The potential loss of chemical sprout inhibitors because of public concern over the use of pesticides underscores the desirability of breeding for long dormancy of potato (Solanum tuberosum L.) tubers. Quantitative trait locus (QTL) analyses were performed in reciprocal backcrosses between S. tuberosum and S. berthaultii toward defining the complexity of dormancy. S. berthaultii is a wild Bolivian species characterized by a short-day requirement for tuberization, long tuber dormancy, and resistance to several insect pests. RFLP alleles segregating from the recurrent parents as well as from the interspecific hybrid were monitored in two segregating progenies. We detected QTLs on nine chromosomes that affected tuber dormancy, either alone or through epistatic interactions. Alleles from the wild parent promoted dormancy, with the largest effect at a QTL on chromosome 2. Long dormancy appeared to be recessive in the backcross to S. berthaultii (BCB). In BCB the additive effects of dormancy QTLs accounted for 48% of the measured phenotypic variance, and adding epistatic effects to the model explained only 4% more. In contrast, additive effects explained only 16% of the variance in the backcross to S. tuberosum (BCT), and an additional 24% was explained by the inclusion of epistatic effects. In BCB variation at all QTLs detected was associated with RFLP alleles segregating from the hybrid parent; in BCT all QTLs except for two found through epistasis were detected through RFLP alleles segregating from the recurrent parent. At least three dormancy QTLs mapped to markers previously found to be associated with tuberization in these crosses.
Quantitative trait loci (QTLs) affecting tuberization were detected in reciprocal backcrosses between Solanum tuberosum and S. berthaultii. Linkage analyses were performed between traits and RFLP alleles segregating from both the hybrid and the recurrent parent using a set of framework markers from the potato map. Eleven distinct loci on seven chromosomes were associated with variation in tuberization. Most of the loci had small effects, but a QTL explaining 27% of the variance was found on chromosome 5. More QTLs were detected while following alleles segregating from the recurrent S. tuberosum parent used to make the backcross than were detected by following alleles segregating from the hybrid parent. More than half of the alleles favoring tuberization were at least partly dominant. Tuberization was favored by an allele from S. berthaultii at 3 of the 5 QTLs detected by segregation from the hybrid parent. The additive effects of the QTLs for tuberization explained up to 53% of the phenotypic variance, and inclusion of epistatic effects increased this figure to 60%. The most common form of epistasis was that in which presence of an allele at each of 2 loci favoring tuberization was no more effective than the presence of a favorable allele at 1 of the 2 loci. The QTLs detected for tuberization traits are discussed in relationship to those previously detected for trichome-mediated insect resistance derived from the unadapted wild species.
In previous studies polygene mapping of a backcross population derived from haploid potato (Solanum tuberosum) and a diploid wild species (Solanum berthaultiij showed at least eight quantitative trait loci (QTLs) associated with tuber dormancy. The same population was mapped for abscisic acid (ABA) content in tubers so that any QTLs identified could be compared with those detected previously. At least three distinct loci on three chromosomes (2, 4, and 7) were associated with variation in ABA content. One of the QTLs was detected only as a main (single locus) effect, and two QTLs were found through two-locus interaction analysis (epistasis). Interaction between QTLs at markers TG234 (chromosome 2) and TG755 (chromosome 4) explained 20% of total phenotypic variante for this trait. The interaction closely resembled one previously detected for dormancy, suggesting an association between high ABA content and long tuber dormancy. Although relationships between ABA leve1 and dormancy could be demonstrated through polygene mapping, there was no indication of a relationship between these traits when they were subjected to a conventional correlation test. This illustrates the usefulness of polygene mapping as a tool to identify possible associations between hormone levels and plant development.A well-tested approach for understanding growth and development is the analysis of differences between two phenotypes that contain different alleles of a single major gene. For example, analyses of dwarf and wild-type siblings have revealed hormonal differences associated with the mutant genotype (Reid and Howell, 1995). Although this approach has been productive, a limitation is that the control of many developmental processes is polygenic, i.e. inheritance is quantitative rather than qualitative. Thus, although it is interesting to know what physiological effect was produced by a dwarf mutation, it might be much more informative to learn about the physiological effects pro- duced by the polygenes that control plant height. In the past the study of quantitative inheritance was so difficult that it yielded only very generalized information, and little could be learned that would elucidate the physiology of growth and development. The situation has changed dramatically with the availability of new methods for mapping polygenes (Tanksley et al., 1989;Tanksley, 1993;Young, 1993). The new mapping methods rely on DNA-based genetic markers, such as RFLP markers. Through such mapping, a number of QTLs may be found that are associated with the control of a trait such as plant height. A QTL denotes a region of chromosome linked to the marker gene that has a significant effect on the quantitative trait (Tanksley, 1993).With the availability of this tool for mapping quantitative traits, polygenes can be used to study the physiology of growth and development in a manner analogous to, but more powerful than, the way single genes have been used in the past. Once the QTLs have been detected that are associated with the control of a given stage of...
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