On the taxonomic identity and status of Silene sericea var. balearica (sect. Dipterosperma , Caryophyllaceae)

. This paper presents a re-evaluation of the taxonomic relationships of Silene sericea var. balearica based on morphological features. Critical examination of herbarium specimens (including type material) and living plants has shown that S. sericea var. balearica should be recognized at species level. Therefore, the new name, Silene migjornensis, is proposed to designate the endemic species growing on maritime sands in southern Mallorca (Balearic Islands, Spain). This taxon is described, illustrated and compared with its morphologically closest relatives from Silene sect. Dipterosperma . morfológicamente más relacionados de Silene sect. Dipterosperma .


Introduction
Silene L. (Caryophyllaceae) is a large genus of flowering plants, distributed mainly across the Northern Hemisphere. The number of species included in the genus varies between taxonomic treatments, but some of the latest studies (Melzheimer, 1980;Greuter, 1995;Zhou et al., 2001;Morton, 2005) estimate between 600 and 700. The genus is particularly diverse around the Mediterranean Basin where more than 350 species have been recorded (Greuter et al., 1984). Section Dipterosperma (Rohrb.) Chowdhuri is a taxonomically complex aggregate comprising 15 species which is diversified around the Mediterranean Basin (Brullo et al., 2017). This section comprises annual plants, characterized by hairy-pubescent (nonglandular) indumentum, erect flowers, arranged in monochasia or dichasia, calyx hairy, not inflated, 10-nerved, anthophore pubescent, petal limb deeply bifid, coronal scales present, seeds orbicular-reniform, laterally flat and dorsally furrowed between two more or less undulate wings (Talavera, 1990;Brullo et al., 2012Brullo et al., , 2017. In the Balearic Islands, the presence of two species of sect. Dipterosperma (S. apetala Willd. and S. secundiflora Otth) is widely accepted and well documented (Bolòs & Vigo, 1990;Talavera, 1990;Chater et al., 1993). On the contrary, the presence in the Balearic archipelago of another species of this section, usually referred to S. sericea All., is controversial. Willkomm (1876) described S. sericea var. balearica Willk. from southern Mallorca. The description provided in the protologue is rather scarce [Differt a forma typical (corsicana) floribus minoribus matutinis, calyce breviore (non nisi 10 mm l.) basi minus attenuato, anthophoro breviore (calycem medium aequante), capsula anthophoro aequilonga]. The taxonomic value of this variety has been considered null (see Talavera & Muñoz Garmendia, 1989) or scarce in recent floras (see e.g., Talavera, 1990). It has been included within the range of variation of S. colorata Poir. (a widespread Mediterranean species), without any formal recognition. On the contrary, Bolòs & Vigo (1990) accepted the presence of S. sericea var. balearica in the Balearic Islands (distributed in southern Mallorca), whereas Chater et al. (1993) listed typical S. sericea for the Balearic arhipelago. The presence of S. colorata in the Balearic Islands was not supported by Bolòs & Vigo (1990), Chater et al. (1993) and, surprisingly, by Talavera (1990). However, it was listed by Pla et al. (1992) as present in the flora of Mallorca. In a detailed taxonomic study (but apparently unnoticed among botanists dealing with the Spanish flora), Valsecchi (1995) circumscribed the distribution area of S. sericea only to Liguria (northwestern Italy). At the same time, several species of this group were described, all of them being restricted to the central and western Mediterranean region. Valsecchi (1995) considered doubtful the presence of S. sericea in the Balearic Islands, doubts that go back to Chater & Walters (1964).
Our study of the type material of S. sericea var. balearica preserved at COI (in the Willkomm herbarium) and living plants from the type locality show that the stems of this taxon are prostrate-ascending, the flowers are solitary and the seed wings are flat. These characters do not match with those shown by S. colorata (erect stems, inflorescences 4-10 flowered and seeds with undulate wings) according to several authors (e.g., Bolòs & Vigo, 1990;Talavera, 1990;Chater et al., 1993). Therefore, our initial identification, based on Bolòs & Vigo (1990) and Chater et al. (1993), led us to consider this plant as S. sericea, without granting taxonomic value to the variety described by Willkomm (Rosselló & Sáez, 2001;Sáez et al., 2017). Valsecchi (1995) and Brullo et al. (2012Brullo et al. ( , 2015Brullo et al. ( , 2017 provided useful information on the main morphological characters discriminating taxa included within Silene sect. Dipterosperma and discussed in detail their taxonomy and distribution. However, no plant material from the Balearic Islands was included in the mentioned studies.
During a revision of the Balearic plants of the genus Silene, the taxonomic position of S. sericea var. balearica has been questioned in light of the recent and detailed revisions of Silene sect. Dipterosperma (Brullo et al., 2015(Brullo et al., , 2017. Our study shows that the Majorcan plants called S. sericea var. balearica differ from the currently recognized taxa by a unique combination of characters, and in our opinion this variation merits their recognition at the species rank.

Material and Methods
Morphological characters recognised as taxonomically discriminant within Silene sect. Dipterosperma (e.g., Talavera, 1990, Valsecchi, 1995Brullo et al., 2015Brullo et al., , 2017, and our own observations) were scored either in the field and herbarium specimens (BC, BCN, COI and MPU; acronyms according to Thiers 2019+). Terminology and delimitation of characters follow Brullo et al. (2017). The morphological comparisons with the allied species are base on detailed data recently published by Brullo et al. (2015Brullo et al. ( , 2017. Morphological observations of materials were carried out under a Zeiss Stemi DV4 binocular stereoscopic microscope. Micromorphology was observed on dry leaves and stems, pollen and seeds which were glued directly to aluminium stubs, coated with 40-50 nm gold, and examined with a scanning electron microscopy (Hitachi 2300-S) at 20 kV. The extent of occurrence and area of occupancy were calculated using the GeoCAT tool (Bachman et al., 2011).

