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12 May 2020 Identity of the Calcarata species complex in Viola sect. Melanium (Violaceae)
Sandra Krause, Joachim W. Kadereit
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Abstract

The Calcarata species complex in Viola sect. Melanium (Violaceae) is a group of species from Italy and neighbouring islands. The complex is of considerable evolutionary interest because several hypotheses about hybrid speciation within the group have been previously proposed. Because the Calcarata complex is not well characterized morphologically, we used 142 samples representing 92 (of c. 120) species of V. sect. Melanium plus three outgroup species. Nuclear ITS and ETS and plastid trnStrnG intergenic spacer sequences were analysed to test the monophyly of the Calcarata complex and to infer relationships among the constituent species. Both nuclear and plastid sequences resulted in very limited phylogenetic resolution. Based on the nuclear dataset, most species of the Calcarata complex were recovered in four clades that also contained species not previously associated with the complex. Results from the plastid dataset recovered most species of the complex in a large polytomy. However, one larger clade containing only Calcarata complex species could be recovered, and species of all four nuclear clades were part of this larger plastid clade. The Calcarata complex clearly could not be resolved as monophyletic. We hypothesize that the lack of phylogenetic resolution may result mainly from frequent hybridization and hybrid speciation, processes that are well documented for Viola and V. sect. Melanium.

Citation: Krause S. & Kadereit J. W. 2020: Identity of the Calcarata species complex in Viola sect. Melanium (Violaceae). – Willdenowia 50: 195–206.

Version of record first published online on 12 May 2020 ahead of inclusion in August 2020 issue.

Introduction

Interspecific hybridization has enormous evolutionary potential and can result in homoploid or polyploid hybrid speciation in addition to introgressive hybridization (Arnold 1997). Homoploid hybrid speciation appears to be rare and has been documented more or less convincingly in fewer than 30 cases (Yakimowski & Rieseberg 2014; Kadereit 2015; Schumer & al. 2014; Nieto Feliner & al. 2017). In contrast, polyploid hybrid speciation may account for 2% to 15% of speciation events in flowering plants (Otto & Whitton 2000; Wood & al. 2009).

Viola L. (Violaceae) is a genus of approximately 525 (Ballard & al. 2014) or 580 to 620 species (Wahlert & al. 2014) distributed in temperate regions and montane areas in the tropics worldwide (Ballard & al. 2014). Several species have long been the subject of intensive “bio-systematic” studies (Briggs & Walters 2016), including studies of hybrid speciation based mainly on chromosome numbers, morphology, ecology and geographical distribution (Clausen 1926, 1927, 1931; Valentine 1950, 1958; Moore & Harvey 1961; Schmidt 1961, 1962, 1963, 1964; Harvey 1966; Küpfer 1971; Merxmüller 1974; Merxmüller & Lippert 1977; Erben 1985).

Several of these studies examined the Italian species of the informally named Calcarata complex (Merxmüller 1974; Merxmüller & Lippert 1977; Pignatti 1994; Erben & Raimondo 1995; Fenaroli & Moraldo 2003). The unranked taxon Calcaratae was first introduced by Becker (1910). In his treatment of Viola in the 2nd edition of Die natürlichen Pflanzenfamilien (Becker 1925), which is the last comprehensive treatment of the entire genus, Becker characterized his V. sect. Melanium Ging. [unranked] Elongatae W. Becker [unranked] Crenatifoliae W. Becker [unranked] Calcaratae W. Becker as violets with large flowers with long or short spurs, often basally dentate sepals and pinnate to dentate stipules. Gams (1925) characterized V. [unranked] Calcaratae as having long and creeping primary axes with leaves and flowers on lateral axes. Becker (1925) divided V. [unranked] Calcaratae into V. [unranked] Eucalcaratae W. Becker with long and acute spurs and V. [unranked] Altaicae W. Becker with long or short but obtuse spurs. These two groups were claimed to be distributed mostly in the Alps and southern European mountains, the Mediterranean islands (V. [unranked] Eucalcaratae) and from North Africa to East Asia (V. [unranked] Altaicae).

