The role of genetic variability and phenotypic plasticity in the invasiveness of Acacia longifolia

Acacia longifolia is a leguminous species native to Southeast Australia and Tasmania. The species was introduced outside Australia and is particularly invasive in Mediterranean-climate regions. Acacia longifolia has two described subspecies – ssp. longifolia and ssp. sophorae – recognized based on differences in morphology and distribution. The invasiveness of A. longifolia has been linked to its prolific seed production leading to the formation of long-lasting seed banks and its capacity to establish symbioses with nitrogen-fixing rhizobia. The population genetic diversity and structure of native and invasive populations of A. longifolia, the identification of native sources of invasive populations, and the contribution of genetic diversity and phenotypic plasticity to the species’ invasiveness are understudied. Moreover, molecular support for the taxonomic classification of the two subspecies of A. longifolia is needed to assist effective management of invasive populations. Therefore, the major objectives of this thesis were to: 1) provide insightsinto the introduction histories of Australian acacias around the world and their resulting genetic consequences; 2) understand the worldwide invasive history, genetic diversity, and population genetic structure of A. longifolia; 3) test whether population genetics support the classification of A. longifolia as two subspecies; 4) clarify the roles of genetic diversity and phenotypic plasticity in the invasiveness of A. longifolia; and 5) provide more detail on the species’ flower development and reproductive success, and how these are influenced by environmental conditions. To accomplish these aims, various species of Australian acacias and their introduction histories outside of Australia were included in a meta-analysis. Results revealed that acacias generally have not experienced genetic bottlenecks or increased inbreeding upon introduction. The population genetic analysis of A. longifolia followed this general observation, making the identification of native sources of invasive populations challenging. Moreover, the native range population structure of A. longifoliais determined by geographic features and not subspecies identity. This was further supported by Species Distribution Models based on bioclimatic niche variables and a common garden experiment subjecting seedlings of both subspecies to different levels of water and nutrient availability. Rather, in its native range, the species consists of two genetic clusters, corresponding to mainland Australia and Tasmania, while the invasive range lacks genetic structure, yet has similar levels of diversity to native range population. Carbon and nitrogen isotope fractionation of A. longifolia phyllodes, which are tracers of plant-environment interactions, indicated different strategies of resource acquisition and conservation. The common garden experiment also revealed that invasive A. longifolia seedlings have more limited responses to stress than native seedlings. The study of the species’ flowering showed a resource trade-off of “quantity over quality”, independent of environmental conditions: the large number of flowers and amount of pollen produced are counterbalanced by their short functional periods and extended receptivity of the stigma for reproductive assurance. In conclusion, the extensive human-mediated introduction of A. longifolia has significantly shaped the species population genetic diversity and structure making it difficult to identify the native sources of invasive populations. Such information is crucial for management strategies, as well as risk assessment and impact prediction.