A Nomenclatural and Taxonomic Revision of the Senecio squalidus Group (Asteraceae)

Senecio squalidus (Asteraceae) currently includes nine subspecies distributed in North and Central Europe and in the Mediterranean basin. Within this taxonomic aggregate, many species have been described, but research on their nomenclatural types is incomplete. A complete nomenclatural survey of 19 names belonging to this taxonomically critical group was carried out. Fourteen lectotypes are here designated. The nomenclatural analysis, complemented by field investigations in the type localities of the taxa described in the Central Mediterranean, allowed us to accept 10 species. Accordingly, we proposed here a new name and a new missing combination at a specific level: S. aknoulensis and S. calabrus.


Introduction
Senecio squalidus L. (Asteraceae) is a species recorded for North and Central Europe and the Mediterranean basin. Currently, nine subspecies are accepted as proposed by [1][2][3]. After [1], other contributions dealt with the nomenclature of this group [4][5][6], but many names still lack nomenclatural types.
In the study of this group, Sicily is a focal point because large part of the taxa have been described or reported from this island. For example, at Mount Etna, three taxa are found at different altitudinal belts. As reported by [7,8], Senecio aethnensis Jan ex DC. (≡S. squalidus subsp. aethnensis (Jan ex DC.) Greuter) grows on recent lava in the upper belt, and S. siculus All. (=S. squalidus subsp. chrysanthemifolius (Poir.) Greuter) in the basal belt. At intermediate elevations, these two taxa currently hybridize, forming a hybrid swarm. About 300 years ago, seeds of these hybrid populations were brought to England. At the end of the 18th century, after a century of cultivation in botanic gardens, stabilized derivatives of this old hybridization escaped and spread rapidly to many parts of the British Isles [9,10]. Accordingly, a homoploid hybridogenous species originated, different from the parentals but also from the original native hybrid [11][12][13][14][15][16][17]. Albeit homoploid hybrid speciation is relatively rare in Asteraceae [18], other examples of speciation due to hybridization are known from Europe, the Mediterranean, and the Americas in other genera of this family (Bidens, Tragopogon), but also in other lineages, such as Oenothera (Onagraceae), Scabiosa (Dipsacaceae), and Vitis (Vitaceae) [19][20][21][22].
Linnaeus [23] described S. squalidus, the oldest available name in this group, from cultivated plants possibly obtained by seeds received from Oxford [24], and thus this name refers to the hybridogenic species of the British Isles. On the contrary, the first name given to the hybrid occurring in the wild in Sicily (Mount Etna) is S. ×glaber Ucria [1,8].
Senecio nebrodensis L. is another species described from Sicily, Spain, and the Pyrenees. Alexander [1] lectotypified this name with a specimen from Spain. Recently it has been proposed as a nomen rejiciendum because it has been shown that the application of this name is ambiguous and based on a type that only partially corresponds to the original description [25]. The name proposed for these plants from Spain is S. duriaei J.Gay. Although there are different taxonomic opinions on this matter, the plants occurring in C and SE Europe were called S. rupestris Waldst. and Kit. (≡S. squalidus subsp. rupestris (Waldst. and Kit.) Greuter). The plants occurring elsewhere in Sicily and North Africa were referred by [1] as S. squalidus subsp. aurasiacus (Batt.) C. Alexander, and those from the high eastern end of the Riff Mountains in Morocco as S. squalidus subsp. araneosus (Emb. and Maire) C. Alexander. Finally, the populations growing in Sardinia and southern Calabria were classified as S. squalidus subsp. sardous Greuter and S. squalidus subsp. calabrus (Fiori) Peruzzi and Bernardo, respectively [6,[26][27][28][29].
The morphological analysis of the original material traced for this study allowed us to better clarify the relationships among the taxa investigated, here considered at the species level, to define new synonyms, as well as to propose new missing names and combinations. This contribution is part of a series of investigations aimed at looking for the nomenclatural types of the plant taxa described in Italy [29][30][31][32].

