Project 5 - Sytematics of Environmental Weeds5.1: Assessing the origins and diversity of Lantana in Australia
UPDATE - The origins and diversity of Lantana in Australia
Biological control of invasive species has had some famous successes but has frequently met with failure. For plants, successful biological control requires the identification of phytophagous or pathogenic control agents that have an appropriate ecology and that attack the target species but not native species in the introduced range. In general, agents with an appropriate host preference and ecology will have co-evolved with the target plant and will be found from the native range of the plant. Unfortunately, many of the world’s most serious invasive plants have complex introduction histories, often involving multiple introductions and repeated hybridization events, and as a consequence their biological and geographic origins are opaque. This lack of clarity imposes a significant taxonomic impediment on the targeted discovery and deployment of effective biological control agents.
Lantana is one of the world’s most serious environmental weeds and epitomizes this impediment to successful biological control. The plant is naturalized in more than 60 countries throughout the tropics and subtropics; forming dense stands in forest edges or open woodlands, and invading pastures. Forms of the plant are variously toxic to stock, resistant to herbicides, or tolerant of fire or defoliation and other types of traditional management approaches. Biological control is then the only practical method of managing this weed, yet after more than 100 years of effort and nearly fifty agents being released attempts to control the plant have met with only limited success world-wide. Resistant forms, tolerance to agent effects, and a narrow ecological tolerance of agents relative to the plant has meant that biological control has had only limited impacts.
Lantana also presents with a complex introduction history. The genus is originally from the Americas, where it has a broad range encompassing the Southern USA, Mexico and Central America, tropical and subtropical South America, and the Caribbean, Bahaman, Greater Antilles and West Indian island groups. The horticultural trade values Lantana for its flowers and foliage, and from the 1700s onwards numerous Lantana varieties were brought to Europe, hybridized and then redistributed around the world. Weedy Lantana is apparently derived from this hybrid material and as a consequence the geographic origins of the weedy forms occurring in various parts of the world are obscure. The outcome is that the targeted discovery of biological control agents for this weed is presently not feasible.
The genus Lantana (Verbenaceae) has been the subject of traditional morphological taxonomic studies. The genus currently contains around 150 named species or subspecies described on the basis of morphological characters and grouped into four sections. There are around 600 horticultural varieties. The weedy form of Lantana is thought to have arisen though the hybridisation of a multitude of Lantana species centred on Lantana sect. Camara. Given the world-wide significance of Lantana remarkably little molecular genetic work has been conducted on the weed. As part of the CERF funded Taxonomic Research & Information Network we have initiated a molecular systematic study of Lantana from both Australia and the native range in order to better understand the history of introduction to Australia.
Our results to date challenge the hybrid swarm model for weedy Lantana while at the same time providing a way forward for greatly improving the targeted discovery of biological control agents. We tested for variation at genetic loci typically used for molecular taxonomic studies and found remarkably little genetic variation both from weedy forms and from native range material. Our results suggest the hybrid swarm model is not appropriate and suggest instead that Lantana sect. Camara may instead consist of a single phenotypically diverse species that has undergone a rapid range expansion in the native range.
In recent years robust methods have been developed for exploring intraspecific relationships and relationships amongst closely related or hybrid taxa. These methods use genetic markers that carry a historical signal that is stable to admixture and subsequent recombination or other genetic homogenization processes. In the case of plants the chloroplast genome is most often used. We have identified genetic variation among the chloroplasts from native range Lantana sect. Camara material and from weedy Lantana in Australia. Chloroplast haplotype mapping suggests at least two origins for Australian Lantana – one centered on Mexico and another around Venezuela and the Dominican Republic. These preliminary findings suggest that chloroplast sequence variation has strong potential to provide a tool for identifying the geographic origins of the weedy forms of Lantana. We are currently improving the sampling density of our chloroplast haplotype mapping. Since many loci important for resistance to biological control agents will be located in the nucleus we are also developing a complementary random genetic marker based program for assessing linkage between nuclear and chloroplast based estimates of geographic origins.
Our results provide a new view into the nature of the Lantana problem and suggest that it may be tractable from a taxonomic stance.
[October 2008]
