Draba L.
Brassicaceae (Cruciferae), Draba family.
Sp. Pl. 642. 1753.
Vegetative morphology. Plants low, densely or loosely tufted perennials with entire or toothed leaves. Aerial stems erect. Leaves distributed along the stems; alternate; simple; existing for a single season or less, or marcescent. Leaf blade bases attenuate.
Reproductive morphology. Flowering stems present. Petals free. Stamens 6. Placentation parietal. Fruit a silique.
Chromosome information. 2n = 16, 32, 48, 64, 80.
Notes. The genus Draba is well know for its taxonomic
complexity in arctic and alpine floras, and the polyploids in particular present
vexing taxonomic problems (Brochmann et al. 1992). It has been suggested that
polyploids in Draba may have formed recurrently from different
populations of the parental species (polytopy), and it is also possible that a
given taxonomic species may actually comprise several polypoid races, each
originating from different progenitor species.
Brochmann et al. (1992)
investigated three morphologically variable Nordic species and their possible
progenitors using enzyme electrophoresis and restriction site analysis of
chloroplast DNA (cDNA) and nuclear ribosomal RNA (rDNA) genes and found the
electrophoretic data showed high levels of fixed heterozygosity in the three
polyploids showing that they are all genetic alloploids. The considerable
electrophoretic variation observed in D. norvegica suggests three, and
probably 13, independent origins. Electrophoretic and rDNA data suggest that
D. lactea and D. corymbosa are polyphyletic polyploids. Brochmann
et al. (1992) concluded that given the widespread geographic distribution of the
three species and their possible progenitors, and that the populations analyzed
represented only a small feracton of their geographic distributions, it is
likely that these species have formed numerous times in different areas.
Brochmann (1992) studied pollen and seed morphology of Nordic Draba
from 54 populations representing 15 species and found pollen size was strongly
correlated with chromosome number, but was unreliable for inferring the exact
ploidal level of individual populations. Five pollen types were recognised based
on the scupturing of the exine. Brochmann (1992) found that seed size and weight
were only weakly correlated with chromosome number, but showed a close
relationship to habitat ecology. The largest seeds were observed in species
typical of closed habitats, in which seedling establishment is probably limited
by competition with mosses. The seed coat surface had a characteristic verrucate
reticulum, but the differentiatioNorth Americaong species was vague or absent.
Seed size and colour distinguishes some species, e.g. in the D. alpina
complex.
Brochmann et al. (1992a) in a study of gene flow across ploidal
levels in Draba found that interspecific hybridization across ploidal
levels in the genus may result in re-establishment of fertility and probably of
euploid chromosome numbers suggesting that the taxonomic complexity in
Draba is also contributed to by the recurrent formation of
allopolyploids.
Brochmann et al. (1993) investigated 101 populations of
Nordic Draba in 16 species and found discrepancies between genetic and
taxonomic relationships in the taxa. The relationships inferred from the
crossing data largely agreed with those inferred from previously published
molecular data, but corresponded poorly to relationship inferred from
morphology.
Brochmann (1993), investigated reproductive strategies of 132
diploid and polyploid populations of arctic Draba (2x -16X) to clarify
possible relationships between reproductive strategies and polyploid evolution
in the genus. The populations were exlusively sexaul and produced viable seed
after spontaneous self-pollination, but showed large variation both in traits
promoting cross-pollination and in autogamous fruit and seed set. Traits
promoting cross-pollination, e.g. floral display, protogyny, and delayed
selfing, were positively correlated, and these traits were negatively correlated
with autogamous seed set. All diploid and many polyploid populations had high
autogamous seed set and small unscented, non-protogynous and rapidly selfing
flowers. In contrast, all populations with low autogamous seed set and large,
scented, and strongly protogynous flowers with distinctly delayed selfing were
polyploid. The result wer consistent with isozyme studies and suggested that the
genetic depauperate diploids are exteme inbreeders and that the highly
fixed-heterozygous polyploids vary from extreme inbreeders to mixed maters.
Brochmann (1993) found that the reproductive data added support to the
hypothesis that allopolyploidy in arctic Draba serves as an escape from
genetic depauperation caused by uniparental inbreeding at the diploid level.
Brochmann and Elven (1992) examined possible differences in ecological
amplitude between the diploids and the highly fixed heterozygous, genetic
variable, polyploids in the Nordic area examining 443 populations of three
diploid and 12 polyploid (4x-16X) species of Draba. They found that the
diploid species occupy largely similar, restricted niches and are predominantly
selfing, stress-tolerators setting few small seeds. Most polyploid species occur
in a wide range of habitats and are either moderately selfing, stress-tolerant
competitors setting few, large seeds or predominantly selfing ruderals setting
many small seeds. They suggested that although the alloploid condition buffers
the effect of inbreeding with respect to loss of viability, mixed mating is
advantageous in habitats with high levels of competition. Brochmann and Elven
(1992) found ecological amplitude as well as heterozygosity and biochemical
diversity in Draba increase significantly with ploidal level suggesting
that the evolutionary success of allopolyploidy in this genus may be based on
increased ecological as well as genetic potential.
The drabas present in the
CAAF area fall into several groups, some of them with unresolved variation, and
a series of single species without other arctic (American) close relatives. The
groups are usually easily recognised but the entities within them are more often
problematic both to identify and to fully characterise. The groups and single
species are:
A. D. alpina group
D. alpina
D.
corymbosa
probably one or more unresolved species
B. D.
micropetala group
D. micropetala
D. pauciflora (= D.
adamsii)
probably one unresolved species
C. D. cinerea group
D. cinerea (perhaps not arctic)
D. arctica
D.
oblongata
D. D. glabella group
D. glabella
D.
juvenilis (=D. longipes, perhaps not in group)
E. D. fladnizensis
group
D. fladnizensis
D. subcapitata
F. D.
nivalis
G. D. lactea
H. D. norvegica group
D.
norvegica
D. arctogena
I. D. oligosperma
J. D.
crassifolia.
Illustrations. • Close-up of plant. Short plants growing between the markers. Nunavut, Baffin Island, Iqaluit. Aiken and Mallory, 02–005. Scale bar in cm. • Close-up of leaves. Leaves of a Draba plant that are glabrous except on the margnis where there is a ciliate fring of hairs. Aiken and Mallory, 02–005. CAN. • Fruit. Genus characterized by siliques; fruit that have a partition with seeds attached to the edges of the partition and not to the ovary wall.
Cite this publication as: ‘S.G. Aiken, M.J. Dallwitz, L.L. Consaul, C.L. McJannet, L.J. Gillespie, R.L. Boles, G.W. Argus, J.M. Gillett, P.J. Scott, R. Elven, M.C. LeBlanc, A.K. Brysting and H. Solstad. 1999 onwards. Flora of the Canadian Arctic Archipelago: Descriptions, Illustrations, Identification, and Information Retrieval. Version: 29th April 2003. http://www.mun.ca/biology/delta/arcticf/’. Dallwitz (1980) and Dallwitz, Paine and Zurcher (1993, 1995, 2000) should also be cited (see References).
