Senecio madagascariensis in Southern South America: evidence for multiple introductions

Abstract

Multiple introductions are a key factor in facilitating the success of biological invasions, as they enhance genetic diversity and adaptability in novel environments. Senecio madagascariensis Poir. (fireweed) is an invasive plant species that has rapidly proliferated across southern South America. To understand the origins and genetic dynamics underlying its invasion, we analyzed 730 individuals from 44 populations using ITS sequences and microsatellite markers. The ITS analysis revealed 19 haplotypes, indicating at least two distinct invasion waves. One wave, dominated by haplotype H3, likely originated in South Africa and reached Argentina in the 1940s. The second wave, characterized by haplotype H1, spread to northeastern Argentina, Brazil, and Uruguay, showing genetic similarities to populations from Australia, the Hawaiian Islands, and South Africa. Microsatellite data demonstrated moderate genetic diversity (FST = 0.067) and a significant correlation between genetic and geographic distances (Mantel test, r² = 0.43), supporting an isolation-by-distance pattern. STRUCTURE analysis identified three primary subpopulations that clustered into two major genetic groups, NOA-BA (Northwest Argentina-Buenos Aires) and NEA-BR-URU (Northeast Argentina-Brazil-Uruguay). Higher gene flow and genetic admixture in the NEA-BR-URU group suggest multiple introduction events. Our findings highlight the roles of genetic diversity, gene flow, and local adaptation in the successful spread of S. madagascariensis across South America. These results underscore the value of population genetics in understanding invasion dynamics and offer insights for targeted management strategies to control this widespread invader.