Following Becker (1925), Viola [unranked] Eucalcaratae contains V. calcarata L. from the Alps, V. bertolonii Pio emend. Merxm. & W. Lippert from Italy and V. corsica Nyman from Corsica, while V. [unranked] Altaicae contains V. altaica Ker Gawl. from southeastern Europe through the Caucasus to China, V. dichroa Boiss. from Turkey, V. arsenica Beck from southeastern Europe and V. eugeniae Parl., V. nebrodensis Presl and V. pseudogracilis Strobl from Italy as well as V. munbyana Boiss. & Reut. from North Africa.

In the course of time, the use of the name Viola [unranked] Calcaratae, as the Calcarata complex (V. calcarata-Gruppe; Schmidt 1964), was limited to mostly Italian species, and the complex was considered by Schmidt (1964) to contain taxa with large flowers. Many of the species considered to belong to the Calcarata complex or found to be associated with it by us, have flowers > 2 cm, and sometimes up to 4 cm wide (Valentine & al. 1968; Pignatti 2017). Pignatti (2017) characterized the complex as often having dimorphic leaves where the lower and upper cauline leaves differ in shape, complex stipules (pinnately or palmately divided), and thin and rather long (20–30 cm) ascending axes. According to Pignatti (2017) the complex occurs at higher elevations in Italy and on Sicily, Sardinia, Elba and Corsica. Following Schmidt (1964) and Pignatti (1994, 2017), the Italian Calcarata complex contains the following 15 species: V. aethnensis Parl., V. bertolonii, V. calcarata, V. corsica, V. culminis F. Fen. & Moraldo, V. dubyana Burnat ex Gremli, V. etrusca Erben, V. eugeniae, V. ferrarinii Moraldo & Ricceri, V. merxmuelleri Erben, V. nebrodensis, V. pseudogracilis, V. tineorum Erben & Raimondo, V. ucriana Erben & Raimondo and V. valderia All. Several of these species were treated by Fiori (1924) as varieties of V. calcarata. These species were grouped by Pignatti (2017) with V. argen-teria Moraldo & Forneris, V. cenisia L., V. comollia Massara and V. magellensis Porta & Rigo ex Strobl, which all have more or less undivided stipules, in V. sect. Melanium group I [unranked] Heterophyllae Pignatti. It is conceivable that some of the species newly described by Ricceri & al. (2018) also belong to this complex. Schmidt (1964) suspected that the Italian Calcarata complex species might be closely related to species from the Balkans and Greece.

The Calcarata complex clearly belongs to Viola sect. Melanium, a group of more than 120 species (Valentine & al. 1968; Merxmüller & Lippert 1977; Erben 1984, 1985, 1986, 1989; Erben & Raimondo 1995; Ballard & al. 1999; Erben 2000; Fenaroli & Moraldo 2003; Blaxland 2004; Moraldo & al. 2011; Marcussen & al. 2015; Margini & Scoppola 2015; Ricceri & al. 2018; Perrino & al. 2018). Viola sect. Melanium is widely distributed in western Asia and Europe, with its centre of diversity in the hills and mountains of southern Europe, especially the Balkan Peninsula and the Apennines (Erben 1996). The only species of V. sect. Melanium to occur in the New World is V. bicolor Pursh from eastern North America (Clausen & al. 1964). Previous studies have recovered V. sect. Melanium as monophyletic (Ballard & al. 1999: two species; Yockteng & al. 2003: 20 species; Marcussen & al. 2015: three species; Slomka & al. 2015: 25 species), but very little phylogenetic resolution within the section was obtained using ITS sequences (Yockteng & al. 2003; Slomka & al. 2015).

In preparation of a detailed study of hybrid speciation in the Calcarata complex using restriction-site associated DNA sequencing (RADseq), we set out to investigate whether this Calcarata complex, which is not well-characterized morphologically, can be resolved as monophyletic using Sanger sequencing data, and whether species from outside Italy and the listed Mediterranean islands also belong to this Calcarata complex. For this purpose, we inferred a phylogeny of Viola sect. Melanium using 92 species and DNA sequence data from the nuclear ribosomal internal transcribed spacer region (ITS), the nuclear ribosomal external transcribed spacer region (ETS) and the plastid trnStrnG intergenic spacer region.

