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 selfincompatibility (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 selfcompatible. 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 C1 S C1 and S C2 S 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 (S 1 S 1 through S 40 S 40 ) generated from plants identified from three mixed populations, including U83. The S C1 S C1 homozygote was reciprocally compatible with all the genotypes examined. The S C2 S C2 homozygote accepted pollen from all but the S 17 S 17 homozygote (identified from the U1 population), but the S 17 S 17 homozygote accepted pollen from the S C2 S C2 homozygote. cDNAs encoding S C2 -and S 17 -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 C1 and S 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.Naturally occurring or induced (e.g. by radiation) self-compatible mutants of self-incompatible species in the Solanaceae have provided useful materials for studying the mechanism of the S-RNase-mediated self-incompatibility (SI) system. For example, molecular genetic studies of a self-compatible line of Lycopersicon peruvianum have suggested that the RNase activity of S-RNases is an integral part of the SI mechanism because mutations in the S-RNase gene rendering its protein product without the RNase activity cause the breakdown of SI (Kowyama et al., 1994a;Royo et al., 1994). Moreover, cytological and molecular genetic studies of x-ray irradiationinduced self-compatible mutants have uncovered a phenomenon, called competitive interaction, where duplication of the S-locus region renders pollen grains carrying two different pollen S-alleles unable to function in SI (de Nettancourt, 1977;Golz et al., 2000). Studies of self-compatible mutants have also led to the identification of loci unlinked to the S-locus that are required for the function of the S-RNase gene or the yet unidentified pollen S-gene. For example, mutations in the HT gene, which encodes a small Asn-rich protein (McClure et al., 1999), are partially responsible for the breakdown o...

Polymerase chain reaction fragment length polymorphisms and nucleotide sequences for a cytochrome P450 gene encoding flavonoid-3',5'-hydroxylase, Hf1, were studied in 19 natural taxa of Petunia. Natural Petunia taxa were classified into six groups based on major insertion or deletion events that occurred only in intron II of the locus. The maximum parsimony method was used to calculate strict consensus trees based on nucleotide sequences in selected regions of the Hf1 locus. Petunia taxa were divided into two major clades in the phylogenetic trees. Petunia axillaris (including three subspecies), P. exserta, and P. occidentalis formed a clade with 100% bootstrap support. This clade is associated with a consistently inflexed pedicel, self-compatibility in most taxa, and geographical distribution in southern and western portions of the genus range. The other clade, which comprised the remainder of the genus is, however, less supported (up to 71% bootstrap); it is characterized by a deflexed pedicel in the fruiting state (except P. inflata), self-incompatibility, and a northeastern distribution. A nuclear gene, Hf1, seems to be a useful molecular marker for elucidating the phylogeny of the genus Petunia when compared with the nucleotide sequence of trnK intron of chloroplast DNA.