[2.9.2] Viola sect. Melanium subsect. Cleistogamae

Viola subsect. Cleistogamae Marcussen & Danihelka, subsect. nov.

—Type: Viola rafinesquei Greene (=V. bicolor Pursh non Hoffm.)

Description.—Annual to biennial. Lamina of basal leaves crenate. Calycine appendages 0.5–2 mm. Corolla with bright yellow throat. Bottom petal (spur included) 8–10 mm. Spur 1–1.5 mm. Cleistogamous flowers produced. Pollen apertures 4. Ploidy 8x. Chromosome number n = 17.

Diagnostic characters.—Cleistogamous flowers produced.

Ploidy and accepted chromosome counts.—8x; 2n = 34.

Age.—Crown node age not applicable (monotypic subsection), stem node age 9.8 (9.1–10.0) Ma [28].

Included species.—1. 

Viola rafinesquei Greene

Distribution.—Eastern North America.

Etymology.—The name Cleistogamae refers to the occurrence of seasonal cleistogamy in the type species.

Distribution.—Section Melanium subsect. Cleistogamae comprises Viola rafinesquei (=V. bicolor Pursh non Hoffm.) only, which differs from all other pansies in two key respects: it has seasonal cleistogamy, i.e., produces chasmogamous flowers in spring (after vernalisation) and cleistogamous ones later in summer, and its native range is in eastern North America while all the other Melanium species have their native ranges in the western Palearctic. Cleistogamy in V. rafinesquei involves reduction of the four lower anthers, unlike in other Viola where the three upper anthers are reduced [256], suggesting cleistogamy in these lineages is not entirely homologous. Viola rafinesquei has the chromosome number 2n = 34 and 4-colporate pollen, and is an octoploid [28]. Different views on the nomenclature of V. rafinesquei have been proposed [257,258] but we hold that of Shinners [257] to be correct. For general taxonomy, see Clausen et al. [256].

The subsections Cleistogamae and Bracteolatae appear to be monophyletic at the octoploid level and may have split 9–10 Ma ago [28]. The two are, however, genetically distant and cannot be crossed successfully [256].