[2.10] Viola sect. Melvio

Viola sect. Melvio Marcussen, sect. nov.

Type: Viola decumbens L. f.

Description.—Perennial subshrubs. Axes not morphologically differentiated. All stems aerial. Stipules somewhat adnate, green, linear, with 1–2 basal teeth. Lamina entire, linear, subapiculate and somewhat succulent. Bracteoles persistent, 1–3 mm. Corolla violet with a white throat. Spur slender, yellow or orange. Style dorsiventrally flattened and tapering towards the tip, in lateral view filiform and sigmoid. Cleistogamous flowers not produced. Allopolyploid (MELVIO).

Diagnostic characters.—Style dorsiventrally flattened and tapering towards the tip, in lateral view filiform and sigmoid. Allopolyploid (MELVIO).

Ploidy and accepted chromosome counts.—Allopolyploid, possibly 6x; chromosome number unknown.

Age.—Crown node age not applicable (monotypic section), stem node age 20.5–22.6 Ma [28].

Included species.—1. 

Viola decumbens L. f.

Distribution.—South Africa: Cape region

Etymology.—Section Melvio is named after the lineage to which it belongs, the diploid MELVIO lineage, for which Viola decumbens is the only extant species. The name was originally applied by T.M. [88] to delimit a clade in the ITS phylogeny of Ballard et al. [2] which comprised sect. Melanium (“MEL”) and sect. Viola (“VIO”) only, as a result of ITS homoeolog loss and limited sampling.

Discussion.—Section Melvio comprises a single species, Viola decumbens (Figure 4), a shrublet with an isolated distribution in the fynbos of the southern Cape of South Africa [260]. It is the sole member of the otherwise extinct Eurasian MELVIO clade and sister to the taxa involved in the dozen of allopolyploidisations that occurred in Eurasia 15–19 Ma ago. Viola decumbens may have been isolated in South Africa since Early Miocene, 20–25 Ma ago [28]. Viola decumbens is allopolyploid, possibly paleohexaploid, based on gene copy number for two nuclear genes [28]. The species was previously included in sect. Xylinosium [1,28], to which it is superficially similar in shrubby habit. It differs, however, from sect. Xylinosium in several key traits. These include the style, which in V. decumbens is characteristically dorsiventrally flattened and tapering towards the tip, in lateral view filiform and sigmoid, vs. clavate in sect. Xylinosium; the leaves which in V. decumbens are entire, linear, subapiculate and somewhat succulent vs. lanceolate and usually crenate in sect. Xylinosium; the bracteoles which are 3–5 mm and persistent in V. decumbens vs. 1–2 mm or caducous in sect. Xylinosium; and the indument of stems and leaves, minutely papillate in V. decumbens and distinctly longer in sect. Xylinosium (sometimes glabrous or ciliate). The inclusion of V. decumbens in sect. Xylinosium by Marcussen et al. [28] was a mistake relating to a chloroplast sequence of V. arborescens (sect. Xylinosium) that had been erroneously assigned to V. decumbens (trnL-trnF; KJ138159). Indeed, another chloroplast sequence (rbcL; AM235165) places this species in agreement with the nuclear homoeologs, of which none are shared between these two taxa