Material and methods

Taxon sampling

A total of 142 samples of Viola sect. Melanium, representing 92 species (including seven species with more than one subspecies or variety), were included in our analysis. Leaf material was taken either from herbarium specimens or was available as silica-dried material collected during fieldwork in 2018. Based on the results by Marcussen & al. (2015), V. hirta L. (V. sect. Viola), V. cazorlensis Gand. (V. sect. Delphiniopsis W. Becker) and V. scorpiuroides Coss. (V. sect. Xylinosium W. Becker) were included as outgroups. These three sections had been identified as close relatives of V. sect. Melanium. Specimen voucher information is listed in  Supplementary Table 1 (wi.50.50205_Supplementary_Table_1.pdf) (see Supplemental content online).

DNA extraction, PCR amplification and sequencing

Total genomic DNA was extracted using the Qiagen DNeasy Plant Mini Kit (Qiagen, Hilden, Germany) following the manufacturer's protocol but with lysis for 30 minutes. We sequenced the nuclear ITS and ETS regions and the plastid trnStrnG intergenic spacer, which included part of the trnS gene (71 bp). PCR amplification of the ITS region was performed using the primer pair ITS1/ITS4 (White & al. 1990). For the ETS region we used the primer pair jKETS-9/ETS-18S (Mitchell & al. 2009; Wright & al. 2001), and for trnStrnG, the primer pair trnS/trnG (Cennamo & al. 2011). PCR products were purified with ExoSap-IT PCR Clean-Up (Affymetrix, Santa Clara, California, U.S.A.). Cycle sequencing was carried out with BigDye Terminator 3.1 (Applied Biosystems, Foster City, California, U.S.A.), using the same primers as for the PCR amplifications. The fluorescence labelled samples were run on an ABI 3130xl Genetic Analyzer at Johannes Gutenberg-Universität Mainz (Germany) for sequencing.

DNA sequence alignment and phylogenetic analysis

Contigs of forward and reverse sequences were assembled and manually edited using Sequencher v.4.1.4 (Gene Codes, Ann Arbor, Michigan, U.S.A.). Sequences were aligned automatically with Mafft7 (Rozewicki & al. 2019) and manually adjusted using MEGA V7.0.21 (Kumar & al. 2016). Phylogenetic analyses were carried out using Maximum Likelihood (ML) and Bayesian inference (BI). The resulting topologies were compared and inspected for supported conflicts as described by Pirie & al. (2008). Since there were no conflicts between the nuclear datasets, we combined ITS and ETS sequences. All sequences obtained in this study were submitted to GenBank ( https://www.ncbi.nlm.nih.gov/genbank/), and accession numbers for the sequences are given in  Supplementary Table 1 (wi.50.50205_Supplementary_Table_1.pdf) (see Supplemental content online). The ML analyses for each individual marker were run on the CIPRES Science Gateway (Miller & al. 2010), using RAxML v.8.2.8 (RAxML-HPC2 on XSEDE) with default settings (Stamatakis 2014). Bayesian inference was performed using BEAST v.1.8.3 (Drummond & Rambaut 2007; Drummond & al. 2012). For each aligned locus, the best-fit substitution model was detected using PartitionFinder2 on XSEDE (Lanfear & al. 2016). HKY + G + I was suggested as the most appropriate nucleotide substitution model for the trnStrnG spacer region, ITS1, ITS2 and ETS, and JC for trnS and 5.8S. The Birth-Death Process was chosen as the tree prior. The individual output log files were examined using Tracer v.1.5 (Rambaut & Drummond 2009) to assess convergence. The first 1000 trees (10 %) were discarded as burn-in, and a maximum clade credibility tree was computed using Tree-Annotator v.1.8.3 (Drummond & al. 2012).

