Panarctic Flora

6410 Potentilla L.

• Subgenus Micropogon Bunge.

Notes: Elven and Murray: The PAF collaborators (mainly Aiken, Elven, Eriksen, Murray, and Yurtsev) initially agreed on very little concerning Potentilla. The current account is a consensus version for some parts, a compromise for other parts. The notes below attempt to include (in some way or other) the opinions of the participants. Many of Yurtsev's views are referred under the groups and species.

Potentilla is a large genus, perhaps 500 species (Ertter et al. 2011; Soják 2007 says ca. 490 species now recognized) and also one of the largest genera in the Arctic with between 40 and 70 species (number depending on taxonomic views). Many of these are restricted to the northern regions or at least described and with types from the Arctic. Extensive speciation in the Arctic and in near-arctic areas is suggested. The problems in the genus are connected to the reproductive modes and the role of hybridization. Potentilla is part agamospermous and prone to hybridize. There is morphological and experimental evidence for widespread hybridization among Potentillas, often involving species assigned to different sections and resulting in entities often interpreted as independent species, see Soják (1986) for one of the largest intersectional hybrid groups. Several large genera in the Arctic are known with numerous hybrids, e.g., the two largest ones - Carex and Salix - but the results of the hybridization are rather different in Potentilla. This is due to the high frequency of agamospermy. Some northern Potentillas are sexual or assumed to be sexual, e.g., P. biflora, P. elegans, the P. multifida group, P. pulchella, and P. villosa. Others combine sexual and agamospermous reproduction, e.g., P. argentea s. lat., P. crantzii, and P. nivea s. lat., the agamospermy perhaps due to past infra and/or interspecific hybridization events. Many are probably entirely or predominantly agamospermous, e.g., the many proposed intersectional hybrid species. Agamospermy was documented early in Potentilla (e.g., Popoff 1935; Gustafsson 1947a, 1947b; Asker 1977, 1986; summary by Asker and Jerling 1992). It is widespread in many sections, among them those with several arctic representatives - Aureae, Niveae, Pensylvanicae - most probably also Chrysanthae. However, few arctic species have been investigated for agamospermy. In the cases studied, seed set is rarely obligately agamospermous, mostly with some sexual seeds. In addition, it depends on pseudogamy, i.e., pollination is needed for fertilization and development of the endosperm and thereby for seed development. Also pseudogamy was documented early in Potentilla (e.g., Müntzing 1928; Gentscheff and Gustafsson 1940). The pseudogamy is often more efficient with pollen from a relative than from the same species (Asker and Jerling 1992; Nyléhn 2002). Two implications of this are that morphologically slightly different apomicts easily may arise within species and attain a geographical range (suggested in, e.g., P. argentea s. lat. and P. crantzii) and that taxonomic hybrids may arise frequently, often in different regions, and become established as agamospermous biotypes and attain ranges. They may therefore appear without much consistency in their geographical ranges except for some correlation with the overlap of the ranges of the putative parents. Potentilla differs in one significant way from other large, northern groups of agamosperms. Whereas many other groups mainly consist of more or less stablilized apomicts with consistent ranges, the hybridization and creation of new apomictic hybrid species seems to very active in Potentilla, often from the same assumed parents in widely distant places. Presence of mixed sexual and agamospermous species within the same groups may be the rule rather than the exception.

There is no perfect way to handle apomictic complexes taxonomically (Asker and Jerling 1992; Eriksen 1999). The matter is simpler in mature and exclusively agamospermous (or nearly so) complexes like in the majority of Alchemilla, Hieracium, Rubus, and Taraxacum where close to nothing is known about the putative parentage of the agamospecies. In these genera, agamospecies are defined by morphological differences ('typologically'), and informal groups of similar and assumedly related agamospecies may be circumscribed to give more practical entities for, e.g., biodiversity surveys. It becomes much more difficult in the genera where there is a mixture of sexuals and apomicts, because the majority of the common taxonomic guidelines applicable to the former - e.g., reproductive system and barriers, inter-breeding, and allopatry vs. sympatry - do not apply to the latter. The main reasons for our problems in reaching a common solution for Potentilla are different viewpoints as to what constitute acceptable taxa (species) in this situation. There are two main approaches:

The typological approach. - The principle behind this approach is that taxa primarily should be defined by (morphological) differential characters and ecological and geographical consistency (as genetic interchange is not a relevant criterion among agamosperms). A species should differ more or less disjunctly from other species in several characters. It should have a consistent or at least explainable range and should preferrably be confined to a set of site types. Hybrid descendants from a certain parental combination might be definable as more than one species, depending on the morphological and eco-geographic pattern. The typological approach could be criticized as partly an eco-geographic approach but a (taxonomically) common descent is assumed even if rarely proved or in some cases improvable.

In this approach, it does not really matter whether the species in question is 'basal' and sexual or 'derived' (hybridogeneous) and agamospermous, as long as it is morphologically and eco-geographically consistent. The origin is of course important but not part of the criteria for the taxon. It also makes it possible, with some efforts and many doubts, to distinguish between stabilized taxa on one side and occasional hybrids and scattered, agamospermously proliferating hybridogeneous biotypes on the other side, as these latter normally do not have the morphological and eco-geographic consistency required. This is the approach preferred by Elven, Eriksen, and Murray.

The parental approach. - The principle behind this approach is relevant only for hybrid species. These are defined by their assumed parentage. In very many cases, the assumedly hybridogeneous and at least partly agamospermous taxa in Potentilla combine characters in a way that suggests specific parents. It is then tempting to assign a single name to all assumed offspring from a specified parental combination, e.g., that P. intermedia (or P. x intermedia) includes all offspring from crosses between P. argentea and P. norvegica. Recognizable variation within P. intermedia can then be treated as racial, if preferred. This is the approach applied by Yurtsev and Soják. Soják have followed these principles in several works, for the northern species especially Soják (1985a, 1986, 2004). Yurtsev has been slightly more prone to accept more than one taxon from the same parental combination, if morphologically different (Yurtsev 1984b, 1993).

Yurtsev: The following is an edited justification by Yurtsev for the parental approach in connection with taxa presumably developed from crosses between species of sect. [Niveae and sect. Pensylvanicae, with items added by Elven in brackets.] There are several species and subspecies which we Soják and Yurtsev interpret as hybridogeneous, having originated from crosses with various species of different sections and even subgenera. The reasons for such supposition (which of course is a hypothesis!) are: (1) Combination of characters of the proposed parents or intermediacy in the values of quantitative characters. (2) The experimental data on hybridization between species of Potentillas. (3) The fact that as a rule the proposed hybrids occur in rare sites where the rare parental species is present together with other, common parents. (4) Presence (in the sites) of populations of the hybrid species, of many individuals, sometimes topographically distinct separate from the parents. (5) The rare cases when the hybrid species has been found separately from at least one of the parents, e.g., P. borealis in the easternmost Chukchi Peninsula, though P. anachoretica occurs west and east of the locality. (6) Sufficient uniformity in the proposed hybrid species throughout their extensive range. (7) Some of the proposed hybrid species have several ploidy levels (usually 4x to 8x). (8) One cannot exclude the polytopic origin of some hybrids, most evident in the case of P. hyparctica x P. stipularis = [P. rubella], see below. Thus the migration was made, most probably, by parental species with enormous extension of the range. (9) Observations in many parts of the Arctic have convinced me that there are true ternate leaved (genotypically ternate) taxa never producing pinnate leaves (but sometimes producing digitate ones) and the genotypically pinnate ones which can produce digitate to ternate leaves under harsh conditions, but able to produce semidigitate to shortly - or normally - pinnate leaves in better environments.

Elven and Murray: The parental approach is a valid one based on several quite plausible hypotheses, e.g.: (a) origin of species by hybridization, (b) combinations of characters as indications of identifiable hybrid origin, (c) stability of characters, and (d) distinctness of taxa after origin, i.e., little or no back-crossing. It also allows hybrid species to have several origins, e.g., the assumed intersectional hybrid species P. rubella which must have arisen independently in Greenland and in several places in northern Asia from crosses between the morphologically very different P. hyparctica and P. stipularis. That species is difficult to defend under the typological approach without further investigations, as it is not geographically consistent even if its range is explainable. Its disjunct "populations" probably never had any gene exchange and will never have. This is a general problem with the parental approach, that it does not assume any coherence in the hybrid species, but this problem may be shared by a majority of agamospecies.