Materials and Methods
Starting from the herbaria hosting the main collections of those authors involved in the Senecio squalidus group, we performed a survey of the original material available for the names involved. For the type of S. squalidus, we consulted the LINN herbarium (acronyms according to [33]); the herbaria of the National Museum (PR) and of the Charles University in Prague (PRC) for the names published by Carl Bořivoj Presl [34]; the Herbarium P for the collections by Jean Louis Marie Poiret and TO for the collections by Carlo Allioni [34]; the Herbarium of the Conservatoire et Jardin botaniques de la Ville de Genève (G) for the collections by Giorgio Jan, Jean Étienne Duby, and Alphonse Louis Pierre Pyramus de Candolle; and the herbarium of the Natural History Museum of Florence (FI) for the original material by Adriano Fiori. The herbarium by Bernardino da Ucria has been lost, so duplicates and other material were searched for in the main Italian and European herbaria BM, BOLO, BR, CAT, E, K, MA, MAF-POURRET, NAP, PAD, PAL, RO, W, WAG, and WU. The articles of the International Code of Nomenclature for algae, fungi, and plants (hereafter ICN) cited in the text follow [35]. In the proposed taxonomic treatment, species are arranged in alphabetical order. For each taxon, the accepted name is reported, as well as the synonyms, the publication details, type information, taxonomic notes, and distribution. The taxonomic revision is based on the morphology of the investigated specimens and supported by field investigations, carried out between 2019 and 2022, in type localities of the taxa described from Sicily, Calabria, and Sardinia. To facilitate identification, a dichotomous identification key and schematic drawings of capitula, upper leaves, and basal leaves arranged in the same order are presented. The lectotype here selected is composed of a floral scape with eight fruiting capitula at different ripening stages and nine leaves. It includes the handwritten label by Carel B. Presl.

Senecio aethnensis
The specimen by Jan (G barcode G00471538 [digital photo!]), also mentioned in the protologue of S. aethnensis, applies to the same unit of diversity.
Taxonomic notes: This perennial species has lanceolate leaves with entire to dissected margin ( Figure 1); capitula 7-13 mm in diameter; cypselae glabrous or puberulous, around 2.3-3.3 mm long.  Alexander [1] indicates the specimen "In Callitrietis et Quercetis, solo margaceo, schistaceo: Aknoul; in monte Nador, Boured 800-1400 m, 19 June 1926, Maire (MPU)" as an isotype and reports it as a poor specimen wherein it is hard to clearly distinguish the leaf characters. The type citation of [1] can be further narrowed to a single specimen by second-step lectotypification according to Art. 9.17 of ICN. In MPU, we found only a single specimen of this gathering (MPU000225), and another one (MPU000224) from the same locality, but with a different date.
Taxonomic notes: It is a perennial mountain species, characterized by abundant hairiness on the whole plant and lyrate leaves ( Figure 1). Distribution: Senecio aknoulensis is a narrow endemic species in the eastern end of the Riff Mts, N Morocco [1,36] (Figure 3     Among the original material found in FI, there are some specimens prepared by other collectors and revised by Fiori. The indication "Nob." in the label stands for "Nobis", to indicate that the name was published by the same reviewer. We chose the specimen FI067113, collected by Hermann Ross, because it includes three well-grown plants with numerous leaves and flower heads at different developmental stages. Taxonomic notes: It is an annual species previously confused with S. siculus [26] from which it differs in the leaves shallowly lobed and not pinnatifids ( Figure 1 Figure 5).   Among the original material found in FI, there are some specimens prepared by other collectors and revised by Fiori. The indication "Nob." in the label stands for "Nobis" to indicate that the name was published by the same collector. We chose the above-mentioned specimen, collected by Fiori himself, because it includes a plant with numerous flower heads at different developmental stages.
Taxonomic notes: It is an annual mountain species with narrowed obovate leaves divided into narrow and spaced laciniae (Figure 1). The lack of deep lobes in the base leaves allows for the distinction of this species from S. rupestris and S. balansae, whose ranges are in contact or, in part, overlap.