ITS substitution rate

In order to compute an ITS substitution rate based on a crown group age of Viola sect. Melanium of between 12.76 and 15.26 (means) million years (ma) as estimated by Marcussen & al. (2015), we used r8s (Sanderson 2003) following the protocol by Lanfear & al. (2013) and using 12.76 ma as minimum and 15.26 ma as maximum crown group ages.

Results

The sequence alignments were 491 bp (ITS), 471 bp (ETS) and 628 bp (trnStrnG, trnS gene) long. The combined nuclear dataset and the plastid dataset contained 132 and 134 accessions, respectively. The ML and BI phylogenies from the combined nuclear dataset and from the plastid dataset showed supported topological differences. In general, the ML/BI plastid phylogenies were far less resolved than the nuclear phylogenies. Apart from the absence of a few clades in the ML phylogeny, the ML and BI topologies did not differ. Accordingly, the phylogenetic trees shown (Fig. 1, 2) represent the BI topology with ML bootstrap support (BS) added. Branches with BS < 50 or posterior probabilities (PP) < 0.95 were collapsed.

Viola sect. Melanium was recovered as monophyletic in all analyses. In the following, we will describe our results with reference to the Calcarata complex as described in the introduction. Most species of the Calcarata complex group in four larger clades in the BI analysis of the nuclear dataset (Fig. 1). Viola aethnensis subsp. aethnensis, V. corsica subsp. limbarae Merxm. & W. Lippert, V. dubyana, V. merxmuelleri and V. valderia were recovered outside these clades in larger polytomies. In the following description, clades with species of the Calcarata complex are given capital letters in the nuclear dataset (Fig. 1) and numbers in the chloroplast dataset (Fig. 2). These clades are also listed in Table 1.

Clade A (PP 1, BS 86) contains Viola nebrodensis, V. ucriana, V. tineorum, V. bertolonii and V. ferrarinii of the Calcarata complex together with V. cornuta L. from the Pyrenees, V. grisebachiana Vis. and V. orphanidis Boiss. both from the Balkan peninsula and V. paradoxa Lowe from Madeira. In this clade, three species of the Calcarata complex from Sicily, V. nebrodensis, V. ucriana and V. tineorum, form a supported subclade (PP 1, BS 76). Of the Calcarata complex species of clade A, V. ferrarinii and V. bertolonii fall into clade 2 (PP 1, BS 64) in the BI analysis of the plastid data (Fig. 2; Table 1), and V. nebrodensis and V. ucriana form a supported clade (1) (PP 1, BS 70; Table 1). Clade B (PP 0.98, BS 62) contains V. eugeniae subsp. levieri (Parl.) A. F. W. Schmidt, V. eugeniae subsp. eugeniae, V. pseudogracilis subsp. cassinensis (Strobl) Merxm. & A. F. W. Schmidt, V. aethnensis subsp. splendida (W. Becker) Merxm. & W. Lippert, V. corsica subsp. ilvensis (W. Becker) Merxm., V. aethnensis subsp. calabra (A. Terracc.) Peruzzi and V. pseudogracilis subsp. pseudogracilis of the Calcarata complex plus the Italian V. magellensis and the Greek V. rausii Erben, V. graeca (W. Becker) Halacsy and V. sfikasiana Erben. Of the Calcarata complex species of clade B, V. corsica subsp. ilvensis is part of clade 2 in the BI analysis of the plastid data (Fig. 2; Table 1). All other Calcarata complex taxa are part of large polytomies in this analysis. Clade C (PP 1, BS 72) contains V. corsica subsp. corsica and V. etrusca of the Calcarata complex together with V. montcaunica Pau from Spain. Of the Calcarata complex species of clade C, V. corsica subsp. corsica is part of clade 2 in the BI analysis of the plastid data (Fig. 2a; Table 1) and V. etrusca and V. montcaunica are supported sister to each other (clade 3; PP 0.99, BS 68; Fig. 2; Table 1). Clade D (PP 1, BS 88) contains V. culminis, V. calcarata subsp. calcarata, V. calcarata subsp. cavillieri (W. Becker) Merxm. & W. Lippert, V. calcarata subsp. zoysii (Wulf.) Merxm. and V. calcarata subsp. villarsiana (Roem. & Schult.) Merxm. of the Calcarata complex. They group together with the widespread V. lutea Huds. subsp. lutea and V. beckiana Fiala from the southern Balkan peninsula. Of the Calcarata complex taxa of clade D, V. calcarata subsp. villarsiana and V. calcarata subsp. cavillieri are part of clade 2 in the BI analysis of the plastid data (Fig. 2; Table 1). Viola culminis is part of a supported clade (4) together with V. comollia in the plastid phylogeny (PP 1, BS 60; Fig. 2; Table 1), and the remaining Calcarata complex taxa of clade D are part of large polytomies in this analysis.

