Invasive fireweed in Australia: exploring the invasion dynamics of Senecio madagascariensis using population genetics.

Abstract

Senecio madagascariensis (fireweed) is an herbaceous flowering plant native to South Africa and known to be invasive in Australia, Argentina, Brazil, Hawaii and Japan. Fireweed first appeared in the Australian herbarium record in 1918 but little is known about its invasion dynamics. This thesis presents detailed molecular genetic analyses of S. madagascariensis in Australia and native South Africa, as well as broad biogeographic analysis of S. madagascariensis invasions from around the world. The thesis aims were to elucidate geographic source, mode of introduction, spread dynamics, hybridisation outcomes with an Australian congener, and potential for adaption to the Australian environment. Using nuclear and chloroplast microsatellites, populations from across the range in Australia and in the KwaZulu Natal province of South Africa were genotyped. Additionally, chloroplast microsatellites were used to genotype all S. madagascariensis specimens held in the Queensland Herbarium, National Herbarium of Victoria and National Herbarium of New South Wales, and from contemporary populations in all other known invasive ranges of the species across the world. Amplified fragment length polymorphisms (AFLPs) were used to study the outcome of hybridisation between S. madagascariensis and a S. pinnatifolius ‘dune variant’ in natural populations, and to look for evidence of potential selection acting on the genome. Results indicate that S. madagascariensis was introduced at least twice to Australia and has resulted in sequential bottlenecks due to seeding of new invasion foci from material within Australia. Global S. madagascariensis invasions have limited diversity and are consistent with secondary invasions originating in Australia. Hybridisation between S. madagascariensis and S. pinnatifolius ‘dune variant’ occurs in natural populations at low levels but does not appear to result adult hybrid plants at least in the populations studied here. Two AFLP loci were highlighted by both population level outlier analyses, and individual level regression analyses against environmental variable data, providing evidence for potential recent selection on the genome and an indication of putative selective agents. In conclusion, this thesis explores the invasion history and contemporary invasion dynamics of one of Eastern Australia’s worst weeds, S. madagascariensis. Findings highlight the ongoing need for appropriate biosecurity measures to limit accidental founding of further invasion foci and flag Australia as a potential bridgehead for S. madagascariensis invasions worldwide. Location of putative source populations points to areas which may prove fruitful for locating suitable biological control agents. The native S. pinnatifolius is unlikely to introgress with S. madagascariensis but risks displacement in native environments where the two occur together. Evidence of potential recent selection on the genome associated with rainfall and light availability could indicate that S. madagascariensis is adapting to Australian conditions, making further encroachment on the range of S. pinnatifolius more likely.