In principle, Yurtsev and Soják prefer to assign one and only one name to all the assumedly hybridogeneous offspring from one specified combination of parents (see especially Soják 2004). This approach is justified in the Code (McNeill et al. 2006 and earlier editions) for binomial hybrid names but it is less appropriate in other cases as in the southern Norwegian Saxifraga opdalensis and the Svalbard S. svalbardensis, both assumed developed from S. cernua x rivularis but differing constantly in several morphological characters and in addition widely allopatric. There is often a conflict in principle between defining and circumscribing taxa based on lineages (phylogenetic) or on morphology and biology. Yurtsev's item (6) above, "Sufficient uniformity in the proposed hybrid species throughout their extensive range", is not fulfilled for some of the taxa proposed below in the "parental approach". The Linnaean system of nomenclature is principally constructed for the morphological species rather than lineages (or combinations of lineages). If species names = specified (assumed) parental combinations, problems arise with the name when the assumptions change. This problem is exemplified in several of the names below. The names Potentilla petrovskyi and P. tschaunensis were synonymized when Soják (2004) assumed them to have the same parentage, without consideration or mentioning of the morphological differences that originally justified them as two species (Soják 1984; Yurtsev 1984b). The Asian P. tolmatchevii and the North American P. pedersenii should be merged according to these criteria as Yurtsev (1984b, PAF proposal) and Soják (1986, 2004) assumed both to have their origin from P. arenosa subsp. arenosa x P. pulchella. Yurtsev (PAF proposal) preferred to treat them as different species. The Asian P. uschakovii and a major part of the North American P. "rubricaulis" auct. should be merged as they both seem to have developed from P. pulchella x P. subvahliana. The morphology is radically different (as emphasized for P. uschakovii by Yurtsev below). Morphology is largely neglected in such a phylogenetic definition of presumed hybrid species. This makes it difficult, or perhaps sometimes impossible, to assign specimens and populations without knowing their full evolutionary history, which is of course impossible.

Another main problem with the parental approach is that it does not distinguish between stabilized taxa and not stabilized hybrid swarms. Its conclusions should therefore be investigated experimentally before they are accepted. The assumed separation between 'basal' or 'primary' species and 'derived' (hybrid) species may be fictive. Hybridization is a fact of life among the angiosperms and is not more 'derived' than speciation by isolation.

Yurtsev's PAF proposal. - Yurtsev proposed entry of ca. 67 species and races of Potentilla for the Checklist (see also Yurtsev 2001). Nearly half of these (33) were indicated to be hybrid species and 19 results of intersectional crosses. Hybridization between distant species is more prone to result in novelties able to stabilize, whereas hybridization between close species is more prone to result in hybrid swarms competing with their parents and perhaps taxonomically unresolvable due to back-crossing. Among other problems, this approach means that nearly a third of the proposed species (and subspecies) do not fit readily into the sectional system which is designed for the 'basal' species. The division on 'basal' and hybrid species might still be an efficient and biologically meaningful way of handling the variation when supported by experimental and populational evidence documenting the following tenets: (a) which taxa are sexual and which are apomictic, and to what degree; (b) which of the apomictic taxa are hybrids between identified species or subspecies and which are not (apomictic strains within a taxonomic species); and (c) which of the hybrid species have attained an independence and a range making them worth attention and recognition. We know close to nothing about the two first items and pretty little about the third one.

Yurtsev's proposal, based on Soják's parental approach, is fragile. It stands or falls with two of its assumptions. The first of these is that some morphological characters are a priori more important for identification of hybrid species than others, e.g., pinnate leaves (sect. Pensylvanicae) vs. ternate ones (sect. Aureae and sect. Niveae), and ternate vs. digitate ones (within sect. Aureae). Intermediacy in these features is nearly uniformly interpreted by Soják and Yurtsev as due to hybridization. The second is that hybrids or hybrid species are morphologically intermediate and not too polymorphic. We know from hybridization experiments that hybrids not necessarily are intermediate. The characters from the parents may combine in many different ways and new expressions may appear. Effects from hybridization between taxa at different ploidy levels (common in Potentilla) are largely unexplored, even if we assume (perhaps erroneously so) that the higher polyploids may show a stronger influence in the hybrids.

