QTL analysis in a complex autopolyploid: Genetic control of sugar content in sugarcane

QTL mapping in autopolyploids is complicated by the possibility of segregat ion for three or more alleles at a locus and by a lack of preferential pair ing, however the subset of polymorphic alleles that show simplex segregatio n ratios can be used to locate QTLs. In autopolyploid Saccharum, 36 signifi cant associations between variation ill Sugar content and unlinked loci det ected by 31 different probes were found in two interspecific F-1 population s. Most QTL alleles showed phenotypic effects consistent with the parental phenotypes, but occasional transgressive QTLs revealed opportunities to pur ge unfavorable alleles from cultivars or introgress valuable alleles from e xotics. Several QTLs on homologous chromosomes appeared to correspond to on e another-multiple doses of favorable 'alleles' at such chromosomal region( s) yielded diminishing returns-such negative epistasis may contribute to ph enotypic buffering. Fewer sugar content QTLs were discovered from the highe st-sugar genotype than from lower-sugar genotypes, perhaps suggesting that many favorable alleles have been fixed by prior selection, i.e. that the ge nes for which allelic variants (QTLs) persist in improved sugarcanes may be a biased subset of the population of genes controlling sugar content. Comp arison of these data to mutations and QTLs previously mapped in maize hinte d that seed and biomass crops may share a partly-overlapping basis for gene tic variation in carbohydrate deposition. However, many QTLs do not corresp ond to known candidate genes, suggesting that other approaches will be nece ssary to isolate the genetic determinants of high sugar content of vegetati ve tissues.