Structure and Expression of Style-Expressed and Pollen-Expressed Components of Gametophytic Self-Incompatibility in Petunia Hybrida

Abstract

Self-incompatibility is a genetic barrier to inbreeding that is based on the ability of the style to discriminate between self and non-self pollen. Historically, this discrimination has been shown to be governed by a single genetic locus known as the S-locus (East and Mangelsdorf, 1925; Bateman, 1955). In gametophytic self-incompatibility systems, like those of Petunia hybrida, the S-locus recognition phenotype is determined by the haploid genotype of the individual pollen grain. Both self and non-self pollen tubes will germinate and begin to grow through the transmitting tract tissue of the style. If there is a match between the S-allele(s) expressed in the style, and that expressed by a pollen tube, the growth of the (self) pollen tube is inhibited in the upper portion of the style. If there is no match between pollen and style recognition specificities, growth of the pollen tube is not inhibited, and it will grow the length of the style to the ovary, where it can function for fertilization and seed set. A contrasting type of self-incompatibility is sporophytic self-incompatibility. Here, the recognition phenotype of the pollen grain is determined by the diploid genotype of the pollen parent. In sporophytic self-incompatibility self pollen grains either fail to germinate at all, or if germination occurs, fail to penetrate through the stigmatic cuticle. In recent years, proteins and genes associated with self-incompatibility alleles have been isolated for both sporophytic systems (such as Brassica) and gametophytic systems (such as Petunia and Nieotiana alata). Current molecular evidence indicates that sporophytic and gametophytic self-incompatibility systems evolved independently and function via different mechanisms (Haring et al., 1990; Nasrallah et al., 1991; Sims, 1992a, 1992b).