Sex ratio variation and sex determination in Urtica dioica

This thesis will first document on variation in progeny sex ratios among individual female plants of Urtica dioica at our field site in Meijendel (Chapter 2). Next, we show that there is also considerable sex ratio variation among male and female flowering shoots in 26 natural populations studied (Chapter 3). Additionally, we studied life history traits of male and female clones to detect sex-specific differences that might have contributed to the sex ratio bias observed in the field. Our results indicate that the sex ratio bias in natural populations may be a consequence of a bias that already originated in the primary sex ratio. Next, we investigated physiological, environmental (Chapter 4) and genetic (Chapter 5) aspects of sex determination. For the latter aspect, a series of crosses including male, female and monoecious plants of U. dioica was performed. These experiments were designed to investigate whether sex ratio variation in the primary sex ratio is due to environmental sex determination (ESD) or genetic sex determination (GSD) solely or due to an interaction of both. Genetic mapping of sex-linked markers was used as a supplementary tool to analyze the genetic mechanism of sex determination (Chapter 6). Finally, to study the inheritance pattern of the sex ratio trait, crosses were performed between individual male and female plants from different sex ratio families (Chapter 7).UBL - phd migration 201

Resistance to three thrips species in <i>Capsicum</i> spp. depends on site conditions and geographic regions

Capsicum species are commercially grown for pepper production. This crop suffers severely from thrips damage and the identification of natural sources of thrips resistance is essential for the development of resistant cultivars. It is unclear whether resistance to Frankliniella occidentalis as assessed in a specific environment holds under different conditions. Additionally, other thrips species may respond differently to the plant genotypes. Screening for robust and general resistance to thrips encompasses testing different Capsicum accessions under various conditions and with different thrips species. We screened 11 Capsicum accessions (C. annuum and C. chinense) for resistance to F. occidentalis at three different locations in the Netherlands. Next, the same 11 accessions were screened for resistance to Thrips palmi and Scirtothrips dorsalis at two locations in Asia. This resulted in a unique analysis of thrips resistance in Capsicum at five different locations around the world. Finally, all accessions were also screened for resistance to F. occidentalis in the Netherlands using a leaf disc choice assay, allowing direct comparison of whole plant and leaf disc assays. Resistance to F. occidentalis was only partially consistent among the three sites in the Netherlands. The most susceptible accessions were consistently susceptible, but which accession was the most resistant differed among sites. In Asia, one C. chinense accession was particularly resistant to S. dorsalis and T. palmi, but this was not the most resistant accession to F. occidentalis. Overall, resistance to F. occidentalis correlated with S. dorsalis but not with T. palmi resistance in the C. annuum accessions. Damage inflicted on leaf discs reflected damage on the whole plant level. Our study showed that identifying broad spectrum resistance to thrips in Capsicum may prove to be challenging. Breeding programmes should focus on developing cultivars suitable for growing in defined geographic regions with specific thrips species and abiotic conditions