Panarctic Flora

6721 Draba L.

• L., Sp. Pl.: 642 (1753).
• Erophila DC., Mém. Mus. Hist. Nat. 7: 234 (1821) nom. cons..
• Nesodraba Greene, Pittonia 3: 252 (1898).
• Excluded: Draba aleutica (see D. tschuktschorum), D. baicalensis, D. lanceolata, D. ogilviensis (see D. sibirica), D. paysonii, D. porsildii, D. praealta, D. ruaxes, D. sambukii, D. villosula.

Notes: Draba is the largest genus of Brassicaceae and among the most species-rich and complicated arctic genera. A subgeneric classification would be helpful. There are, however, too many ambiguities about where some species belong, e.g., in the system of series by Tolmachev (1939). Allopolyploidy may confuse the limits between sections or series based on the diploids. We have refrained from acceptance of formal subgeneric categories and rather treated some of the more obvious groups as aggregates.

An updated account of the North American and Greenlandic species, as far as they currently are known, is Al-Shehbaz et al. (2010a). The most updated account for the northern Russian areas is still Tolmachev (1975b).

Large parts of northern Draba is still problematic taxonomically. One of these is the group of yellow-flowered "Chrysodraba" species (01-14 below). All these have been denoted as D. alpina in one or more contexts. The two features keeping the taxa previously assigned as D. alpina together are that they are scapose (without stem leaves) and with yellow or yellowish petals. The now designated Scandinavian type of the name D. alpina connects it to a decaploid and probably not very broadly amphi-Atlantic species (see below). The majority of the material previously named as such fits into other named species but a significant part remains unnamed, especially in American Beringia. The "Chrysodraba" group is almost certainly polyphyletic and consisting of several fairly independent evolutionary lineages. One of these is the D. micropetala aggregate, containing at least three species, two of them documented at comparatively low ploidy levels (4x-6x). The many other yellow-flowered species are at higher levels (8x-18x among those species counted) and may be more or less distinct allopolyploids from hybridization among numerous and only distantly related low-ploid lineages. This is documented for the three species investigated by combined molecular and cytological methods (Brochmann et al. 1992): D. alpina, D. corymbosa (extremely polyphyletic), and D. oxycarpa. Two of the morphologically more similar species - D. alpina and D. oxycarpa - are quite distant genetically (Brochmann et al. 1992). It should be noted that the difference between yellow and white flowers (reflected in the old division between "Chrysodraba" and "Leucodraba") may be less important than assumed. The most convincing hybrids between species with yellow and white flowers - D. arctica (white) x D. corymbosa (yellow), and D. lactea (white) x Svalbard D. oxycarpa (yellow), have white flowers. It is also possible (and even probable) that more than one group of yellow pigments are involved.

Another partly unresolved group is centered on the white-flowered D. cinerea aggregate (species 25-29) and on the also white-flowered D. rupestris (30). In the D. cinerea aggregate, the number of proposed species may be larger than the number of "real" taxa. Draba rupestris (= D. norvegica auct.) is probably an unresolved complex of several species (morphological and genetic data).

The Checklist treatment of Draba is preliminary and tentative in many aspects. It is probable that several taxa (probably species) remain to be recognized, characterized, and named, especially in American Beringia.

Erophila must be included in Draba (Koch et al. 2001; Koch and Al-Shehbaz 2002). Two investigated parts of Erophila (E. verna and E. spathulata) were assigned within a major clade of Draba in analyses based on several gene sequences, which means that some other groups of Draba (e.g., the D. aizoides group) must be recognized as genera if Erophila is accepted. This is not surprising. The status of Erophila - as a genus or as part of Draba - has been disputed almost throughout its nomenclatural history.

Yurtsev: Nesodraba is often treated as part of Draba (series Hyperboreae Gilg) but we accept the genus due to monopodial growth with lateral scapes, inflated siliquas, lobate leaves, and a deviant flower type.

Elven: A molecular analysis of American Drabas (Koch and Al-Shehbaz 2002) did not include this species. Al-Shehbaz et al. (2010a) included it in Draba for Flora of North America with an argument. We accept their reasons aand treat it as Draba for the Checklist. Note, however, the indication of a base chromosome number of x = 9.