Elevational gradient in herbivory and plant defence in wild Brassicaceae
by Janisse Deluigi
Supervisor: Prof. Yvonne Willi, Alessio Maccagni
An important open question in ecology is what drives species spatial distribution and species diversity (Gaston, 2003). While the importance of climate is well known and supported by many studies (Lee-Yaw et al., 2016), the role of biotic factors and the interaction between abiotic and biotic factors is still unclear (Sexton, 2010; Wisz et al., 2013; Louthan et al., 2015). Here, I investigated herbivore abundance, diversity and herbivore damage on an elevational gradient. At the same time, I studied differences in plant defence strategies among plant species with different elevational distribution, as plant defence is predicted to depend both on the herbivory regime and other stressors. The aim was to evaluate how elevation and herbivory pressure interact to shape plant defence strategies in species with different elevational distribution.
I performed two separate experiments, one in the field to assess herbivory over an elevational gradient, and one in climate chambers to assess plant defence under manipulated temperature conditions. The first was a transplant experiment on Calanda mountain (Chur, Switzerland) involving 30 Brassicaceae plant species differing in elevational distribution. Seeds were sown and plants raised at 5 sites over an elevational gradient of 1400m a.s.l. Variation in herbivore abundance, diversity and herbivore damage was assessed and tested for an association with the elevational gradient. The climate chamber experiment included 12 Brassicaceae species, which were subjected to one of three temperature treatments (cold, mid, warm) and one of two defence induction treatments (with or without application of jasmonic acid; 3x2 factorial design). I tested whether there was a relationship between defence strategy (physical or chemical defence) and elevational distribution of the species (high-elevation vs. mid-elevation vs. low-elevation) and whether temperature and defence induction treatment affected the type and amount of defence. In addition, I investigated the relationship between plant growth, plant defence, and herbivore resistance estimated in a bioassay with a generalist caterpillar (Spodoptera littoralis).
Although we did not find evidence for an elevational gradient in herbivore abundance and diversity and in herbivore damage, we observed an association between herbivory and temperature and their interaction in shaping plant defence. In the field experiment, increasing mean temperature at site was associated with increased damage. In the controlled-conditions experiment (climate chamber), plant defence traits were mostly affected by the warm temperature and the induction treatments, both generally increasing plant defences. In both approaches however, variability across plant species was high, suggesting different degrees of response across plant species. Interactions between both abiotic and biotic factors seem therefore to play an important role in shaping plant defence and therefore plant-herbivore interactions. Further studies are needed to better understand the influence of these interactions on the species-specific traits.
Intraspecific differentiation upon secondary contact between two subspecies of the Erebia euryale (Esper, 1804) species complex
by Selim Bouaouina
Supervisor: Dr. Kay Lucek, Prof. Yvonne Willi
Zones of secondary contact between closely related species or lineages are a common legacy of the Quaternary ice ages. Despite the abundance of such contact zones, little is known about the factors that shape interspecific gene flow and hybridization. In this study the two closely related subspecies Erebia euryale adyte (Huebner, 1818) and Erebia euryale isarica (Heyne, 1895), which are known to form zones of secondary contact in the Swiss Alps, are tested for genomic differentiation in sympatry and for morphological differentiation in both sympatry and allopatry. The formation of zones of secondary contact and previous studies suggesting strong phenotypic segregation in the contact zones, propose reinforcement as a driver of speciation between the two subspecies. Reinforcement is the process through which prezygotic reproductive isolation evolves in response to selection against costly hybridization in secondary contact zones. I analyzed morphological data of wing patterns and male valves between individuals from sympatric and allopatric populations of the two subspecies to assess the degree of morphological difference between the two subspecies. Furthermore, based on genomic data, gathered through restriction site associated DNA sequencing (RADseq), of sympatric populations I estimated the population structure across secondary contact zones. My study shows high occurrence of morphological hybrids and provides evidence for a high level of gene flow between the two subspecies in secondary contact and therefore the lack of strong prezygotic barriers.