Brassica tardarae (Brassicaceae), a New Species from a Noteworthy Biotope of South-Western Sicily (Italy)

A new species of Brassica sect. Brassica is described here from Sicily (Italy), which is known to be one of the centers of the diversification of wild taxa of this group. The new species (named Brassica tardarae) is restricted to the carbonate cliffs in the Tardara Gorges between Menfi and Sambuca di Sicilia (Agrigento province), an area with a peculiar geological history and where another strictly endemic species was recently described. The morphological relationships between the new species and other similar taxa are discussed, and an analytical key to the Sicilian taxa belonging to the genus Brassica sect. Brassica is also provided.

Brassica sect. Brassica consists of 14 species (with several subspecies) characterized by x = 9 genome, a suffrutescent perennial habit, large size (up to two meters tall when flowering), flowers with yellow-white petals, cylindrical or subcylindrical/tetragonous fruits (siliques), globose and reticulate seeds. The life cycle is usually between two and five to six years [3]. The populations occur mainly around the Mediterranean basin, from Israel and Lebanon in the East to the Canary Islands and the British Isles in the West [4]. The morphological characteristics of these taxa show great variability within the populations. Some taxa were indeed split into new species or subspecies, with some populations described as new taxa, mostly in the last forty years [4][5][6][7][8][9][10][11]. At the same time, the ability of these taxa to intercross and generate greater or fewer fertile hybrids led to them being considered as few natural taxa, even up to the extreme of recognizing only one species [12,13].
As is currently known, Sicily is one of the centers of the diversification of wild taxa of Brassica sect. Brassica [3,4]. The Raimondo and Geraci). More recently, new species were described on the basis of their morphological traits, namely, B. raimondoi, which occurs only in the cliffs of Castelmola (Messina) [10] and was recently considered a subspecies of the sympatric B. incana [14], if not synonymous, and B. trichocarpa, which is endemic to a single locality near Mt. Cuccio, close to Palermo [11].
Brassica rupestris and B. villosa are endemic to western Sicily [4,7,9,15], with a single disjunct population of B. rupestris in Calabria [15]. All populations of these taxa grow on cliffs and rocky habitats (mainly on limestone) from the sea level to 1000-1200 m above sea level (a.s.l.). Recently, Tatout et al. [16] proposed considering B. villosa subp. drepanensis as a distinct species on the basis of molecular analyses, hypothesizing a hybrid origin of this taxon involving B. incana and B. villosa.
Since most of the taxa are endemic and/or with a restricted or highly fragmented area of distribution, their populations are often threatened by anthropic activities, grazing, fires, etc. Brassica macrocarpa is considered to be Critically Endangered (CR) [17][18][19], whereas B. rupestris s.l. and B. villosa s.l. are included as Near Threatened (NT) species on the IUCN Red List [20,21]. In previous national and regional Red [22,23].
During field investigations in south-western Sicily, the first author noticed and for years observed a population of Brassica located at "Gole della Tardara", a Gorge about 11 km North-NorthWest from the town of Sciacca (Agrigento province). Further investigations by the current authors led to the conclusion that this population was different from all other known similar taxa, therefore, it is described here as a new species. Table 1 describes the main features of the Tardara Gorges Brassica population and compares them with those of the most similar taxa, i.e., B. rupestris (subsp. rupestris and subsp. hispida) and B. villosa subsp. brevisiliqua. The studied population seems to be very close to these taxa regarding the indumentum of the leaves, which are glabrous, or with rare bulbose hairs, but it differs from B. rupestris in terms of the very short fruit (up to half the length) with an evident dorsal rib. The fruiting pedicels are long, making the ratio of the siliqua to the pedicel rather small compared to the other taxa. The petals are smaller than in B. rupestris and similar to the ones of B. villosa subsp. brevisiliqua for both size and color. Nevertheless, the studied population differs from B. villosa subsp. brevisiliqua in terms of the morphology and thickness of the basal leaves, the diameter of the seeds, and the habit of the reproductive plants, since the former shows long flowering and fruiting branches.