With this approach, studies based on herbarium specimens and on field investigations (where the putative parents and offspring can be observed "in activity") can give very different results. For arctic Potentillas, Soják's evaluations are mainly based on herbarium studies, whereas Yurtsev's also are field based in northern and northeastern Asia and to some degree in North America. The main criticism raised (and experimentally supported) against some of the taxonomic proposals of Soják, and to a lesser degree those of Yurtsev, concerns non-Russian taxa studied on herbarium vouchers alone. It may therefore be less relevant for the Russian taxa that really have been studied as populations in the field and in context with their putative parents.

Yurtsev and Soják have sometimes circumscribed the 'basal' species more narrowly than done by the majority of North American and northwestern European botanists. They have allowed less variation than other authors do within the limits of a species in, e.g., P. arenosa, P. crantzii, P. hyparctica, and P. nivea. It is not known to what degree their conclusions are based on the a priori significance of characters and on evaluation of single or a few 'typical' specimens, or on populational and regional variation in the material. Their acceptance of some taxa ('basal' and hybrid) based on very few or sometimes single specimens (e.g., P. protea Soják, P. nubilans Soják, P. insularis Soják, P. insularis subsp. spissa Soják, and the previous Svalbard record of P. lyngei Jurtz. & Soják) points towards the former, especially in regions from where they only have had a restricted herbarium material available.

Morphological and experimental investigations. - Comparatively few studies have tested the parental approach hypotheses for northern taxa (more for temperate taxa); none have looked at more than very restricted parts of the genus and of the ranges. Eriksen (1997) studied the morphological variation in Alaskan members of sect. Niveae. She accepted, e.g., one wide P. nivea s. lat. and one wide P. uniflora s. lat. Murray and Elven are now, after a closer study of the northwestern North American material, very reluctant to accept her conclusions and find that both her species are divisible into several morphologically definable and morphologically non-overlapping taxa. Eriksen and Nyléhn (1999) documented that the change from ternate via semidigitate to semipinnate leaves, assumed by Soják and Yurtsev essential for separation between sections and for identifying intersectional hybrids in some groups, could be environmentally governed in single plants and not necessarily results of hybridizations. Eriksen and Yurtsev (1999) described hair types in detail (SEM) and found them to be potentially very valuable in characterization but not to follow the lines of assumed hybridizations. This was followed up by Hamre (2000) for some North Atlantic species. Murray and Elven have utilized the hair types to separate between hybrids and non-hybridogeneous species in northwestern North America, with success, they think (Ertter et al. 2011). Eriksen and Töpel (2006) discussed the phylogeny of the circumpolar complexes of P. pulchella, P. nivea, P. arenosa (in that paper as P. hookeriana), and P. uniflora s. lat. based on a combination of hair micro-characters and molecular markers. They found much variation commented on under the species below but did not elucidate the two approaches. Töpel et al. (2006) found support for frequent allopolyploid origins in northern Potentillas but that study is preliminary.

Hansen et al. (2000) tested the hybrid hypothesis of Svalbard P. insularis (sect. Pensylvanicae x sect. Niveae according to Soják and Yurtsev) with isoenzymes and RAPDs as molecular markers. The markers assigned P. insularis close to ternate-leaved P. arenosa subsp. chamissonis, in spite of its semidigitate leaves. There were no sure signs of participation from other species of other, pinnate-leaved sections. This result still stands but needs some revision due to subsequent finds (see P. insularis and P. lyngei below).