Distribution: Senecio calabrus in Calabria on the Sila Mts, Serra San Bruno, Aspromonte, and in Sicily in the higher Madonie Mountains ( [28], field checks and specimens in FI) (Figures 2 and 3). In the protologue, this species is reported from Sicily, Spain, and the Pyrenees. This species is not known from the Pyrenees, and the populations from Spain and Sicily have been attributed to two different taxa [1]. Alexander [1] lectotypified this name with a specimen presumptively coming from southern Spain. Only recently has this name been proposed as a nomen rejiciendum [25] because it has been shown that its application is ambiguous and based on a type that only partially corresponds to the original description.
Taxonomic notes: This perennial species is characterized by basal lyrate leaves, lobed with crenate margins (Figure 1). It differs from S. rupestris by the presence of glandular hairs in the upper part of the stem.
Distribution: Senecio duriaei occurs from 1200 to 3000 m a.s.l. in central and southern Spain [37] (Figure 3 In the protologue of this name, after a short diagnosis, a synonym by Cupani is cited along with the reference to a plate of his Panphyton Siculum. No suitable original specimen by Ucria was found. The Herbarium Ucria has been lost and no specimen was found in PAL, FI, G, and in a small herbarium kept at "Società Siciliana per la Storia Patria di Palermo" attributed to Ucria [39]. For this reason, we here designate the plate 139 of the Panphyton Siculum by Cupani (Copy A, housed in the Central Library of Sicily Region) as the lectotype of the name. This iconography was known by scholars through the consultation of a dozen copies preserved in the main European libraries. These copies consisted of several engravings with different numbering [38]. Cupani's work was published for the first time in 2003 by collating the main existing copies [38]. No original specimen has been found in the investigated herbaria. We here designate as the lectotype of the name the iconography included in the copy by Bonanno reported in the protologue. The plate 167 of Bonanno's copy corresponds to the plate 139 of Cupani's copy and to the plate 364 of the 2003 edition [38]. After lectotypifications, Senecio ×glaber Ucria and Jacobaea incisa C.Presl are homotypic.
Distribution: This morphologically variable hybrid occurs in Mount Etna in the contact zone between S. aethnensis and S. siculus, at altitudes between 1300 and 1900 m a.s.l. [8] ( Figure 2 Taxonomic notes: This perennial species shows lanceolate basal leaves, deeply laciniate with toothed margins (Figure 1). S. rupestris, compared to the other species of the group, shows a relative morphological homogeneity throughout its range.
Distribution: Senecio rupestris is a widely distributed species, found in central Europe, peninsular Italy, and the Balkan peninsula ( Figure 3).
Senecio sardous ( homotypic with S. leucanthemifolius var. nemoralis Gennari, described in 1867. Hence, the name by Greuter is not a new combination but a replaced name at a different rank (Art. 58.1 of the ICN), as already evidenced by [29].
Taxonomic notes: It is an annual species, which differs from S. balansae in having non-lyrate basal and upper leaves.
Distribution: Senecio sardous is endemic to Southern and Central Sardinia, where it grows from sea level to 1200 m a.s.l. [6] (Figure 3).  (Figure 6). Only the specimen here designated as lectotype was found in the Herbarium Allioni in TO. It consists of a single plant, without roots, with three flower scapes rich in deeply laciniate leaves and about 20 flower heads at different degrees of development. This specimen corresponds well to Boccone's iconography [41] (pl. 36 at p. 67) related to the diagnosis by Boccone himself [41] (p. 66) and reproduced in Allioni's protologue. It was chosen Only the specimen here designated as lectotype was found in the Herbarium Allioni in TO. It consists of a single plant, without roots, with three flower scapes rich in deeply laciniate leaves and about 20 flower heads at different degrees of development. This specimen corresponds well to Boccone's iconography [41]  In G-DC, only one specimen was found, but in the impossibility of knowing whether there were more duplicates, this specimen is prudently designated here as a lectotype. Senecio chrysanthemifolius var. microglossus was distinguished for having small, revolute ligules, slightly exceeding the flower head. This taxonomic character is not constant in nature, and the low-altitude populations of Mount Etna all refer to S. siculus.
