No abstract

Late blight caused by the oomycete Phytophthora infestans is the economically most important and destructive disease in potato cultivation. Quantitative resistance to late blight available in tetraploid cultivars is correlated with late maturity in temperate climates, which is an undesirable characteristic. A total of 30 DNA-based markers known to be linked to loci for pathogen resistance in diploid potato were selected and tested as polymerase chain reaction-based markers for linkage with quantitative trait loci (QTL) for late blight resistance and plant maturity in two half-sib families of tetraploid potatoes. Most markers originated from within or were physically closely linked to candidate genes for quantitative resistance factors. The families were repeatedly evaluated in the field for quantitative resistance to late blight and maturity. Resistance was corrected for the maturity effect. Nine of eleven different map segments tagged by the markers harbored QTL affecting maturity-corrected resistance. Interactions were found between unlinked resistance QTL, providing testable strategies for marker-assisted selection in tetraploid potato. Based on the linkage observed between QTL for resistance and plant maturity and based on the genetic interactions observed between candidate genes tagging resistance QTL, we discuss models for the molecular basis of quantitative resistance and maturity.

The phylogeny of Anisoptera, dragonflies in the strict sense, has proven to be notoriously difficult to resolve. Based on morphological characters, several recent publications dealing with the phylogeny of dragonflies proposed contradicting inter- and intrafamily relationships. We explored phylogenetic information content of mitochondrial large-subunit (LSU) and small-subunit (SSU) ribosomal gene fragments for these systematic problems. Starting at published universal primers, we developed primer sets suitable for amplifying large parts of the LSU and SSU rRNA genes within dragonflies. These fragments turned out to harbor sufficient phylogenetic information to satisfyingly resolve intrafamily relationships, but they contain insufficient phylogenetic structure to permit reliable conclusions about several interfamily relationships. We demonstrate that decay of phylogenetic signal progresses from intrafamily to interfamily to outgroup relationships and is correlated with an increase of genetic distances. As expected, signal decay is most pronounced in fast-changing sites. Additionally, base composition among fast-changing sites significantly deviates from the expected homogeneity. Homogeneity of base composition among all included taxa was restored only after removing fast-changing sites from the data set. The molecular data tentatively support interfamily relationships proposed by the most recent publication based on morphological characters of fossil and extant dragonflies.

SummaryRGL sequences showed higher nucleotide diversity when compared with other sequences, suggesting evolution by divergent selection. Information on sequences, sequence similarities, SNPs and InDels is provided in a database that can be queried via the Internet.

Single-nucleotide polymorphisms (SNPs) are the most frequent DNA sequence variations, and they have become increasingly popular markers for association studies. Allelic discrimination of the mostly binary SNPs has been reported for diploid species, mainly the human, but not for polyploid genomes such as the agriculturally important crops. In the present study, we analyzed the applicability of pyrosequencing to genotyping SNPs in tetraploid potatoes. Out of 94 polymorphic loci tested, 76 (81%) proved to be amenable to allelic discrimination by pyrosequencing. An additional locus could be genotyped by the addition of an ssDNA binding protein to the pyrosequencing reaction. Of the remaining 17 loci, two failed because of the presence of paralogs in the genome, while in the other cases, self-annealing of the primer or template at the low reaction temperature (28 degrees C) employed in pyrosequencing rendered allelic discrimination impossible. The quantitative precision ofpyrosequencing was found to be similar to that of conventional dideoxy sequencing and single-nucleotide primer extension. Exceptfor some sequencespecific limitations, pyrosequencing appears to be an appropriate method for genotying SNPs in polyploid species because it is possible to distinguish not only between homoand heterozygosity but also between the different heterozygous states.