Genetic characterisation of agronomic and morphological traits and the development of DNA markers associated with total glycoalkaloid content in the tubers of tetraploid potato (Solanum tuberosum L.)

Glycoalkaloids are secondary metabolites that are characterised by an undesirable taste, and which are known to be toxic when consumed in large quantities. Some wild potato germplasms that are used for the introgression of genes encoding heat tolerance contain high concentrations of glycoalkaloids in the tubers. Breeding and selection should therefore take place to lower the glycoalkaloid content in potato tubers and DNA markers can significantly facilitate the selection process.    This study was designed to identify DNA markers associated with the total content of glycoalkaloids (TGA) in the potato tuber. Two segregating resource populations were constructed, one of 216 clones using the tetraploid clones Cara and LT7 as parents and one of 176 clones using the tetraploid parents NT8 and LT7. The two populations were characterised by divergent TGA-content of the parental clones, where LT7 was the mutual male parent with high TGA-content, and both Cara and NT8 the parental clones with low TGA-content. The populations were used, in replicates, to estimate genetic parameters for total glycoalkaloid (TGA) content in the tubers.    Besides TGA-content in the tubers, over thirty other agronomic and morphological traits were assessed for genetic characterisation of the two populations used. Among agronomic traits assessed were tuber dormancy, tuber weight and tuber number. Several traits were calculated based on data obtained, such as tuber size distribution and tuber number distribution. Morphological characteristics assessed included plant height, number of nodes on the main stem, and terminal leaflet shape.    In both populations, the parental clones differed markedly in TGA-content and the progeny population was normally distributed for this trait after logarithmic transformation. Broad sense heritability estimates for TGA-content were 0.54 in CaraxLT7 and 0.50 in NT8xLT7 and the trait proved to be inherited in a non-dominant manner. The minimum number of genes contributing to TGA-content was estimated to be between 3 and 7. Broad sense heritability was also estimated for all other traits assessed. Heritability estimates that were similar in both populations ranged from 0.1 for the number of main stems to 0.5 for total tuber glycoalkaloid content. In both populations, most heritabilities estimated were about 0.3. Main agronomic traits, such as tuber dormancy, tuber weight, and maturity, appeared to be controlled by several additive genes. None of the traits recorded in this study showed a statistically significant genetic association with TGA-content. This suggests that tuber TGA- content may be genetically modulated without any significant adverse effects on other agronomic traits. The genetic parameters estimated in this study indicate that these populations are suitable for DNA-marker analysis for TGA-content in tubers.    Three hundred forty two Inter-Simple Sequence Repeat (ISSR) single primers or combinations of two of such primers were used to amplify random PCR products. Parental clones of both populations were used to identify polymorphic PCR products for further linkage analyses with TGA-content. In these analyses, two single ISSR markers were found to be significantly associated with TGA-content. A multiple regression analysis was also carried out using a 'stepwise' procedure. In this analysis TGA-content was the dependent variable whereas the polymorphic PCR products and all possible two-way interactions among them were the independent variables. The resulting best model consisted of an interaction between two loci in addition to a single locus effect. This interaction suggests that the expression of TGA-content is partially modulated by two interacting loci.    A single copy DNA marker for a morphological characteristic, leaflet vein depth, was generated using two ISSR-primers to generate random PCR products followed by the design of a Cleaved Amplified Polymorphic Sequence (CAPS) marker, A multiple regression analysis showed that the best model consisted of the ISSR-PCR product and a CAPS-PCR product. The results suggest that these products represent separate alleles at the same DNA locus. Vein depth is apparently controlled by one or several genes that modulates the lamina expansion, either during cell division or during cell elongation. Vein depth is shown to be regulated in an additive way and hence allelic differences are supposedly accountable for small differences in lamina growth. The feasible generation of a single copy DNA marker in tetraploids and its possible use in the study of tetrasomic inheritance are thereby demonstrated.