SummaryTo efficiently transfer important disease and insect resistance traits from diploid Mexican wild species Solanum pinnatisectum to tetraploid cultivated Solanum tuberosum, molecular cytogenetic and karyotypic analyses were carried out to examine the degree of chromosomal and genomic variation between these 2 species. The results demonstrated that the chromosome complement in S. pinnatisectum was predominantly metacentric and submetacentric, while S. tuberosum showed prominent subtelocentric and telocentric chromosomes. It appears that karyotype evolution from diploid to tetraploid originates through chromosome rearrangements, involving mainly deletions or multiplication at the same ploidy level. Fluorescence in situ hybridization (FISH) using 18S-5.8S-26S ribosomal RNA (rDNA) as a probe demonstrated that both diploid and tetraploid potato species had a single nucleolus organizer region (NOR) locus. However, the localization of the rDNA sites in one of the 2 accessions of S. pinnatisectum was characterized by a pair of morphologically distinct chromosomes, in which one was a submetacentric chromosome, while its homologous partner was a subtelocentric chromosome, which was very similar to that observed in tetraploid potato. This indicated that chromosome variation in structural alterations and nuclear DNA content, as well as loss or addition of highly repetitive sequences could play a role in potato evolution and development of new cultivars. This study offers a useful molecular cytogenetic marker for species identification, chromosome inheritance and potential introgression in future intergenetic hybridization experiments with Mexican wild potato species.

Genomic in situ hybridization (GISH) using S genomic DNA as a probe was applied to study inheritance of blue seed color and characterize the chromosome constitution of blue-grained lines selected from derivatives of Chinese Spring (CS)ϫAgrotana, a multiple disease resistant wheat-Th. ponticum partial amphiploid line. The results showed that the expression of blue seed color in the CS-Agrotana advanced hybrid lines was always linked with a pair of translocation chromosomes derived from J and J s genomes of Th. ponticum. The blue-grained lines had 43-44 chromosomes, which included 2 J-J s ϩ2 (or 1) J s-J alien translocation chromosomes, while the red-grained lines had 41-42 chromosomes, only carried 2 J-J s translocation chromosomes. This suggested that the J s-J translocation chromosomes might carry the major gene(s) that controlled the blue grain color. Since the J-J s translocation chromosomes were always present with the J s-J translocation chromosomes in blue-grained lines, it is possible that the blue grain color resulted from an interaction between the 2 translocations. Meiotic analyses of chromosome pairing in the blue-grained lines indicated that the 2 pairs of the alien translocation chromosomes did not pair each other, nor did they pair with the wheat chromosomes. The occurrence of 2 bivalents and no alien quadrivalents in the bluegrained lines with J s-J and J-J s translocation chromosomes, demonstrated that the 2 alien chromosome translocations were not reciprocal translocations. The J-J s translocation chromosomes present in the blue-grained lines were more stable at meiosis than the J s-J translocation chromosomes. The J s-J chromosomes were easily lost, resulting in an instability in grain color that could change from light blue to red. Our results determined the molecular cytogenetic characteristics and inheritance of the alien translocation chromosomes present in the blue-grained lines carrying resistance genes to common root rot. This study further confirmed that the chromosome translocations among the J s and J genome were responsible for blue grain color.