In summary, species of the Calcarata complex fall into four larger clades in the BI analysis of the nuclear dataset. Interestingly, some species from these four clades are part of the only larger supported clade (2) in the plastid phylogeny, which also contains Viola corsica subsp. limbarae, which is not part of any larger clade in the nuclear phylogeny. All species recovered in clade 2 of the plastid phylogeny are part of the Calcarata complex.

The ITS substitution rate calculated using the crown group age of Viola sect. Melanium estimated by Marcussen & al. (2015) is 0.57 × 10-9 substitutions/site/year.

Discussion

Considering the phylogenetic topologies obtained by us from the analysis of nuclear (ITS/ETS) and plastid (trnStrnG) sequences (Fig. 1, 2), it is obvious that neither the Italian Calcarata complex as understood by Schmidt (1964) and Pignatti (1994, 2017) nor Becker's (1925) Viola [unranked] Calcaratae was recovered as a monophyletic group. In the nuclear dataset, most taxa of the Italian Calcarata complex – except V. aethnensis subsp. aethnensis, V. corsica subsp. limbarae, V. dubyana, V. merxmuelleri and V. valderia – fall into four larger clades which, however, all also contain additional species that had not previously been considered part of the Calcarata complex. In the plastid dataset, some species of all four nuclear clades fall into clade 2, which contains only Calcarata complex species. However, the majority of Calcarata complex species are found in unresolved positions in a large polytomy.

Table 1.

Clades recovered in the analyses of the ITS/ETS and trnStrnG datasets. Species of the Calcarata complex are marked in bold script. Placement of species of ITS/ETS clades in trnStrnG clades and of species of trnStrnG clades in ITS/ETS clades is indicated in parentheses.

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Fig. 1.

Bayesian inference (BI) phylogeny of Viola sect. Melanium based on the combined nuclear dataset (ITS/ETS). Values above branches are Bayesian posterior probability and maximum likelihood bootstrap values (PP/BS). Only posterior probabilities ≥ 0.95 and bootstrap values ≥ 50 are shown. A hyphen (-) represents posterior probabilities < 0.95 or bootstrap values < 50. Absence of clades is indicated by n.p. (not present). Capital letters refer to clades discussed in the text. Species in bold script have been considered part of the Calcarata complex in the past.

