Genetic Background, Ecological Causes, and Evolutionary Consequences of Flowering Initiation in Linum bienne Mill

  • Beatrice Landoni

Student thesis: Doctoral Thesis

Abstract

Plant responses to climate change partly depend on past adaptations to the historical environment across their distribution. The presence and distribution of phenotypic and genotypic variation within and across the distribution of a species can determine the viability of populations in the present, but also their responses to future climatic scenarios. A central question in evolutionary ecology is to understand the role of natural selection in maintaining phenotypic and genotypic variation across species’ distributions. Species with wide geographical distributions are ideal to study plant adaptation under a variety of environmental conditions. In temperate regions, predictable fluctuations in environmental variables, such as photoperiod, temperature, or precipitation co‐vary with latitude, and are used by plants to time their flowering phenology. This mechanism ensures that reproduction happens under optimal conditions. At the same time, plants can evolve optimal phenotypes along environmental gradients such as the one represented by latitude. Thus, differentiation in flowering initiation along environmental gradients can be interpreted as the signature of plant adaptation to local climatic conditions. Individuals can also adjust to specific environmental conditions within their lifetime via phenotypic plasticity, for example by adjusting the initiation of flowering in response to temperature. Plasticity can also produce geographical patterns of variation across species´ distributions and can be itself adaptive. In the present thesis, I studied the variation of flowering time and correlated traits in Linum bienne Mill., a species with a distribution spanning the Mediterranean Basin and the Atlantic coasts of Europe, up to the British Isles. The goal of the thesis was to understand what environmental factors shape flowering time variation and life history at large across the distribution of L. bienne. By means of field and greenhouse experiments, as well as genetic characterization of populations, the thesis aims to: 1) assess the presence of latitudinal variation in flowering initiation and whether it reflects adaptation to local climatic conditions at the western edge of the species distribution; 2) assess whether flowering time variation underpins variation in life history at large and investigate the presence of ecotypes relative to drought seasonality based on life history traits; 3) describe the wider evolutionary context in which populations have differentiated using a phylogeographic approach. An important result of the thesis is that flowering
initiation varies along latitude in the western range of L. bienne. Southern populations flower earlier, while northern populations flower overall later, but advance flowering if vernalized, consistent with latitudinal adaptation. This
variation associates with variation in other life history traits by which populations can be grouped into putative ecotypes, Mediterranean and Atlantic. The phylogeographic analysis also shows these two groups correspond to two
out of three ancient lineages identified in L. bienne. Despite these lineages are distinct, there are signs of chloroplast capture, which probably result from the history of recolonization and secondary contact during the Quaternary across
its latitudinal range. Populations are also differentiated for plasticity in responses to temperature, drought, and its timing, sometimes in a way consistent with the expectation of adaptation. Overall, patterns of variation in flowering initiation and life history along the latitudinal range of L. bienne (and beyond) are partly a result of spatially varying selection, thus reflect adaptation, but also arise because of ecogeographical isolation and neutral processes.
Date of Award22 Sep 2022
Original languageEnglish
SupervisorRocio Perez-Barrales (Supervisor) & William Scott Armbruster (Supervisor)

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