Distribution: From the studied herbarium specimens housed in FI, P, PAL, and the field investigations, we can state that Senecio siculus is endemic to Sicily, southern Italy, and northern Tunisia. It occurs in Mount Etna in the basal belt, up to about 1500 m a.s.l., and grows also in northern and southern Sicily, the Aeolian islands, Calabria (S Italy), and Cape Bon (N Tunisia) (Figures 2 and 3 Taxonomic notes: It is a perennial stabilized hybrid species that originated in England from seeds of S. ×glaber Ucria collected on Mount Etna [11]. This species has elliptic to oblong basal leaves, pinnatifid to pinnatipartite, rarely entire and dentate, middle and upper stem leaves pinnatifid to pinnatipartite, less frequently entire (Figure 1). Distribution: Starting from the UK, S. squalidus became later naturalized in France, northern Europe, the USA, and Canada [42] (Figure 3). The wide distribution reported throughout Europe (especially in Mediterranean countries), Algeria, and Morocco [42,43] refers to the whole taxonomic group.
We agree with the proposal [25] of rejecting the name S. nebrodensis because it has often been treated as a nomen confusum. Furthermore, in the European Herbaria, the Spanish specimens are either casually identified as S. nebrodensis or S. duriaei.
It is possible to discriminate the taxa through the morphology of the basal leaves. This character is also easily observed in the photos of herbarium specimens. Some taxa are very recognizable by their diffuse hairiness (S. aknoulensis), by the presence of glandular hairs in the upper part of the stem (S. duriaei), or by their complete absence of hairs (S. aethnensis). The size of cypselae is also a relevant and important discriminatory feature [1], but it is not always observable in herbarium specimens.
Senecio squalidus is morphologically very variable, and S. ×glaber shows a range of morphological variations from one parental taxon to the other. On the contrary, S. rupestris is morphologically very homogeneous throughout its range.
The taxonomic relationships among the taxa investigated are not clear yet, and in several cases, there are geographical and/or ecological overlaps. The previous taxonomic accounts [1][2][3], which have seen all taxa at the subspecific level within S. squalidus, is not adequate to explain the variability that has emerged so far. Compared to previous taxonomic treatments, we deem more appropriate to treat these taxa at the species level, given their morphological diagnosability but unclear systematic and phylogenetic relationships. Accordingly, a new name (S. aknoulensis) and a new missing combination at a specific level (S. calabrus) are here proposed.
Consequently, the current taxonomic treatment accepts 10 species: S. squalidus, a hybridogenous species known only as a naturalized alien in England, northern France, the USA, and Canada [42,45]; S. ×glaber (S. aethnensis × S. siculus), a hybrid endemic to Mount Etna between 1300 and 1900 m a.s.l.; S. aethnensis endemic to the upper belt of Mount Etna; S. siculus endemic to Sicily, the extreme southern peninsular Italy, and Tunisia, from the sea level up to 1500 m a.s.l.; S. duriaei endemic to Spanish mountains; the south-eastern European S. rupestris; S. balansae endemic to Sicily, Tunisia, and Algeria; S. calabrus endemic to the mountains of Calabria and Sicily; S. sardous endemic to Sardinia; and S. aknoulensis endemic to the mountains of N. Morocco. From a biogeographical point of view, Sicily is the territory that hosts the largest number of taxa within this group. This could be explained by adaptive radiation [46,47], a phenomenon that is however still poorly understood in Mediterranean plants [48]. On the basis of the data available in the literature, S. squalidus and all the species native to Central Mediterranean are seemingly diploid with 2n = 20 chromosomes [1,[49][50][51][52][53][54][55][56][57], while S. duriaei is reported as tetraploid [56,57]. A molecular phylogeny should be carried out to properly address the systematic relationships among these morphospecies.