Invasive Senecio madagascariensis Poir. and the Senecio pinnatifolius A.Rich. complex (Senecioneae): Evolutionary relationships and their implications for biological control research

Abstract

Fireweed (Senecio madagascariensis, Senecioneae) is native to southern Africa and Madagascar but has become an invasive weed in southern South America, Japan, Australia, and Hawai'i. One strategy for reducing the impact of fireweed is classical biological control (biocontrol), i.e. the release of natural enemies of a weed such as pathogens or insects sourced from its home range. Before release, candidate agents are subjected to rigorous host specificity testing to minimise the risk of collateral damage to non-target species. It is impor tant to include non-target species that are closely related to the target weed in these experiments, because candidate biocontrol agents are more likely to attack them than distantly related species. However, Australian biocontrol research on fireweed has for a long time suffered from confusion about the taxonomy and evolutionar y relationships of fireweed and its presumed closest Australian relatives from the Senecio pinnatifolius complex. We provide an over view of the histor y of taxonomic and phylogenetic perceptions and relevant studies and illustrate that although S. pinnatifolius belongs to the Australasian clade of Senecio most closely related to fireweed, the two are not closely related in the context of the overall evolutionar y histor y of the genus. We conducted a phylogenetic analysis of Senecioneae incorporating new sequence data for 38 specimens including all seven extant varieties of the S. pinnatifolius complex. The varieties were placed in different clades, suggesting that the species as currently circumscribed does not constitute a natural group, and that the varieties cannot be used interchangeably in biocontrol research. Fur ther research into the complex is needed to arrive at a more appropriate taxonomy. Senecio skirrhodon was sequenced for the first time, confirming it to be closely related to fireweed, S. har veianus, and S. inaequidens.