Plant reproduction in the alpine landscape : reproductive ecology, genetic diversity and gene flow of the rare monocarpic Campanula thyrsoides in the Swiss Alps

Abstract

Aims & Objectives The work presented in this thesis forms part of a larger project “How patchy habitat and isolation affect alpine plant life: genetic diversity, gene flow and mating systems”, which includes the PhD studies of Patrick Kuss and the author under the supervision of Professor Jurg Stocklin. This doctoral thesis investigates the consequences of the natural fragmentation and patchiness of alpine landscapes on the life of alpine plant populations. The central focus of the thesis is on the mating system, the role of inbreeding and/or outbreeding depression, genetic diversity and geographic structure within and among populations of the rare Alpine monocarpic perennial Campanula thyrsoides. The main objectives and research questions addressed are: • Is Campanula thyrsoides self-compatible (SI) and if not, does the SI system break down with flower age? Do inbred C. thyrsoides offspring in the common garden suffer from inbreeding depression? • Do we find a distance related inbreeding depression (poorer reproducive output) or outbreeding depression (increased reproductive output) in field populations of C. thyrsoides following crosses of different crossing distances (selfing, 1m, 10m, 100m and among distant populations)? • How much genetic diversity exists within populations of C. thyrsoides and how does it relate to population size and altitude? Has the natural habitat fragmentation let to strong genetic differentiation and restricted gene flow among populations of C. thyrsoides resulting in a pronounced geographic structure? Study species In order to seek answers to our research questions, we choose to study a yellow bellflower; Campanula thyrsoides. The choice was based on the information that C. thyrsoides is a rare plant species, which is only found on calcarious soils within the European Alps and adjacent mountain ranges (Aeschimann et al. 2005). The plants selectiveness for carbonate bearing soils together with the fact that its seeds are not adapted to long-distance dispersal (Tackenberg 2003) are the main reasons for the isolation and small sizes of many of its populations. These population characteristics, therefore, made C. thyrsoides a suitable study species. Another important characteristic of C. thyrsoides, and one of the main reasons for its inclusion in the study is because it is a monocarpic perennial which flowers once and subsequently dies (Jager 2000). Monocarpic plants species, which are more commonly found in subtropical and tropical mountain systems (e.g. the giant rosettes of Puya spp, Espeletia spp., Echium spp. etc., Smith & Young 1987; Young & Augspurger 1991) are rare amidst the temperate alpine flora (for the Alps, see Aeschimann et al. 2005). Monocarpy can promote genetic differentiation between populations by reducing the effective population size due to a shorter generation time and lower density of populations (Loveless & Hamrick 1984; Vitalis et al. 2004). When studying the effects of population isolation and habitat fragmentation on plant reproduction (e.g. mating system and inbreeding depression), it is, moreover, ideal to study a Campanula species. Although most Campanula species are selfincompatible and allogamous (Nyman 1993), both a break-down in the SI system with flower age (Vogler et al. 1998) and an evolution towards complete self-compatibility (AEgisdottir & Thorhallsdottir 2006) have been recorded. Design We studied the reproductive ecology and genetic diversity of Campanula thyrsoides by firstly setting up pollination experiments in the common garden and in the field and secondly by sampling leaf material in 32 field populations in Switzerland. In the common garden study, we set up a pollination experiment in order to study the breeding system of C. thyrsoides, including the consequences of selfing, half-sibling crossings and outcrossing on reproductive output and seedling performance. Moreover, field experiments in four populations were set up in the Swiss Alps in order to study the effect of different crossing distances on reproduction in C. thyrsoides and to see if evidence would be found of hidden inbreeding depression or outbreeding depression following large-distance crossings compared to within-population crossings. In addition, we studied the genetic diversity, gene flow and geographical structure within and among 32 field populations of C. thyrsoides in Switzerland, covering both large geographical and altitudinal ranges. The genetic study was conducted using 5 co-dominant microsatellite markers. In addition, we studied the genetic diversity in C. thyrsoides and two other alpine plants using random amplified polymorphic DNA (RAPD) marker as well as studing the evolutionary demography of C. thyrsoides.