Panarctic Flora

500725-27 The Saxifraga rivularis aggregate S. bracteata, S. hyperborea, S. rivularis

Geography: Circumpolar-alpine.

Notes: Jørgensen, Gabrielsen, Brochmann, and Elven: The Saxifraga rivularis aggregate is a small, arctic-alpine group where 6-7 species have been named, five of arctic relevance: S. rivularis described from Europe and now considered broadly amphi-Atlantic; S. bracteata described from the Russian Far East and coastal amphi-Pacific/Beringian; S. hyperborea described from northern Canada and high-arctic circumpolar; S. flexuosa described from East Chukotka and amphi-Pacific; and S. arctolitoralis described from East Chukotka and until recently considered arctic Asian Beringian but now found to be much more widespread.

The lowest chromosome number in the group, 2n = 26, is functionally diploid but genetically allotetraploid. All taxa are well documented as to chromosome numbers. Saxifraga bracteata, S. flexuosa, and S. hyperborea are diploids (2n = 26), S. arctolitoralis and S. rivularis are tetraploids (2n = 52) but with some aneuploidy (see Guldahl et al. 2005).

This group and the next one (the S. sibirica aggregate) have been studied for molecular and morphological variation. Results for the S. rivularis aggregate are presented by Jørgensen et al. (2006; molecular, cytological, and morphological data, revision etc.), and Guldahl et al. (2005; molecular, ploidy, and morphological data) and can be summarized as follows. Saxifraga bracteata is morphologically and genetically distinct from the others. The others are more similar both in morphology and molecular markers (RAPD, AFLP) but are separable, except for S. flexuosa which we can not separate in molecular markers and scarcely in morphology from S. hyperborea. One important diagnostic feature in the aggregate is that the diploids S. flexuosa and S. hyperborea are without underground runners, whereas the diploid S. bracteata and both the tetraploids S. arctolitoralis and S. rivularis have such runners. The other diagnostic characters are found in shape of hypanthium and sepals, in coloration, and in the pubescence (see, e.g., Guldahl et al. 2005). Jørgensen et al. (2006) presented descriptions and a key. Saxifraga rivularis is an allotetraploid with S. hyperborea or a progenitor as one of its parents (Guldahl et al. 2005) and S. bracteata or a progenitor as the other (Jørgensen et al. 2006). Saxifraga arctolitoralis has the same parentage taxonomically as has S. rivularis (Jørgensen et al. 2006). The origin of the original allotetraploid seems to be Beringian. The geographical separation and divergence of amphi-Atlantic S. rivularis and amphi-Beringian and North American S. arctolitoralis is probably secondary. Our conclusion is that they merit rank as (parapatric) subspecies but not as species. More comments below.