[2.8] Viola sect. Leptidium

Viola sect. Leptidium Ging. in Mém. Soc. Phys. Genève 2(1): 28. 1823 ≡ Viola subg. Leptidium (Ging.) Peterm., Deutschl. Fl.: 66. 1846—Type: Viola stipularis Sw.

Description.—Subshrubs or perennial herbs. Axes not morphologically differentiated. Stems reclining to erect, sometimes branched (in Viola scandens and V. stipularis at least 1 m long). Stipules lanceolate to ovate, laciniate, partially sheathing the stem. Lamina linear-lanceolate to reniform, margin crenate, short- to long-petiolate. Corolla whitish to violet with a white throat (corolla entirely red in V. arguta). Spur short and saccate (spur thick and bulbous in V. arguta). Bottom pair of stamens with apical “u”-shaped connective appendage. Style filiform, straight undifferentiated, with a simple stigmatic opening. Cleistogamous flowers produced, sometimes subterranean in V. arguta and possibly other species; cleistogamy facultative. Allotetraploid. Inferred secondary base chromosome number [x’ = 13.5].

Diagnostic characters.—Aerial stems AND laciniate sheathing stipules AND short saccate or thick bulbous spur AND “u”-shaped connective appendage on bottom pair of stamens AND filiform style.

Ploidy and accepted chromosome counts.—4x, 8x; 2n = 54 (V. dombeyana).

Age.—Crown node age 8.7 (3–16) Ma [28].

Included species.—18. 

Violaarguta Humb. & Bonpl. ex Schult., 

V. atroseminalis H. E. Ballard, ined., 

V. boliviana Britton, 

V. bridgesii Britton, 

V. cerasifolia A. St.-Hil., 

V. dombeyana DC. ex Ging., 

V. fuscifolia W. Becker, 

V. gracillima A. St.-Hil., 

V. lehmannii W. Becker ex H. E. Ballard & P. Jørg., 

V. mandonii W. Becker, V. saccata Melch., 

V. scandens Humb. & Bonpl. ex Schult., 

V. steinbachii W. Becker, 

V. stipularis Sw., 

V. subdimidiata A. St.-Hil., 

V. thymifolia Britton, 

V. uleana W. Becker, 

V. veronicifolia Planch. & Linden

Distribution.—Southeastern Mexico to Bolivia; northwestern Venezuela; southeastern Brazil (Figure 19). 

Discussion.—Section Leptidium is an allotetraploid (4x) lineage, derived from ancient hybridisation and chromosome doubling of the common ancestor of subgenus Viola and the most recent common ancestor of sect. Leptidium and sect. Tridens; this allopolyploidisation may have happened c. 15 Ma ago [28]. A comprehensive phylogeny of sect. Leptidium has not been published. While V. arguta appears to be 4x, further allopolyploidisation has occurred in V. stipularis (8x). The count of n = 27 in V. dombeyana (as V. humboldtii Tr. & Pl. [53]), presumably referring to the 8x level as well, is the only count for the section and needs confirmation.

This widely distributed Latin American lineage encompasses 17 species and possibly the mysterious V. producta W. Becker. Viola scandens and V. stipularis account for the Mesoamerican and Antillean portions of the range of the section, with four species in southeastern Brazil and 13 (14?) occupying middle and higher elevations of the northern and central Andean Mountains in South America. All species have petals glabrous within, and all share a peculiar synapomorphy of prolonged “u”-shaped anther connective appendages on the bottom pair of stamens, first documented in two Brazilian species by Freitas and Sazima [171]. The Mesoamerican and southeastern Brazilian lineages may have diverged 8.7 (3–16) Ma ago [28].

A transition from nectar to pollen flowers and “buzz” pollination has been suggested for the majority of the species within sect. Leptidium; the unique “u” shape of the connective stamen appendages appears to be an adaptation to this [171]. The flowers are pollinated by Anthrenoides bees (Andrenidae) that hold onto the connective appendages while harvesting pollen by vibration (“buzz-pollination”) [171], and may thus be analogous to similar structures in unrelated genera (e.g., Arbutus, Ericaceae). Curiously, a secondary transition to hummingbird pollination, unique in the genus, seems to have occurred in V. arguta; this species produces copious amounts of nectar, 4 µL per 24 h [174] but it has also preserved the “u”-shaped connective appendages indicative of ancestal “buzz”-pollination.