Polymorphisms and Evolution of Solanum bulbocastanum Genes for Broad-Spectrum Resistance to Phytophthora infestans

For two decades, Solanum bulbocastanum Dun. has been challenging potato geneticists with its durable resistance to Phytophthora infestans Mont. de Bary, the causal agent of late blight, the most devastating potato disease. In this study, highly specific SCAR (sequence characterized amplified region) markers for three S. bulbocastanum genes of broad-spectrum resistance to P. infestans,RB/Rpi-blb1,Rpi-blb2 and Rpi-blb3, were employed to screen a vast collection of wild tuber-bearing species from several series of Solanum L. section Petota Dumort. and also potato hybrids and varieties. In S. bulbocastanum and S. stoloniferum Schlechtd. et Bché, the sequences of the markers Rpi-blb1-820 and Rpi-sto1-890, which recognize the LRR and CC regions respectively differed inconsiderably in the orthologous genes RB/Rpi-blb1 and Rpi-sto1. These two markers were tightly linked in diverse wild Solanum genotypes. These markers as well as RB-226, another marker recognizing the LRR region of the Rpi-blb1 gene, were obviously related to high resistance to P. infestans. One more marker recognizing the CC region of the gene, RB-629, was found in much wider range of Solanum genotypes; it was not related to resistance to P. infestans and is therefore considered non-specific. In addition to Rpi-blb1 = Rpi-sto1, the Rpi-blb2 and Rpi-blb3 genes were registered in both S. bulbocastanum and S. stoloniferum accessions. In most cases, three bulbocastanum genes were not linked together. The markers specific for Rpi-blb1 = Rpi-sto1 genes were found in the Solanum series Bulbocastana (Rydb.) Hawkes (genome designation BB) and Longipedicellata Buk. (genome designation AABB) and were absent from five Pinnatisecta Rydb. species also comprising genome B. The marker for Rpi-blb2 was found in the series Bulbocastana and Longipedicellata and also in three accessions of Pinnatisecta, while the Rpi-blb3 gene, the orthologue of widely spread R2 gene for resistance to P. infestans, was present in all three Solanum series. In contrast with Rpi-blb1 = Rpi-sto1, two other functional RB homologues, Rpi-bt1 in S. bulbocastanum and RBver in S. verrucosum Schlechtd. (genome AA), apparently diverged earlier in the evolution of tuber-bearing Solanum species. Our evidence on functional Rpi-blb1 = Rpi-sto1 genes in three Petota series substantiates the assumption that the B genome of tetraploid Longipedicellata evolved from diploid Bulbocastana rather than from diploid Pinnatisecta progenitors. Similar to wild Solanum species, the markers Rpi-blb1-820 and Rpi-sto1-890 were tightly linked in advanced potato hybrids and established varieties bred from S. stoloniferum as the conventional source of bulbocastanum genes for late blight resistance, but did not always match the markers for the Rpi-blb2 and Rpi-blb3 genes and the marker for the Rysto gene as an independent indicator of stoloniferum germplasm.