3430 Dupontia R. Br.
- R. Br., Chlor. Melvill.: 32 (1823).
Notes: Elven: From a morphological viewpoint, Elven, Murray, and Tzvelev would prefer to accept two or perhaps three taxa. From a molecular viewpoint, the investigations of Brysting et al. (2003, 2004) throw strong doubts on whether one should accept more than one undivided species in the genus Dupontia.
Brysting and Aiken: The treatments of the genus Dupontia are different in North America, Greenland-Svalbard, and Russia. Cayouette and Darbyshire (2007b) recognized only one species in the genus for North America. Bay (1992) and Elven and Elvebakk (1996) recognized two species - D. fisheri and D. psilosantha - for Greenland and Svalbard, respectively. Czerepanov (1995) treated the three taxa recognized by Tzvelev (1976) for Russia as full species without further comments.
Brysting et al. (2003) studied Dupontia by numerical taxonomy from herbarium specimens, including types and chromosome vouchers. They concluded that morphological characters used in the literature to divide the genus into more than one taxon, cannot be reliably applied to distinguish most North American plants. In North America, we might see two endpoints, which correspond more or less to D. psilosantha and D. fisheri, respectively, as these two are recognized in the floras of, e.g., Greenland and Svalbard. However, as suggested by Polunin (1940), there appears to be a continuum of variation among North American specimens and too many intermediate forms exist, which are impossible to place in two or three categories. Even though only a limited sample of Eurasian specimens were included in the analyses, there is no reason to believe that inclusion of more specimens from these parts of the Arctic will change this conclusion. On the contrary, the continuity in measurements will probably be even more evident. Until further evidence is available, Brysting et al. suggest that the genus is treated as monotypic.
Molecular evidence supports the use of a wide species circumscription (Brysting et al. 2004). DNA sequence data (trnL-trnF, ITS; including plants from Russia, Svalbard, Canada, and Alaska) revealed almost no variation within Dupontia, only a couple of base pair changes, which were related neither to ploidy levels, nor to geography. DNA fingerprinting analysis (AFLP) sorted the Dupontia material first of all according to geography (Russia, North America, Svalbard). Within each geographical region, there was a tendency that plants were sorted according to ploidy level, suggesting that the higher ploidy levels most likely have arisen several times and in different regions within the Arctic.
Two main ploidy levels have been reported in the literature and associated with different taxa: 2n = 42 and 44 (4x) with "psilosantha", 2n = 84 and 88 (8x) mostly with "fisheri". The variation in chromosome numbers within each ploidy level is probably the result of Bchromosomes, which are obviously present in these plants (Flovik 1938). Two counts of 2n = 132 have previously been reported from Southampton and Devon islands in the Canadian Arctic (Bowden 1960a; Löve and Löve 1975c). Results from recent chromosome counts and flow cytometry (Brysting et al. 2003, 2004) have revealed that this high-ploid level (2n = 126-132) is widely distributed in arctic parts of Canada and Alaska and mainly associated with tall, coarse plants with more or less spreading branches and therefore usually identified as ["psilosantha" in spite of their coarseness].
Löve and Ritchie (1966) assigned the three ploidy levels to three different species: D. psilosantha was considered tetraploid with 2n = 44, D. pelligera octoploid with 2n = 88, and D. fisheri dodecaploid with 2n = 132. Lipkin (1983) identified, however, D. fisheri subsp. fisheri with the octoploid cytotype (2n = 88), the type material of which is from Melville Island in the Canadian Arctic Archipelago. This is in agreement with recent chromosome counts of 2n = 88 for Melville Island plants morphologically inseparable from the type material (Brysting et al. 2003). The dodecaploid is therefore without name. The name [D. pelligera most probably belongs to the octoploid and is synonymous with D. fisheri].
Counts of 2n = 66 have been obtained from Alaskan (Lipkin 1983) and Svalbard plants (Brysting et al. 2004), probably as a result of hybridization between plants at different ploidy levels. In both places, the possible hybrid plants were growing in close proximity to plants with 2n = 44 and 2n = 88. In the Alaskan plants, seed production and percent germination of the presumed hybrid plants seemed comparable to nonhybrid plants (Lipkin 1983). The evidence so far (including preliminary results from isoenzyme analysis, Brysting et al. 2004) suggests that the ploidy levels within Dupontia have arisen by autoallopolyploidy rather than allopolyploidy.
In the literature, it has been suggested, e.g., by Tzvelev, that the genus Dupontia has an intergeneric hybrid origin, involving at least the closely related Arctophila fulva or a predecessor. The sequencing data (cpDNA and rDNA) confirm a close relationship between Dupontia and Arctophila. The cpDNA (trnL-trnF) sequences of Dupontia are identical or nearly identical to those of Arctophila, suggesting that the latter is the likely maternal parent in a possible hybridization event. The rDNA sequences (ITS) of Dupontia are similar to the A. fulva sequences, as well. Even though several copies are present (and separated by cloning of the PCR products), these vary by only two to three base pair changes from each other and from the A. fulva sequences. From two of the Dupontia plants, one diverging sequence copy was obtained. However, this sequence copy was found in very low frequency in the plants (one out of 20-25 sequenced colonies), and did not point out any second parental species in particular. In the parsimony analysis, this sequence copy resulted in instability and changed position in the tree, sometimes grouping together with the Poa taxa, sometimes together with A. fulva and the remaining Dupontia copies. Genomic in situ hybridization (GISH) experiments, using DNA from A. fulva as a labelled probe, did not result in any discrimination within the Dupontia genome. The Arctophila probe hybridized strongly to the entire Dupontia genome, supporting the results from the molecular analyses: 1) The very close relationship between Dupontia and A. fulva; and 2) if Dupontia has a hybrid origin, it is certainly not a recent event, and the Dupontia genome must have been homogenized towards the mother genome (Brysting et al. 2004).
From a Panarctic perspective, the best solution will be to operate with one variable species within Dupontia (D. fisheri). There obviously is more variation than what fits into the two-three species or subspecies proposed in recent times, and at the moment the available evidence does not suggest a subdivision of D. fisheri.
Elven: Another corollary from the molecular and hybridization evidence is that it might be problematic to retain Arctophila and Dupontia as two genera. The priority genus name is Dupontia. Until the identification of a second parent, we propose no nomenclatural change.
Elven, Murray, and Tzvelev: There is a pattern in the morphological variation that is difficult to ignore. The plants at the lowest ploidy level (2n = 42, 44) seem to have the slender "psilosantha" morphology throughout, predominate in the southern Arctic, and prefer very wet sites, also brackish marshes. Those at the middle level (2n = 84, 88) have the stout "fisheri"-"pelligera" morphology, predominate in the northern Arctic, and prefer moist sites: shallow mires and sediment flats. Those at the highest ploidy level (2n = 126-132) have a coarse "psilosantha" morphology, are found mainly in the boreal to southern arctic zones, and are typical of wetlands and brackish marshes. This pattern is repeated in several parts of Eurasia and North America.