Morphological Analysis
Concerning the micromorphological observations on the leaf, pollen, and seed surfaces, the studied population shows bulbose hairs and numerous stomata in the lower blade (694/mm 2 ). Pollen grains are elliptical, tapered at the poles, and bigger than in the other compared taxa, with a size similar to those of the B. villosa group [30].

Taxonomic Outcome
According to our observations on morphology and phenology, and our comparative analysis, the Tardara Gorges Brassica population must be referred to a new species, hence the description herein.
Conservation status: Although its habitat is naturally protected from most human activities (but not from quarries, for example), the unique population (with an area of occupancy of about 8 km 2 , and a single location) is "prone to the effects of human activities or stochastic events within a very short time period in an uncertain future, and is thus capable of becoming Critically Endangered or even Extinct in a very short time period" [33]. On the basis of currently available data, we apply the "D" criterion [33], assessing the species as Vulnerable (VU D2).
Eponymy: The specific epithet refers to the name of the gorges (see above) where the taxon lives. Phenology: Flowering late January to early February, fruiting mid-May. Distribution and ecology: The species is limited to the limestone cliffs of the "Gole della Tardara" in Sambuca di Sicilia territory (Agrigento, Sicily, Italy), between 100 and 400 m a.s.l. and about 10 km from the sea coast ( Figure 2). It lives in a chasmophytic community, described as the endemic association between Brassico rupestris-Centauretum saccensis, where it is one of the characteristic species [31]. The geographical, climatic, bioclimatic, and vegetational descriptions of this biotope can be found in Bazan et al. [31] and Raimondo et al. [32].
Conservation status: Although its habitat is naturally protected from most human activities (but not from quarries, for example), the unique population (with an area of occupancy of about 8 km 2 , and a single location) is "prone to the effects of human activities or stochastic events within a very short time period in an uncertain future, and is thus capable of becoming Critically Endangered or even Extinct in a very short time period" [33]. On the basis of currently available data, we apply the "D" criterion [33], assessing the species as Vulnerable (VU D2).
Eponymy: The specific epithet refers to the name of the gorges (see above) where the taxon lives.

Analytical Key to the Taxa of Brassica sect. Brassica in Sicily
This key is largely modified from those proposed by previous authors [7,10,15]

Analytical Key to the Taxa of Brassica sect. Brassica in Sicily
This key is largely modified from those proposed by previous authors [7,10,15].

Discussion
The finding of the new species supports Sicily as one of the main centers of diversification of Brassica sect. Brassica [11,25].
The Tardara population was known (as "Brassica rupestris") since the first work of Snogerup et al. [4]; in fact, the authors of that work talked about an isolated population of B. rupestris "10 km N Sciacca". After that, several authors referred to that population as B. rupestris [15,31,32]. As has already happened for other populations in Sicily (e.g., Brassica bivonana, previously included in B. villosa, and B. villosa subsp. brevisiliqua, previously considered under B. rupestris), our deeper investigations revealed that the Tardara population is clearly different from the other taxa occurring in Sicily and, according to a species concept currently used in this group [10,11], we here describe the new taxon as a new species.
The differences between Brassica tardarae and other morphologically similar taxa, namely B. rupestris (subsp. rupestris and subsp. hispida) and B. villosa subsp. brevisiliqua, are shown above ( Table 1). The new species is also well separated from the mentioned similar taxa geographically; the nearest population of a species of Brassica sect. Brassica (a few km South-East) belongs to B. villosa subsp. bivonana. From this point of view, the species is isolated and limited to this special site.
The Tardara Gorges deserve some additional remarks. Its cliffs host important and rich plant communities, where our new species is not the only strictly endemic taxon. Centaurea saccensis Raimondo et al. [32] is, in fact, another endemite restricted to this site. Interestingly, it seems to be phylogenetically very isolated (and probably older) when compared to the other species of the Centaurea cineraria group occurring in Sicily and surrounding areas [34]. If we look at the geology of the site, this part of Sicily was recently considered as part of the foreland, somewhere similar to the south-eastern Sicilian Hyblean area but with a different history [35,36] (Figure 3). Even though other geologists disagree on this view [37,38], there seems to be general agreement that some restricted sites could have (at least partially) emerged in this area over the last few millions years, after the deposition of the "Trubi" pelagic carbonates [39]. This is just a first remark, and further investigations on this topic are ongoing, but it is clear that the paleogeography and phytogeography of Sicily could be revised if this area is confirmed to be crucial for the geological and biological evolution of this part of the Mediterranean; in fact, it could be one of the oldest areas to have emerged during the complex history of Sicily in the last few million years. In other cases, the geology suggested possible solutions to explain the current genetic structure of some species (e.g. Ambrosinia bassii, see [40,41]), or was fundamental in the definition of the borders of phytogeographical units [42], but in this case, the importance of this part of Sicily "risks" being greater.
Finally, the Tardara Gorges are a (proposed) geosite but are not protected due to their biological heritage. They are, in fact, not included in any of the parks or reserves or Natura2000 sites occurring in that area. It is clear that its inclusion as a protected area is fundamental to guarantee its peculiar flora and vegetation, so that the enlargement of the nearby Natura2000 site "Complesso Monte Telegrafo e Rocca Ficuzza" (code ITA040006) will be requested to the competent authorities.  [34], with modification to the color of the island of Pantelleria). Regarding the aim of the present contribution, we note the "Foreland Units" in the area around Sciacca (in western Sicily).

