A system of subtribe Potentillinae J. Presl (Rosaceae Juss.)

Horkelia Cham. et Schltdl.


Introduction
Development of molecular-genetic methods in the end of XX century prompted phylogenetic research in Rosaceae Juss. and Potentilla s. l., in particular. One of the first works in this field was ITS-based phylogenetic analysis which included 14 Potentilla s. l. species and members of 19 related genera (each represented by a single species), except for Fragaria L. Relevant tree of 34 Rosaceae taxa demonstrated lack of monophyly of Potentilla. In a subsequent, ITS-and trnLF-based study of 44 Rosaceae (including 18 Potentilla s. l.) species (Eriksson et al., 2003), monophyly of Potentilleae Juss. has been confirmed.
Potter et al. (2007) Rydb. (Potter et al., 2007). The authors emphasized the lack of reliable non-molecular features for delimitation of the above subtribes.
Such a character has been reported soon by Soják (2008) who discovered that representatives of Potentillinae have anthers with two thecae while members of Fragariinae possess anthers with one theca. Thus, results of molecular phylogeny have been supported by morphological data. More recently, Faghir et al. (2014) reconstructed the phylogeny of Iranian species of Potentilla and some related genera (over 60 spp. in total) using ITS and trnLF markers. This study confirmed the monophyly of Potentilla and pored light on evolution of certain taxonomically important morphological characters. In particular, independent evolvement of anther structure, style shape and position has been demonstrated while parallel evolution was revealed for leaf morphology.

