Spatio-ecological segregation of diploid and tetraploid cytotypes of Galium valdepilosum in central Europe.

The Galium pusillum agg. (Rubiaceae), with four species native to the Czech Republic, is a taxonomically challenging complex. Of these, G. valdepilosum is particularly interesting because this relict species shows both ploidy (the incidence of diploid and tetraploid cytotypes) and habitat differentiation (occurrence on different soil types, including serpentines). With the aid of DNA flow cytometry, analysis of vegetation samples and a hydroponic cultivation experiment we addressed the cytogeographic pattern, ecological preferences of different cytotypes both across the entire range of distribution and in the contact zone and the plant’s response to serpentine edaphic stress. Ploidy distribution in G. valdepilosum is parapatric, with a narrow contact zone in southern Moravia. Neither triploids nor mixed 2x-4x populations were found, which together with the restriction of the species to isolated relict habitats, suggest the static character of the contact zone. In general, tetraploids occupied a wider range of habitats and colonized larger geographic areas. Diploids typically occurred in open low-competitive oak-pine forests on acidic soils while their tetraploid counterparts were also able to survive in open basiphilous grasslands with a comparatively higher competitive pressure. Serpentines did not play an important role in ecological sorting of the cytotypes. Cultivation experiments showed that G. valdepilosum is likely to be constitutively tolerant to serpentine chemical stress. Relative genome size and ecological data indicate that the serpentine populations from western Bohemia, traditionally referred to as G. sudeticum, differ from the type subalpine populations from the Krkonoše Mts and suggest their merger with G. valdepilosum.