QTL mapping in diploid potato by using selfed progenies of the cross S. tuberosum × S. chacoense.

Usually, mapping studies in potato are performed with segregating populations from crosses between highly heterozygous diploid or tetraploid parents. These studies are hampered by a high level of genetic background noise due to the numerous segregating alleles, with a maximum of eight per locus. In the present study, we aimed to increase the mapping efficiency by using progenies from diploid inbred populations in which at most two alleles segregate. Selfed progenies were generated from a cross between S. tuberosum (D2; a highly heterozygous diploid) and S. chacoense (DS; a homozygous diploid clone) containing the self-incompatibility overcoming S locus inhibitor (Sli-gene). The Sli-gene enables self-pollination and the generation of selfed progenies. One F2 population was used to map several quality traits, such as tuber shape, flesh and skin color. Quantitative trait loci were identified for almost all traits under investigation. The identified loci partially coincided with known mapped loci and partially identified new loci. Nine F3 populations were used to validate the QTLs and monitor the overall increase in the homozygosity level.

The molecular basis of co-evolution between Cladosporium fulvum and tomato.

Cladosporiumfulvum is a semi-biotrophic pathogen, which causes leaf mold of tomato (Lycopersicon spp.). In our laboratory this pathosystem serves as a model to study gene-for-gene interactions between plants and pathogenic fungi (Joosten & De Wit 1999). Many avirulence (Avr) genes and matching resistance (CQ) genes have been cloned and we are now beginning to understand how their products can induce an array of plant defense responses, including the classic hypersensitive response (HR). Here, we will discuss the latest results of our molecular studies on this interaction. These include the isolation of: (i) two new Avr genes, Avr2 and Avr4E, (ii) the determination of the specificity determinants within the Cf-4 and Cf-9 genes by artificial domain swaps and introduction of point mutations, (iii) the analysis of polymorphism occurring in AVR9-responsive Cf genes occurring in natural populations of L. pimpinellifolium, and finally (iv) the description of an efficient method to identify early HR-related genes.

Somaclonal variation in tomato: effect of explant source and a comparison with chemical mutagenesis.

Plants were regenerated from leaf, cotyledon, and hypocotyl explants of tomato cv Moneymaker. Various phenotypic alterations were observed among regenerated plants (R1), but were not transmitted to the progenies, except for ploidy variation. Variation in ploidy level, mainly tetraploidy, occurred in R1 plants and their R2 progenies, and the frequency of polyploid plants depended on the explant source. More than 50% of the regenerants derived from hypocotyl explants were found to be polyploid. A correlation was observed between the percentage of polyploid cells present in the explant material in vivo and the frequency of polyploid plants. Several monogenic mutations were recovered in the R2, four of which were shown to be allelic to known, recessive, single-gene mutants. No significant effect of explant source or duration of tissue culture period on mutant frequency or spectrum was found. For several mutant types that could be scored unambiguously, somaclonal variation was compared to variation induced by treatment of seeds with ethyl methane sulphonate (EMS). The results showed that the mutant frequencies were higher after EMS treatment than those generated through tissue culture. With respect to the mutant spectrum, no clear differences were observed between the spectra obtained after EMS treatment and those after tissue culture. However, tissue culture gave rise to polyploid plants, whereas no ploidy variants occurred after EMS treatment.