Materials and Methods
Our work was based on field surveys to study the Brassica population of the Tardara Gorges. During the field trips, from October 2018 to May 2020, the morphology, ecology, and phenology of the plants in the wild were observed, and were specimens collected. During additional surveys, other populations of similar Brassica taxa (B. villosa subsp. brevisiliqua and B. rupestris) were visited and studied. Qualitative characteristics were observed on all reachable plants of the population during the trips. Quantitative characteristics were measured using 20 plants or items (such as pollen grains, stomata, etc.) collected from at least 5 different plants, with calculations of the mean ± standard deviation. Morphology was observed using a stereomicroscope (LeicaMZ9.5, maximum magnification of 60×). Analysis of relevant literature and revision of herbarium specimens were also performed; in detail, specimens preserved in the Herbarium Mediterraneum (PAL) were checked.
For scanning electron microscope (SEM) investigations, air-dried material (pollen grains or fruits) remained untreated and were put on stubs, coated with gold/palladium, and examined using an Oxford Leo 440 SEM.

Materials and Methods
Our work was based on field surveys to study the Brassica population of the Tardara Gorges. During the field trips, from October 2018 to May 2020, the morphology, ecology, and phenology of the plants in the wild were observed, and were specimens collected. During additional surveys, other populations of similar Brassica taxa (B. villosa subsp. brevisiliqua and B. rupestris) were visited and studied. Qualitative characteristics were observed on all reachable plants of the population during the trips. Quantitative characteristics were measured using 20 plants or items (such as pollen grains, stomata, etc.) collected from at least 5 different plants, with calculations of the mean ± standard deviation. Morphology was observed using a stereomicroscope (LeicaMZ9.5, maximum magnification of 60×). Analysis of relevant literature and revision of herbarium specimens were also performed; in detail, specimens preserved in the Herbarium Mediterraneum (PAL) were checked.
For scanning electron microscope (SEM) investigations, air-dried material (pollen grains or fruits) remained untreated and were put on stubs, coated with gold/palladium, and examined using an Oxford Leo 440 SEM.
Author Contributions: Conceptualization, methodology, formal analysis, and field and lab investigations, V.I., A.G., and A.T; comparison with previous data about Brassica sect. Brassica, A.G.; writing-original draft preparation, A.G.; writing-review and editing, A.T. and A.G. All authors read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Acknowledgments: Thanks to Attilio Sulli (DISTEM, University of Palermo) for the useful discussions about the geology of the Tardara Gorges area, Francesco Furnari (DISTEM, University of Palermo) and Carmela Di Liberto (STEBICEF, University of Palermo) for their assistance with scanning electron microscopy, and the Director and personnel of the Herbarium Mediterraneum (PAL) for the access to the examined specimens.

Conflicts of Interest:
The authors declare no conflict of interest.