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The question arises as to why the Calcarata complex/Viola [unranked] Calcaratae is not resolved as monophyletic and why, more generally, phylogenetic resolution is rather poor in both our nuclear and plastid phylogenies, as also found by others (e.g. Ballard & al. 1999;Yockteng & al. 2003; Wahlert & al. 2014; Slomka & al. 2015). The lack of resolution could be caused by insufficient sequence divergence (Maddison 1989), incomplete lineage sorting, or hybridization (Doyle 1992; Maddison 1997). In this case, we postulate that hybridization is the major problem in reconstructing phylogenetic relationships in Viola in general and in V. sect. Melanium in particular. First, there exist a large number of studies which have either documented interspecific hybridization in the genus and in V. sect. Melanium (Clausen 1927; Fothergill 1938; Erben 1985, 1996; Pettet 1964; Kakes 1979; Krahulcová 1996; Marcussen & al. 2001; Conesa & al. 2008) or hybrid speciation (Erben 1996; Küpfer 1971; Merxmüller & Lippert 1977; Pignatti 1994; Erben & Raimondo 1995; Erben 1996; Fenaroli & Moraldo 2003; Siuta & al. 2005; Marcussen & al. 2012), and the role of hybridization in the origin of supraspecific lineages in Viola, suspected previously by Ballard & al. (1999), has been convincingly demonstrated by Marcussen & al. (2015). An example of hybrid speciation in V. sect. Melanium inferred by nuclear-plastid incongruence may be highlighted by V. corsica subsp. ilvensis. This taxon has been hypothesized by Pignatti (1994) to have originated from hybridization between V. bertolonii, with which it groups in clade 2 of the plastid phylogeny (Fig. 2), and V. eugeniae or V. pseudogracilis, with which it groups in clade B in the nuclear phylogeny (Fig. 1). Second, evidence for hybridization may also be provided by the limited amount of ITS sequence divergence in the section. If indeed, as estimated by Marcussen & al. (2015), Viola is of Oligocene origin and the crown group age of V. sect. Melanium is between 12.76 and 15.26 (means) ma, the ITS substitution rate calculated by us is 0.57 × 10-9 substitutions/site/year. Such a rate clearly falls outside the known range of ITS substitution rates for herbaceous annual or perennial plant species of 1.72 × 10-9 to 8.34 × 10-9 substitutions/site/year and is similar to the lowest rates otherwise found only in woody plants (Kay & al. 2006). The low substitution rate found by us is then perhaps best explained by hybridization across the entire section, resulting in homogenization of younger ribotypes leading to the exclusion of older ribotypes.

Fig. 2.

Bayesian inference (BI) phylogeny of Viola sect. Melanium based on the plastid dataset (trnStrnG). Values above branches are Bayesian posterior probability and maximum likelihood bootstrap values (PP/BS). Only posterior probabilities ≥ 0.95 and bootstrap values ≥ 50 are shown. A hyphen (-) represents bootstrap values < 50. Numbers refer to clades discussed in the text. Species in bold script have been considered part of the Calcarata complex in the past.

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Based on our findings, we hypothesize that hybridization has been so frequent in the evolution of Viola sect. Melanium that tree building methods such as ML and BI are not suitable for the reconstruction of phylogenetic relationships (Posada & Crandall 2001).

Irrespective of the likely great importance of hybridization in the evolution of Viola sect. Melanium, we will briefly examine (1) those species of the Italian Calcarata complex that did not fall into the four major nuclear clades, (2) those species that fell into these nuclear clades but had not been explicitly associated with the Calcarata complex or V. [unranked] Calcaratae before, and (3) those species that fell into the plastid clade but had not been explicitly associated with the Calcarata complex or V. [unranked] Calcaratae before. The four major nuclear clades found by us do not correspond to the major morphological groups among Italian Calcarata complex species identified by Pignatti (2017).

As for the five taxa not falling into the four larger clades of Calcarata complex species in our nuclear phylogeny, i.e. Viola aethnensis subsp. aethnensis, V. corsica subsp. limbarae, V. dubyana, V. merxmuelleri and V. valderia, their relationship to the Calcarata complex has never been doubted from a morphological point of view (Pignatti 2017). In the case of V. aethnensis subsp. aethnensis and V. corsica subsp. limbarae, other subspecies of these two species group in the four major nuclear clades found by us, and V. corsica subsp. limbarae falls into clade 2 of our plastid phylogeny.

