in others to the pollen tubes failing to penetrate the full length of the style. Style‐grafting and temperature experiments indicate that the stoppage of pollen‐tube growth is due to a reaction between a substance in the pollen tube and a complementary substance in the style, the pollen and style substances being reactively different for each allele. The reaction blocks some process necessary for tube growth and is irreversible.

6. Incompatibility in species crosses and their derivatives shows that species without an incompatibility system have polygenic modifiers which weaken the action of the S gene. From Nicotiana and Petunia hybrids there is evidence that the S gene can have an additional effect of inhibiting pollen from a self‐compatible species.

7. The spontaneous mutation rate of the S gene in Oenothera and Prunus, including all changes, is of the order of one in a million pollen grains, but no mutations to a fully operative incompatibility allele have been found in seventy million tested grains.

Mutant alleles produced by X‐rays are all of a type which have their pollen‐controlling activity destroyed but their stylar activity unimpaired. The X‐ray results and the evidence from polyploidy and interspecific hybrids all point to a dual structure of the S gene. The solution of the problem of the extremely low mutation rate with the high number of alleles in natural populations may throw light on the origin of new genie material.