Multiple allelism as a control mechanism in metabolic pathways: GBSSI allelic composition affects the activity of granule-bound starch synthase I andstarch composition in potato

Multiple allelism in heterozygous autopolyploid species like potato not onl y occurs for genes that affect morphological characteristics but also for g enes involved in metabolic pathways. Based on a combination of Southern and PCR analyses, at least eight alleles encoding granule-bound starch synthas e I (GBSSI), which is responsible for amylose biosynthesis, have been ident ified in potato. These alleles were grouped into four classes, distinguisha ble by Southern analysis, and subdivided based on PCR. Despite the heterozy gous and polyploid character of potato it was possible to assign variation in GBSSI activity to the allelic composition at the GBSSI loci within a lar ge population of Solanum tuberosum cultivars and Solanum breeding lines. Mo reover, the availability of an amf allele made it possible to reduce hetero geneity and enabled us to demonstrate an effect of GBSSI allelic compositio n on amylose content. The major difference between the alleles identified w as the absence or presence of a 140-bp fragment at a site 0.5 kb upstream o f the ATG start codon of the gene for GBSSI. The absence of this 140-bp fra gment had a major effect on GBSSI activity and amylose content, while the p resence of small deletions and simple sequence repeats had no obvious effec t.