Species not strictly associated with the Italian Calcarata complex or Viola [unranked] Calcaratae are V. cornuta, V. grisebachiana, V. orphanidis and V. paradoxa in clade A, V. graeca, V. magellensis, V. rausii and V. sfikasiana in clade B, V. montcaunica in clade C and V. beckiana and V. lutea subsp. lutea in clade D. Of these 11 species, the widespread V. lutea subsp. lutea (clade D) has been considered closely related to V. calcarata by Reiche & Taubert (1895), and Becker (1925) included V. dubyana, a species of the Calcarata complex, in his V. [unranked] Luteae W. Becker, which also included V. lutea. Viola lutea was found in a clade containing Calcarata complex species also by Hildebrandt & al. (2006). Viola beckiana (clade D), a species from serpentine or calcareous rocks in the southern Balkan peninsula, is large-flowered and has palmately or pinnately divided stipules (Valentine & al. 1968). The Spanish V. montcaunica (clade C) and V. cornuta (clade A; both with 2n =22), of which the former was described as essentially a smaller form of the latter by Valentine & al. (1968), have palmately divided stipules like V. valderia (see above) and share relatively large flowers and long spurs with species of the Calcarata complex. Viola magellensis was included in V. sect. Melanium group I [unranked] Heterophyllae, which also includes the Calcarata complex, by Pignatti (2017). Of the Greek species (all clade B), V. graeca has large flowers and long spurs, V. rausii has long spurs and dimorphic leaves and V. sfikasiana has dimorphic leaves, all characters found in at least part of the Calcarata complex. Viola orphanidis and V. cornuta, grouping with species of the Calcarata complex in clade A, are similar to each other in leaf and stipule shape. Following Gams (1925), Becker included V. orphanidis, a species from the Balkans, in V. lutea, and V. lutea had previously been linked to the Calcarata complex (see above). Viola grisebachiana (clade A) from the Balkans is acaulescent according to Valentine & al. (1968) and has no obvious similarities to the Calcarata complex. Finally, V. paradoxa (clade A) from Madeira has been postulated to be a close relative of V. calcarata by Lowe (1868); the species has long axes, dimorphic leaves and large flowers (Short 1994). In the plastid clades, only V. montcaunica (clade 3) and V. comollia (clade 4) fell into supported clades with Calcarata complex species (Fig. 2). Of these, V. montcaunica has been discussed above, and V. comollia was included in V. sect. Melanium group I [unranked] Heterophyllae by Pignatti (2017).

With the exception of Viola grisebachiana, a relationship of all those species that have not been associated with the Calcarata complex but falling into nuclear Calcarata complex clades seems possible on the basis of their morphology or on the basis of their classification by earlier authors. However, in the absence of clear morphological characters defining the Calcarata complex – all characters used by Pignatti (2017) to characterize the Calcarata complex can also be found in other species of V. sect. Melanium (Valentine & al. 1968) – association of non-Calcarata species with the Calcarata complex based on morphological characters is not fully convincing.

In summary, tree-building methods such as ML and BI do not appear to be suitable for the reconstruction of phylogenetic relationships in Viola and V. sect. Melanium because of rampant interspecific hybridization. Reconstruction of relationships instead requires both larger DNA sequence datasets and tree-building methods that take hybridization into account (e.g. Wen & al. 2018).

Acknowledgements

We would like to thank the directors and curators of B, FI, FIAF, HBBS, M and MSB for permission to use their material for DNA extraction. We are grateful to Dr. Lorenzo Cecchi (Firenze, Italy), Prof. Gianniantonio Domina (Palermo, Italy), Prof. Dmitar Lakušić (Beograd, Serbia) and Dr. Filippo Scafidi (Palermo, Italy) for sending us leaf material collected in the field or taken from herbarium specimens. We are grateful to Dr. Andreas Fleischmann and Dr. Matthias Erben (both München, Germany) for assistance during visits to their institution by one of us (SK). We thank Doris Franke (Mainz, Germany) for help in preparing the figures. Dr. Markus S. Dillenberger (Mainz, Germany) is gratefully acknowledged for help in data analysis. We are also grateful to Dr. Kevin Thiele (Perth, Australia) and an anonymous reviewer for their very helpful comments on an earlier version of this article.

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© 2020 The Authors · This open-access article is distributed under the CC BY 4.0 licence
Sandra Krause and Joachim W. Kadereit "Identity of the Calcarata species complex in Viola sect. Melanium (Violaceae)," Willdenowia 50(2), 195-206, (12 May 2020). https://doi.org/10.3372/wi.50.50205
Received: 21 January 2020; Accepted: 6 April 2020; Published: 12 May 2020
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KEYWORDS
Calcarata species complex
ETS
hybrid speciation
hybridization
ITS
trnS–trnG
Viola
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