## 1 Introduction  
The genus _Genista_ L. (Fabaceae, Genisteae) includes about 150 species and over 180 taxa, including subspecies [1], of spiny or harmless shrubs (rarely small trees) (see [2] for a detailed morphological description). It is widespread from the Euro-Mediterranean region, where the genus is greatly diversified, with over than 120 species [3], to western Asia [4]. A greater concentration can be observed in the central and western sectors of the Mediterranean Basin (e.g., [5, 6, 7, 8]). As known, its basin is one of the richest mega hot spots of plant biodiversity worldwide [9, 10, 11, 12], and numerous taxa are narrow endemic and often limited to islands [13, 14]. In various cases, distributions show disjunctions that are suggestive of past tectonic or climatic events [9, 15, 16, 17, 18, 19, 20]. This scenario has been reported for several _Genista_ species [8, 21], but, at least in _G. anglica_ L. s.l. [22] and in some representatives of the _G. elphedroides_ DC. group [23], the distribution has been partly attributed to anthropogenic dispersal in protobistoric or historical times. The well-knownarroboreal broom _G. ettennsis_ (Raf.) DC. ([24], and the nomenclatural section therein, for the correct name of the taxon) shows one of the most intriguing native distributions within the genus, as it occurs only in the three largest islands of the Mediterranean: Sicily, Sardinia, and Corsica [3]. In Sicily, it was discovered on Mt. Etna in 1810 or before by [PERSON] ([25] p. 27, sub _Spartium_), where it grows on lavas up to 2000 m a.s.l. ; but it had been introduced for forestry purposes on the Peloritani massif a long time ago [26] and later in some neighboring localities of the Nebrodi Massif and in Lachea (Cyclopean Isles) [27]. [PERSON] [28] first observed _G. ettennsis_ in Sardinia, where it is known for several siliceous mounts of the central and northern sectors of the island (rarely descending to the basal plain along riverbeds) [29]. Only in 1961 was the species reported as present in Corsica [30], where it occurs very locally along the south-eastern coasts [31; 32; 33]. For a detailed distribution, see Table 1. In addition, this plant was first introduced into Campania (southern Italy), i.e., in Ischia Island, the Sorrentine Peninsula, and, especially, Vesuvius [34; 35], where it has become invasive [36]. Nowadays, as a naturalized or casual alien, it is also reported for several other regions of Italy, from Emilia-Romagna to Calabria [37]. On the basis of ecological considerations, some authors [38] hypothesized that _G. ettennsis_ originated in Sardinia, and only later spread to Sicily. Indeed, [PERSON] [39] seemed to suggest that the plant had been introduced to the latter island, where apparently it occurs mainly in man-influenced habitats. Despite this, [PERSON] et al. [40] regard _G. ettennsis_ as an archaic species characterized by a relic distribution both in Sardinia and Sicily; in this latter island, it may have survived on the relatively recent Mt. Etna on account of the peculiar activity of this volcano, which periodically regenerates its cacuminal vegetation. In Corsica, it was believed to be introduced (e.g., [31; 41; 42; 43; 44]), but this statement has been vigorously confuted by the most recent studies [45; 33]. In addition to its atypical native range, the species shows a remarkable taxonomic distinctness, which is suggested also macroscopically by its arboreal habitats, which is very unusual within the genus. This feature inspired [PERSON] ([46] p. 152) in publishing the new monotypic genus _Dendrospartum_ Spach, including _D. ettenense_ ([PERSON].) [PERSON] (\"aetnense\"), and also characterized by several other features, among which is the calyx shape. Immediately after, [PERSON] ([47] p. 568) segregated _D. ettennsis_ s.l. into the new genus _Drymospartum_, including _D. purgans_ ([PERSON]) [PERSON], _D. ettenense_ (Raf.) [PERSON] (\"aetnense\"), and the new species _D. sardum_ [PERSON]. [PERSON], which included the Sardinian population described by [PERSON] as _G.