[2.1] Viola sect. Abyssinium

Viola sect. Abyssinium Marcussen, sect. nov. 

Type specie: Viola abyssinica Steud. ex Oliv.

Description.—Perennial herbs. Axes not morphologically differentiated. All stems ascending or trailing, rooting at proximal nodes. Stipules deeply dentate-laciniate to entire. Lamina crenulate, petiolate. Flowers c. 1 cm, peduncles produced only from some leaf axils. calycine appendages very short or absent. Corolla violet or white, with a white throat, bottom petal with violet striations. Spur saccate. Style clavate, laterally compressed, at base geniculate, at apex galeiform and distally margined, beardless. Cleistogamous flowers not produced. Allododecaploid (CHAM + MELVIO). Secondary base chromosome number x’ = c. 36. ITS sequence of MELVIO type.

Diagnostic characters.—All stems ascending or trailing AND corolla violet or white with white throat AND style clavate.

Ploidy and accepted chromosome counts.—12x; 2n = c. 72 (Viola abyssinica).

Age.—Crown node age c. 2 Ma; stem node age 3.6 (1.8–5.0) Ma [2].

Included species.—3. 

Viola abyssinica Steud. ex Oliv., 

Viola eminii (Engl.) R. E. Fr., 

Viola nannae R. E. Fr.

Distribution.—Highmountains of central and eastern Africa and Madagascar (Figure 1): Viola abyssinica throughout the range; V. eminii in eastern Congo, Rwanda, Burundi, Uganda to central and southern Kenya, northern Tanzania south to the Uluguru Mountains; V.  nannae in central and southern Kenya [3].

Etymology.—The name Abyssinium refers to the main distribution area in and around Ethiopia (=Abyssinia).

Discussion.—Sect. Abyssinium is one of just two endemic African lineages of Viola (the other is the South African sect. Melvio). The count of 2n = c. 72 in V. abyssinica [4] is the only count for the section and needs confirmation. Section Abyssinium has an African distribution but is phylogenetically nested within the north hemisphere tangle of allopolyploid lineages. It appears to have originated in the Pliocene, from an allopolyploid of sect. Spathulidium (8x) and one of the 4x ancestors of that lineage , which is distributed in southwestern Asia. The relatively recent origin of sect. Abyssinium from Eurasian ancestors fits a pattern commonly observed in Afrotemperate/Afromontane floral elements [5]. Becker [6] made a note that this group of species would merit a separate section, but he did not provide one. Possible hybridisation among the three species of sect. Abyssinium is briefly discussed by Grey-Wilson [3].

References

1. Marcussen, T. et al. A Revised Phylogenetic Classification for Viola (Violaceae). Plants 11, 2224 (2022)p. 42-43 

2. Marcussen, T.; Heier, L.; Brysting, A.K.; Oxelman, B.; Jakobsen, K.S. From gene trees to a dated allopolyploid network: Insights from the angiosperm genus Viola (Violaceae). Syst. Biol. 2015, 64, 84–101

3. Grey-Wilson, C. Notes on African Violaceae. Kew Bull. 1981, 36, 103–126

4.  Morton, J.K. Chromosome numbers and polyploidy in the flora of Cameroon Mountain. Opera Bot. 1993, 121, 159–172.

5. Gizaw, A.; Brochmann, C.; Nemomissa, S.; Wondimu, T.; Masao, C.A.; Tusiime, F.M.; Abdi, A.A.; Oxelman, B.; Popp, M.; Dimitrov, D.; et al. Colonization and diversification in the African ‘sky islands’: Insights from fossil-calibrated molecular dating of Lychnis (Caryophyllaceae). New Phytol. 2016, 211, 719–734

6. Becker, W. Viola L. In Die Natürlichen Pflanzenfamilien 21. Parietales und Opuntiales; Engler, A., Ed.; Wilhelm Engelmann: Leipzig, Germany, 1925; Volume 21, pp. 363–376.

Photogallery