Inferring the complex origins of horticultural invasives: French broom in California

Investigating the origins of invasive populations provides insight into the evolutionary and anthropogenic factors underlying invasions, and can inform management decisions. Invasive species introduced for horticultural purposes often have complex origins typified by multiple introductions of species, cultivars, and genotypes, and interspecific and intraspecific hybridizations in introduced ranges. Such complex introduction histories may result in complex genetic signatures in the invaded range, making inferences about origins difficult, particularly when all putative sources cannot be sampled. In this study, we inferred the origins of the invasive French broom complex in California using 12 nuclear microsatellite markers. We characterized the genetic diversity and population structure of invasive and horticultural brooms in their invaded range in California and of Genista monspessulana in its native Mediterranean range. Overall, no significant differences in allelic richness, observed heterozygosity, inbreeding, or genetic structure were observed between the invaded and native ranges, but differences existed among populations within ranges. Bayesian STRUCTURE analysis revealed three genetic clusters in the French broom complex. Nearly all native G. monspessulana assigned highly to a single cluster. Many invasives assigned to a second cluster that contained Genista canariensis, Genista stenopetala, and ornamental sweet broom, and the remaining invasives assigned to a third cluster that also contained some G. monspessulana individuals from Sardinia and Corsica. Admixture between the second and third clusters was detected. Approximate Bayesian Computation analysis of six alternative scenarios supported the hypothesis that some invasive French broom is derived from an unsampled population branching from ornamental sweet broom. A combination of factors, including multiple introductions, escapes from cultivation, and inter-taxon hybridization, likely contribute to the invasive success of French broom in California and may have important implications for management, in particular biological control.