Results and Discussion
Silene migjornensis L. Sáez Etymology. The specific epithet migjornensis is derived from "Migjorn", the name of the area where the type locality is found, in southern Mallorca (Balearic Islands, Spain).

Geographical distribution and habitat
Silene migjornensis is known from Es Trenc, a coastal area in Southern Mallorca. The population at this location is restricted to a narrow strip of dune vegetation, along 2 km, at an altitude of 1-5 m asl. A considerably smaller population occurs at Ses Covetes, about 2 km at the NW of the first location. In both locations Silene migjornensis is part of a therophytic ephemeral sandy community.

Taxonomic relationships
Taxa of Silene sect. Dipteropserma constitute a difficult taxonomic aggregate in need of further work. Most species of this section are morphologically closely related. The group of plants that has been referred to S. sericea in a broad sense (Chater & Walters, 1964;Bolòs & Vigo, 1990;Chater et al., 1993) displays a geographically structured morphological variation, which allows the recognition of taxonomic entities at the species level (Valsecchi, 1995;Brullo et al., 2015Brullo et al., , 2017 should be considered as a separate taxon because of the well-defined morphological characters discriminating them from the other species currently recognized. Selected characters of the most similar and likely most closely related taxa of S. migjornensis are shown in Tables 1 and 2, including S. colorata, which apparently seems to be non-closely allied. We also discuss the morphological relationships with S. sericea, which seem to be remote, so it is not included in Tables 1 and 2. The other taxa recognized within Silene sect. Dipterosperma show a quite different assemblage of characters (see Brullo et al., 2015Brullo et al., , 2017 and are not need not to be dealt with. Petal limb length (mm) 5.0-7.0(7.5) 7.5-8 8-10 6.5-7.0 7-9 Petal lobe size (mm) (3.5)4-5 × 2-3.5 5-5.5 × 3.5-4 6-6.5 × 3-5 4.8-5. Stamen filament length (mm) 8.0-9.5 9-10 8-10 6.5-9.0 6.5-8.5 Anther colour pinkish greenish-white pinkish pink-lilac yellowish-green Anther length (mm) 1.3-1.5 1.8 2 1.6-1.8 1.8-2.0 Ovary length (mm) 3.0-3.5 3 3.5 2.2-3.0 2.4-2.8 Capsule length (mm) 5-7 4.5-6.5 8-9 7-8 7.5-8.0 Carpophore 4.5-5.5 6-6.5 5-5.5 4.5-5.2 5-6 On morphological grounds, S. migjornensis is mainly related to S. nummica Vals., endemic to Sardinia and Sicily (Peruzzi et al., 2014), with which it shares most of the vegetative characters (prostrate-ascending stems, leaves moderately succulent and rounded) and some reproductive features (solitary flowers, and flat seed wings). However, a careful comparison of their morphological features (Tables 1 and 2) shows relevant differences: S. migjornensis is easily distinguished from S. nummica by its longer calyx and calyx teeth, smaller petals lobes, shorter and pinkish anthers, shorter carpophore and by its glabrescent abaxial surface claw (vs. totally hairy in S. nummica) (Figures 1, 4). Regarding seed characters, several discontinuities also exist, like seed size, and dorsal furrow epidermal cell size and shape, among others (Table 2). Morphological relationships with S. colorata (endemic to mountains of C and S Europe) appear to be more remote. This species can be easily discriminated by several characters, including erect stems, 4-10 flowers in dichasia, longer anthers and strongly undulate seed wings (Table  1). In addition, several seed characters allow an easy distinction between both species (Table 2).
Geographic isolation is, most likely, linked to the speciation processes occurred within Silene sect. Dipterosperma. Apparently, polyploidy is not present within the section and all cytologically know species are diploid (2n=24). In this context, the geographic isolation of S. migjornensis with respect to the morphologicallyrelated S. nummica (endemic to Sardinia) is interesting. The importance of restricted gene flow and genetic drift as a major evolutionary force driving plant diversification in Mediterranean continental islands is well known (Mayol et al., 2012)

Conservation status
Following the categories and criteria of IUCN (Anonymous, 2012), our data so far indicate that S. migjornensis should be listed as EN (Endangered): B1ab(iii)c(ii-iv) + 2ab(iii) c(ii-iv) based on: i) its geographic restriction: the extent of occurrence and the area of occupancy (calculated on a 2 × 2 km grid) is 12 km 2 (both values are 1.5 km 2 on a 0.5 × 0.5 km grid), ii) the number of locations or subpopulations (two locations are here recognized), iii) continuing decline inferred in area, extent and/or quality of habitat, and iv) extreme fluctuations in the number of mature individuals exist. The Balearic population is found in maritime dunes, so it is likely that anthropogenic disturbances (trampling and circulation of vehicles) can cause impact on S. migjornensis.

Identification key
In order to incorporate S. migjornensis into the recent identification key of the taxa of Silene sect. Dipterosperma (Brullo et al., 2017), thereby facilitating its identification, we present here a partial modification of this key. The key is the same until couplet #14, where it should be modified as follows: