Breakdown of self-incompatibility in a natural population of Petunia axillaris caused by loss of pollen function

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 S_C1- or an S_C2-haplotype. S_C1S_C1 and S_C2S_C2 homozygotes were generated by self-pollination of two of the self-compatible plants, and they were reciprocally crossed with 40 self-incompatible S-homozygotes (S₁S₁ through S₄₀S₄₀) generated from plants identified from three mixed populations, including U83. The S_C1S_C1 homozygote was reciprocally compatible with all the genotypes examined. The S_C2S_C2 homozygote accepted pollen from all but the S₁₇S₁₇ homozygote (identified from the U1 population), but the S₁₇S₁₇ homozygote accepted pollen from the S_C2S_C2 homozygote. cDNAs encoding S_C2- and S₁₇-RNases were cloned and sequenced, and their nucleotide sequences were completely identical. Analysis of bud-selfed progeny of heterozygotes carrying S_C1 or S_C2 showed that the SI behavior of S_C1 and S_C2 was identical to that of S_C2 and S_C2 homozygotes, respectively. All these results taken together suggested that the S_C2-haplotype was a mutant form of the S₁₇-haplotype, with the defect lying in the pollen function. The possible nature of the mutation is discussed.