A molecular phylogenetic analysis of invasive and ornamental brooms and their relationships within the Genistoid legumes

The Cytisus-Genista complex includes species that have become invasive following introduction into new geographic ranges as ornamental shrubs. Despite their impacts, the evolutionary relationships among invasives, ornamentals, and native-range species have never been investigated. Our objective was to examine relationships within the Cytisus-Genista complex to determine (1) the taxonomic identity of invasive “French broom” and ornamental “sweet broom” and (2) whether “sweet broom” contributes to “French broom” populations directly or via hybridization. We used sequence data from chloroplast and nuclear regions to gain insight into evolutionary origins and to confirm taxonomic status. Our phylogenetic analyses suggest a complex evolutionary history that includes hybridization events. Placement of invasive and ornamental individuals within the Cytisus-Genista complex resolves taxonomic uncertainty in these groups, as our phylogenetic analyses recovered separate “French broom” and “sweet broom” clades within the G. monspessulana clade in the genus Genista. Extensive cloning and sequencing of the ITS region revealed that, although the majority of invasive “French broom” in California is Genista monspessulana, hybridization with individuals from the ornamental “sweet broom” clade likely occurs in populations throughout the state.

Graphical abstractDownload full-size imageHighlights► We investigated the evolutionary origins of invasive and ornamental brooms within the Genisteae. ► We used sequence data from two cpDNA and two nrDNA regions. ► Invasive French broom and ornamental sweet broom are phylogenetically nested within Genista. ► Our results suggest that most invasive French broom in California is G. monspessulana. ► Ornamental sweet broom can contribute to invasions directly and via hybridization.