Nyléhn (1999) investigated northern European representatives of the P. nivea group with RAPDs as molecular markers. She found three nearly equidistant groups: southern Norwegian P. nivea (P. nivea s. str.), Svalbard P. nivea s. lat. (considered by Soják and Yurtsev as the hybrid species P. subquinata), and Fennoscandian and Svalbard P. arenosa subsp. chamissonis. This conclusion was revised in a subsequent, larger investigation where Nyléhn et al. (unpubl.) found P. arenosa subsp. chamissonis to be distinct and not very polymorphic in molecular markers, whereas a large and widespread sample of Scandinavian and Svalbard P. nivea s. lat. constituted 2-3 groups but not corresponding to the three species or subspecies proposed by Yurtsev from the same regions.

Nyléhn (2002) studied P. crantzii and found that the leaf variation used by Soják for separation of northern European plants on two species - P. crantzii s. str. with five leaflets and P. gelida subsp. boreo-asiatica with three leaflets - and their offspring hybrid species P. scandica with 3-5 leaflets per leaf - was a variation within one species, often within a population, and sometimes within an individual.

Nyléhn and Hamre (2002) investigated reproductive systems in P. nivea, P. arenosa subsp. chamissonis, and the tentative intersectional and assumed agamospermous hybrid species P. insularis. They found the two former (assumed by Soják and Yurtsev to be parental in many hybrid combinations) to be nearly fully pseudogamous apomicts, whereas the last-mentioned species (assumed by Soják and Yurtsev to be a hybridogeneous endpoint) in addition produced seeds sexually. Some sexuality in hybrid species would probably be enough for the stabilization and consistency we usually prefer to have in a taxon recognized as species.

The results from the few experimental investigations throw some doubts on the basic tenets of the model of Soják and Yurtsev but at present there is no alternative to it. Such an alternative should be close to the typological approach and should not depend on any specific evolutionary theory. It should take into account the effects of hybridization and agamospermy but should not assume hybridogeneous origins of species before they are experimentally proved. Sexual taxa should be allowed to vary, perhaps more so than in many other genera due to possible effects of occasional back-crossing with not fully apomictic offspring. Taxa which appear to be (or are documented to be) apomictic should be characterized and grouped morphologically. Their possible parentage should be taken into account when several disjunct populations show similar morphological features, but parentage hypotheses should be subordinate to morphological consistency. They should be considered for acceptance as worthwhile taxa due to their morphology, their mode of reproduction, whether they form coherent populations, their ecological consistency, and the size and (not always possible for hybrids) coherence of their ranges. The last point marks a possible threshold for recognition between local agamospermous segregates and practically worthwhile taxa. Occasional agamospermous biotypes appearing here and there without much consistency in morphology or coherence should, in such an approach, not be accepted as taxa.

It is at present not possible to follow this approach due to lack of relevant information from almost the entire Arctic and especially from the large Russian area (from where much of the variation is reported), northeastern North America, and Greenland.

Only seven of the 67 proposed taxa have been non-controversial as to definition and circumscription during the PAF discussions (see also Eriksen et al. 1999). Of these seven, three are even adventives and not confirmed to have established population in the Arctic (P. neglecta, P. intermedia, P. thuringiaca), leaving four native arctic taxa (7% of those proposed) uncontested. Different opinions have been forwarded concerning nearly every major arctic species or species group: P. crantzii-gelida, the P. hyparctica aggregate, P. pulchella, and the P. multifida, the P. nivea, the P. pensylvanica, and the P. uniflora aggregates. The version presented below includes nearly all of the taxa proposed by Yurtsev but those the majority of us (editorial committee) suspect may be single or not stabilized swarms of hybridogeneous, agamospermous biotypes are entered provisionally and with our doubts expressed.

Yurtsev and/or Soják have proposed the following additional taxa or hybrids for consideration but these are omitted below: P. anachoretica x ?pulchella (Wrangel Island), P. arctolitoralis (P. arenosa x litoralis, western Alaska), P. arenosa x stipularis (West Chukotka), P. ?arenosa x subvahliana (Wrangel Island and northern Alaska-Yukon), P. arenosa x uniflora (Wrangel Island), P. hyparctica x litoralis (Hudson-Labrador), P. hyparctica x nivea (Greenland), P. taigae (P. crantzii x litoralis, Hudson-Labrador), and P. villosula var. congesta x ?vulcanicola (western Alaska).

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  Source: Dysmachus at Wikispecies