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This well supported clade corresponds to an enlarged version of the “C. rotundifolia clade” sensu Borsch & al. (2009), and comprises two main entities. A first subclade with seven North American species of bellflowers is sister to a second large subclade, encompassing the so-called “C. rotundifolia aggregate” or “alliance”, or section Heterophylla (Greuter & al. 1984, Fedorov & Kovanda 1976).

Within the first subclade all species but C. lasiocarpa (trans-pacific distribution) are North American endemics. The composition of this group matches the “Rapunculus 1a clade” of Wendling & al. (2011), to which the rare C. shetleri must be included. Despite some karyological homogeneity, most investigated species sharing a somatic number of 2 n = 34, the subclade appears morphologically heterogeneous. Nonetheless, a clade with low support for branches was depicted to comprise C. piperi and C. shetleri, two perennial species with more or less dentate margins of the mucronate leaves, occurring in alpine habitats of the northern California - southern Washington mountain ranges. More detailed biogeographic analyses remain necessary to understand the origin of this American clade, whose ancestor was hypothesized to have colonized the New World via the Beringian route (Wendling & al. 2011).

The second subclade includes most species assigned to section Heterophylla (Kovanda 1968), a particular group of long-recognized campanulas (harebells) morphologically characterized by the presence of dimorphic leaves, with reniform and petiolate basal leaves and subsessile linear cauline ones, and a basal dehiscence of the capsule (De Candolle 1830, Boissier 1875, Federov 1957). Phylogenetically, the subclade encompasses up to eight lineages, most of them monospecific, and unresolved with each other. A majority of these lines includes dwarf mountain species, morphologically well-circumscribed such as C. cenisia, C. excisa, C. cespitosa, and C. cochleariifolia, the latter two inferred as sister species. Of interest is the presence in this subclade of some isophyllous species such as C. elatinoides, C. fragilis, and C. isophylla, as already mentioned under clade Cam08. From a taxonomic point of view, the presence of C. isophylla in the Heterophylla clade can render problematic the distinction of potential isophyllous and heterophyllous groups.

Finally, a large and well-supported subclade contains c. 23 species related to C. rotundifolia, which cannot be segregated based on petD phylogenetic reconstruction alone. Several explanations can be proposed to explain such polytomy. First, polyploidy is known to occur in this otherwise well-delimited karyological group (x = 17), some species exhibiting up to 6x valence levels, overall rendering the specific limits difficult to assign (Geslot 1973, Gadella 1964). Further, most Heterophylla species show great distributional range overlap thus increasing the likelihood of genetic exchanges via introgression or homoploid/polyploid hybridization. Last but not least, the inferred crown age of that clade (1,01 Ma [0,32–3.29]) suggest very recent diversification, and does not rule out the possibility of incomplete lineage sorting between clades. Taken as a whole, these evidences explain both the phylogenetic and taxonomic confusion in section Heterophylla and particularly C. rotundifolia, a species for which some 96 heterobasionyms have been published (Lammers 2007).

Overall, this subclade should be considered a large polyploid complex similar to the many ones exemplified in both the Mediterranean and Arctic-Alpine regions of Europe, including e.g. Centaurium, Draba, or Primula (Mansion & al 2005, Guggisberg & al. 2006 a, b, Koch & Bernhardt 2004), the detailed study of which would imply particular analytical strategy (Guggisberg & al. 2009).

From: Mansion & al. (2012: 17)


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