Why study the population genetics of ragweed?
Ragweed displays large genetic variability in its native range, North America. As ragweed colonizes disturbed and agricultural habitats, the genetic structure of ragweed populations is strongly influenced by human-driven dispersal processes. These processes results in genetic admixture (interbreeding between genetically differentiated sub-populations). Novel genetic combinations are produced, which may favor invasiveness.
In Europe, ragweed is known to have been introduced repeatedly at multiple locations. As a result, its population genetic structure is more complicated than in the native range. Previous studies have shown that populations from Eastern and Western Europe are, to some degree, genetically distinct, which suggests that they originate from geographically distant sources in the native range. In both areas, however, colonization is not associated with any detectable genetic bottleneck, and genetic admixture is ubiquitous. As a result, the genetic variability of Ambrosia in even higher in Europe than in the native range, and one can hypothesize that this large genetic variation is the source for novel combinations of adaptive traits values and enhanced ability to adapt to a diversity of climatic conditions and habitats.
Past population genetic studies have been successful in documenting how the colonization of Europe proceeded for Ambrosia artemisiifolia. However, many questions remain to be addressed.
First, how colonization does proceed at finer geographical scales is still not well known. What is the relative importance of short distance versus long-distance dispersal events? What are the respective roles of different landscape elements in the colonization processes?
Second, several details about the genetic structure of populations remain to be investigated. Do recently founded populations have a lower genetic variability than more ancient ones? Is genetic admixture present only the core of the present range or also at the colonization front? Is genetic variation spatially structured at regional to local scales?
Third, adaptive genetic variation has not been much investigated. Is there a relationship between neutral genetic variation as revealed by molecular markers and genetic variation for traits? Are established populations locally adapted to their environmental conditions? Can we detect some pre-adaption to the environmental conditions of areas still not colonized? Or, conversely, will some genetic constraints (e.g. limited trait variation or antagonistic correlations among traits) limit adaptation?
Population genetic studies require co-dominant, polymorphic and repeatable molecular markers. At present only a handful microsatellite markers are available. Our first objective is to develop new sets of markers. For this, we will rely on latest next-generation sequencing technologies.
The genetic structure of populations and dispersal processes will be studied at regional to local scales, with a particular emphasis on the role of agricultural landscapes, waterways and road networks.
Common garden experiments are being conducted to investigate genetic variation of phenological traits such as flowering time, and pre-adaption to the climatic conditions of Northern Europe. The genetic determinants of pollen allergenicity and associated genetic variation are also investigated.
At present, about 12 researchers from 7 countries (Armenia, Austria, France, Hungary, Italy, The Netherlands and Sweden) participate in the Task Force.