Breakdown of Self-Incompatibility in a Natural Population ofPetunia axillaris Caused by Loss of Pollen Function

Abstract

Abstract Although Petunia axillaris subsp.axillaris is described as a self-incompatible taxon, some of the natural populations we have identified in Uruguay are composed of both self-incompatible and self-compatible plants. Here, we studied the self-incompatibility (SI) behavior of 50 plants derived from such a mixed population, designated U83, and examined the cause of the breakdown of SI. Thirteen plants were found to be self-incompatible, and the other 37 were found to be self-compatible. A total of 14 S-haplotypes were represented in these 50 plants, including two that we had previously identified from another mixed population, designated U1. All the 37 self-compatible plants carried either an SC1- or anSC2-haplotype.SC1SC1andSC2SC2homozygotes were generated by self-pollination of two of the self-compatible plants, and they were reciprocally crossed with 40 self-incompatible S-homozygotes (S1S1throughS40S40) generated from plants identified from three mixed populations, including U83. TheSC1SC1homozygote was reciprocally compatible with all the genotypes examined. The SC2SC2homozygote accepted pollen from all but theS17S17homozygote (identified from the U1 population), but theS17S17homozygote accepted pollen from theSC2SC2homozygote. cDNAs encoding SC2- and S17-RNases were cloned and sequenced, and their nucleotide sequences were completely identical. Analysis of bud-selfed progeny of heterozygotes carrying SC1 orSC2 showed that the SI behavior of S  C1 and S  C2 was identical to that of S  C1 andS  C2 homozygotes, respectively. All these results taken together suggested that the S  C2-haplotype was a mutant form of the S  17-haplotype, with the defect lying in the pollen function. The possible nature of the mutation is discussed.