Results and discussion
The current paper presents the global system of subtribe Potentillinae comprising 12 genera possessing anthers with two thecae. Majority of genera of Potentillinae are endemic to North America and characterized by exclusively pinnate leaves (often with 4-15 pairs of deeply incised and whorled leaflets) and small, usually rather narrow petals. One of the most diverse New World Potentillinae genera, Horkelia, is distributed in the west of North America. Species of Horkelia have 10 stamens with flat broadened filaments predominantly attached to the hypanthium mouth. Carpels often numerous (20-50 and more), usually attached to the hypanthium base; inflorescence laxly racemose. The closest to Horkelia is Horkeliella represented by three species also distributed in the west of North America (Rydberg, 1908). The latter genus differs from the former by having 20 filaments and sepals recurved at anthesis.
Ivesia is another relatively big North American genus of Potentillinae which is characterized by 15-20 (rarely 10 or 5) stament with filiform filaments situated in the upper part of hypanthium, 4-5 to 10-15 carpels, often dense, subcapitate inflorescence and small flowers. Monotypic genus Purpusia (arid mountains of Nevada) is represented by cespitose-cusion plant lacking "outer calyx" and having 5 stamens attached to the hypanthium mouth and 5-7 carpels. Another monotypic American genus of Potentillinae is Stellariopsis endemic to the mountains of California and characterized by 15 stamens attached closer to the hypanthium base. It also has anthers opening by subterminal pores to short lateral slits and a single 1-seeded carpel in an open hypanthium. Leaves compound, pinnate, with numerous pairs of tightly sitting and densely pubescent small leaves.
The above six exclusively North American genera of Potentillinae demonstrate unique patterns of leaf morphology and generative organs not characteristic to the rest of subtribe including Potentilla.
Original morphology within Potentillinae is demonstrated by Duchesnea (two species and a hybrid: Soják, 2012). It is separated from Potentilla and other Potentillinae members by big, apically 3-5-dentate epicalyx scales ("outer sepals") and glabrous torus 116 with numerous ovaries, considerably enlarging and becoming globose, fleshy or spongy, red but, unlike in Fragaria, not detaching from the hypanthium. Phylogenetic studies show that Duchesnea (D. indica) falls into one, almost 100 % supported, clade with P. reptans L. and P. erecta (L.) Raeusch. (Kechaykin, 2016). R. V. Kamelin (2001) and Th. Wolf (1908) merged Duchesnea with Potentilla. The prior author placed P. indica into the section Duchesnea (J. E. Smith) B. K. Dikshit et G. Panigrahi, as Panigrahi and Dikshit (1985) have done while the latter treated the same species as a member of subsection Gomphоstylae Th. Wolf and group Tormentillae, along with P. erecta, P. reptans, P. flagellaris Willd. ex D. F. K. Schltdl., etc. (Wolf, 1908). Indeed, Duchesnea species share some characters with members of Potentilla subgen. Tormentilla (L.) Kechaykin, e. g., single flowers on long stalks. Besides, representatives of both groups are perennial herbs with short rhizome and long, creeping shoots rooting at nodes (except for P. erecta having erect stem). This morphological similarity apparently reflects common origin of Duchesnea and Tormentilla. At the same time, having in mind resemblance of fruits of Duchesnea and Fragaria and inadequate information on phylogenetic position of D. chrysantha (Zoll. et Moritzi) Miq. and D. hara-kurowae Naruh. et Sugim. it seems reasonable to treat Duchesnea for the time being as a distinct genus.
Rather separate position in the system of Potentillinae is occupied by Dryadanthe Endl. (Endlicher, 1840). The only species of this genus, D. tetrandra (Bunge) Juz., occurs predominantly in high mountains of Central and Middle Asia. It is a densely cespitose to cusion-forming perennial with woody rhizomes and stems to 15 cm tall. The genus Dryadanthe differs from Potentilla s. str. by having 4 stamens, 4 pistils, and lateral styles. In contrast to the other genera of Potentillinae, flowers in Dryadanthe are unisexual.
The second biggest genus of Potentillinae is Argentina. The centre of its diversity and speciation (and, probably, origin) is situated in mountains of Palaeotropics (South-East Asia). 20 species of Argentina (over 25% of the whole number) are endemic to New Guinea (Soják, 2012). Majority of Argentina species occur at the elevations above 3000 m a. s. l., have leaves with many (in average 10-20) pairs of leaflets; 10 species produce terrestrial creeping stolons rooting at nodes, others develop erect to ascending stems. One of the characters separating Argentina from Potentilla and other genera of Potentillinae is narrow style attached to the ventral side of achene. This feature is, however, not reliable enough as in more than 10 Asian species the styles are subterminal (Soják, 2010). Instead, the difference in stipule structure detected by Soják (2010) appears to be more sufficient for sepatarion of Argentina from Potentilla. While stipule auricles in Potentilla s. str. are lateral (stretching along the leaf petiole margins), in Argentina they are ventral, and this difference is consistent (Soják, 2010). Molecular studies (Kechaykin, 2016;Feng et al., 2014) also support distinctness of Argentina from other genera of Potentillinae. Based on these findings, we follow Hill (1756), Rydberg (1898Rydberg ( , 1908 and Soják (2004Soják ( , 2010Soják ( , 2012 in accepting Argentina as a separate genus. Fragariastrum Heist. ex Fabr. and Tormentilla L. are also accepted here as distinct genera and the need of a new typification of Potentilla L. with subsequent conservation of the latter name is stressed for which the following agruments can be put forward. First, majority of Potentilla species are taprooted polycarpic herbs (Serebryakov, 1964) with erect stems and many-flowered terminal inflorescences. It was mentioned in the literature (e. g., Kamelin, 2001) that Rydberg's (1908) lectotypification of Potentilla by P. reptans L. was not a good decision, an opinion with which we fully agree. Potentilla reptans is a stoloniferous polycarpic herb (Serebryakov, 1964) without developed inflorescence (represented by single axillary flowers on long pedicels). This life form is, with few exceptions, untypical for Potentilla. R. V. Kamelin (2001) noted that P. reptans is similar to some species of Duchesnea Sm.
Second, and most important, retention of P. reptans as the lectotype of Potentilla and acceptance of the genus Tormentilla (which contains P. reptans), well-sepatated both morphologically and molecularly, would bring severe instability in nomenclature. Namely, it would request several hundreds of transfers of species from Potentilla to Tormentilla and drastic change of the concept of Potentilla. The same action is inevitbale if the above American genera are accepted unless the paraphyletic concept of Potentilla is adopted (Ertter, Reveal, 2014).
Hence, a new typification of Potentilla is needed. The best potential type would be P. argentea L., another, along with P. reptans, original member of the genus (Linneaus, 1753). Potentilla argentea exhibits basic features typical for most Potentilla s. str. species. It is a taprooted polycarpic herb with erect stems, apical many-flowered inflorescence, and terminal styles (subterminal in Tormentilla). Unlike P. reptans, P. argentea readily hybridyze with members of variuos sections of P. subgen. Potentilla.
Kechaykin A. A., Shmakov A. I. A system of subtribe Potentillinae (Rosaceae) Traditional concept of Potentilla is adopted here which includes P. argentea and excludes its current lectotype, P. reptans. Proposal to conserve Potentilla with a new type is in preparation.
Potentilla students emphasized that among the species of the genus intensive hybridization takes place which complicates systematics and identification of individual specimens. However, no natural hybrids between representatives of Potentilla s. str. with species of Drymocallis or Schistophyllidium from subtribe Fragariinae (where P. rupestris L. and P. bifurca L., respectively, belong) are known to the date. Furthermore, no native hybrids between the genera of Potentillinae and active hybridization (often intersectional) is restricted to the genus Potentilla. This is an additional argument in favor of acceptance of Argentina, Drymocallis, Fragariastrum, Schistophyllidium, Tormentilla, etc. A key for the genera of Potentillinae as well as the system of the subtribe with some comments are given below. 3. Elements of epicalyx ("outer sepals") 2-3 as wide as sepals, apically 3-5-dentate. Torus in fruit considerably enlarged, globose, fleshy, red or pink (reminding fruits of Fragaria L.) ......... Duchesnea + Elements of epicalyx ("outer sepals") narrower than or, rarely, as wide as sepals, apically entire or rarely 2-3-dentate. Densely to loosely cespitose plants, sometimes forming cusions to 40 cm diam. Leave blades usually more than 1.5 cm long, with (3)4-6 (7)   Ten species in mountains of South and South-East Asia including New Guinea.

Argentina sect. Parvulae Kechaykin et Shmakov, sect. nov.
Usually non-cespitose plants. Leaves with more than 5 pairs of leaflets. Lateral leaflets round in outline, often with 2-6 teeth on each side. Central costa of lateral leaflet more than 3 mm long.
About 40 species in Northern Hemisphere.