Impact of dysploidy and polyploidy on the diversification of high mountain Artemisia (Asteraceae) and allies

Molecular cytogenetics and the study of genome size have been used to understand evolutionary and systematic relationships in many species. However, this approach has seldom been applied to alpine plants. A group of dysploid–polyploid high mountain Artemisia species, distributed from the European Sierra Nevada to Central Asian mountains, through the Pyrenees, the Alps and the Caucasus, is a good model to consider changes at the genome and chromosome levels. These small perennial Artemisia, found frequently in isolated populations, present highly disjunct distributions. Some are considered rare or even endangered. Here, we show results for nine species and 31 populations, including genome size (2C-values), fluorochrome banding and fluorescent in situ hybridisation of ribosomal RNA genes (rDNA). Significant intraspecific genome size variation is found in certain populations of A. eriantha and A. umbelliformis, but without taxonomic significance due to the absence of morphological or ecological differentiation. The number and position of GC-rich DNA bands is mostly coincidental with rDNA although there is an expansion of GC-rich heterochromatin in centromeres in some taxa. Ancestral character states have been reconstructed and x = 9 is inferred as the likely ancestral base number, while the dysploid x = 8 has appeared repeatedly during the evolution of Artemisia. On the basis of cytological observations, Robertsonian translocations are proposed for the appearance of dysploidy in the genus. A remarkable presence of x = 8-based species has been detected in the clade including high mountain species, which highlights the important role of dysploidy in the diversification of high mountain Artemisia. Conversely, polyploidy, though present in the alpine species, is more common in the rest of the genus, particularly in arctic species. Hypotheses on the mechanisms underpinning the relative abundance of dysploids and scarcity of polyploids in high mountain Artemisia are discussed.