Origins of invasive French broom in California

Background/Question/Methods

The horticultural trade provides a major pathway for the introduction of invasive plants. These invaders are often difficult to identify due to hybridization among ornamental cultivars and species, and naturalized populations. Evidence of hybridization is important because it can increase invasiveness and make management, particularly biological control, difficult. French broom, believed to be Genista monspessulana, was introduced into California by the horticultural industry and has caused serious environmental damage throughout the state. It is no longer available commercially, but its close relative, sweet broom, assumed to be Genista racemosa, is a popular ornamental and may be contributing to invasive populations. The goals of this research are to: i) identify the cultivated sources of invasive broom populations in California, and ii) determine whether hybridization between ornamental cultivars, species, and naturalized populations has occurred. To address these objectives, we collected samples from 25 invasive French broom populations from a range of altitudes and habitats throughout California, six landscape plantings, seven horticultural outlets, and nine botanical gardens and arboreta from its native range. These samples were used to reconstruct a phylogeny of brooms using two chloroplast (trnL-F spacer and tRNA-leu intron) and two nuclear (ITS and ETS) DNA regions.

Results/Conclusions

Our results suggest multiple origins of invasive French broom in California. Chloroplast and nuclear phylogenetic analyses revealed a clade containing G. monspessulana samples from its native range and the majority of invasive French broom samples from California. High altitude French broom individuals from throughout the state formed their own well-supported group within this invasive clade. Chloroplast phylogenetic reconstructions revealed an ornamental sweet broom clade containing a small number of invasive French broom individuals, all collected from naturalized populations in urban areas, and G. racemosa. Thus, preliminary results suggest that the majority of invasive French broom in California originated from G. monspessulana but that ornamental sweet broom can contribute to invasive populations, particularly in urban locations. To confirm parentage and assess hybrid origin, we are currently cloning nuclear ITS sequences from suspected hybrids. ITS sequences are non-coding DNA regions that occur several hundred times throughout the genome. The copies are usually homogenized so that only a single sequence is found within an individual. However, this homogenization process may not be complete in recent hybrids. Thus, analyzing multiple copies of ITS from suspected hybrids can give information about parentage and hybrid origin.