Preadaptation, hybridisation, and breeding system shape the invasion of three Rumex species in New Zealand : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Research has previously shown that Rumex (Polygonaceae) species introduced to New Zealand have multiple potential drivers behind their success, such as phenotypic plasticity, enemy release, and niche shift. However, it is not known whether these changes were caused by post-introduction evolution, or what other drivers could explain the success of these agricultural weeds. I combined demo-genetic traits and processes to assess how hybridisation and introgression, genetic differentiation, and breeding system contribute to the invasiveness of three introduced Rumex species. I compared plants from the species’ native (Europe, mainly the UK) and introduced (New Zealand) range and assessed whether the success is more likely due to prior adaptation or post-introduction evolution. Ploidy is associated with increased invasiveness, and if a species has multiple geo-cytotypes, higher ploidies are often found within the introduced range. Similarly, self-compatibility can help introduced populations to counter mate limitation and mixed mating can introduce new alleles to populations. I found no differences in genome sizes or chromosome numbers between plants from the two ranges using flow cytometry and manual chromosome counts. In addition, a comparison between bagged and unbagged Rumex conglomeratus plants showed no consequences from selfing, indicating mixed mating strategies. Surprisingly, the overall seed viability was lower for provenances from the introduced range compared to the native range. Hybridisation and introgression can increase genetic variation and help with adaptation to new environments. In a field survey, hybrid plants were found in New Zealand. However, the majority were likely first-generation hybrids, making introgression an unlikely driver behind the invasiveness. In addition, the parent species co-occurrence was lower in New Zealand compared to the UK. Lastly, genetic differentiation can indicate the origin of the introduction, as well as how likely a post-introduction evolution is a driver behind the invasiveness. A minimal differentiation was revealed by genotyping-by-sequencing both within but also between the native and introduced ranges. The population genetic analyses suggest that the UK is a likely origin for these species but admixture from elsewhere was also found. This would have likely helped the introduced populations to maintain comparable level of genetic variation to the native populations. As limited differences were found between the native and introduced populations, the investigated traits and processes are unlikely to explain the invasiveness in New Zealand. Rather, the success of these species is likely caused by prior adaptation. In addition, as these species are primarily agricultural weeds within both provenances, anthropogenically induced adaptation to invade is likely the main driver behind the success of these species. This method of adaptation, likely coupled with jack-of-all-trades genotypes, have allowed the species to thrive in manmade habitats, all around the globe. Thus, similar weeds within these habitats need to be carefully monitored to prevent further invasions in the future.