[2.12.5] Viola sect. Nosphinium subsect. Nosphinium

Viola subsect. Nosphinium (W. Becker) Marcussen & H. E. Ballard, stat. nov. 

≡ Basionym: Viola sect. Nosphinium W. Becker in Nat. Pflanzenfam., ed. 2 [Engler & Prantl], 21: 374. 1925—Lectotype (Espeut 2020 [61], page 34): Viola chamissoniana Ging.

≡Viola [unranked] (”Gruppe”) Sandvicenses W. Becker in Beih. Bot. Centralbl., Abt. 2, 34: 209. 1917.

Lectotype (designated here): Viola chamissoniana Ging.

Description.—Branching or non-branching shrubs or treelets, rarely perennial herbs (Viola kauaensis). Axes morphologically differentiated into erect stems, rarely rhizomes (V. kauaensis), and lateral floriferous stems or branches (very rarely absent). Leaves of floriferous stems in most species reduced to a pair of stipules, giving the floriferous stem the appearance of a leafless, bracteose, 1–4-flowered inflorescence; rarely floriferous stems with normal-sized leaf laminas (V. chamissoniana and V. kauaensis) or reduced leaf laminas (V. tracheliifolia). Stipules triangular, free, sheathing the stem, glandular-lacerate. Laminas crenulate, undivided. Calycine appendages short and truncate to rounded. Petals on the inside violet or whitish, concolourous and lacking violet striation, lateral sometimes bearded; petals often violet on the back side. Style cylindrical or slightly clavate with a weak dorsolateral swelling and thick blunt or short conic rostellum. Cleistogamous flowers produced in V. kauaensis only. Allo-14-ploid (10x with one additional 4x genome from sect. Plagiostigma). Inferred secondary base chromosome number [x’ = 40].

Diagnostic characters.—Woody (rarely herbaceous) AND aerial stems AND stipules triangular, acute or acuminate, glandular-lacerate and sheathing the stem AND cleistogamy absent (rarely present).

Ploidy and accepted chromosome counts.—14x; 2n = c. 76, c. 85, c. 86.

Age.—Crown node 5.0 (3.4–6.5) Ma (Figure 1), stem node age 3.9–7.2 Ma

Included species.—9. 

Viola chamissoniana Ging., 

Viola helena C. N. Forbes & Lydgate, 

Viola kauaensis A. Gray, 

Viola lanaiensis W. Becker, 

Viola maviensis H. Mann, >iNat

Viola oahuensis C. N. Forbes, iNat

Viola robusta Hillebr., >iNat

Viola tracheliifolia Ging., >iNat

Viola wailenalenae (Rock) Skottsb.

Distribution.—Hawaiian Islands.

Discussion.—This endemic Hawaiian Island subsection arose from a secondary allopolyploidisation including genomes of the allodecaploid ancestor of the Nosphinium lineage and a Pacific sublineage of allotetraploid subsect. Stolonosae (different from that leading to the Mexicanae) [45]. Subsection Nosphinium is represented by nine species, most of which are woody and produce lateral 1-4-flowered leafless inflorescences. These species have entirely rayless wood, which agrees with the phylogenetic inference that woodiness is secondary [45,81,278]. Viola tracheliifolia, the largest species, is a branched shrub or treelet with lateral inflorescences with reduced (but not absent) leaf laminas. Only V. kauaensis has retained the presumably ancestral, herbaceous habit and lateral floriferous stems with solitary flowers in the axil of normal leaves (i.e., peduncles not clustered together on leafless lateral axes) and is the only species producing cleistogamous flowers. The predominantly woody habit and racemose inflorescence, broad semi-sheathing stipules, style with apex bent into a tall short rostellum, and near-absence of cleistogamy define the subsection. An initial phylogenetic study using ITS [81] indicated V. langsdorffii erroneously as a direct sister taxon to subsect. Nosphinium, but the relationships were later shown to be more complex due to separate allopolyploid origins in the Langsdorffianae and Nosphinium lineages [45].

Ballard et al. [81] indicated that the initial diversification occurred on the oldest island of Kauai, with speciation occurring along ecological gradients, and later dispersal and further speciation to younger islands eastward. Havran et al. [85] reanalysed biogeography of subsect. Nosphinium with more sophisticated models and arrived at a scenario involving initial dispersal to Maui Nui. A reanalysis of the molecular data set by T.M. arrived at the original finding of colonisation beginning on Kauai (Figure 25), as supported by both ancestral state reconstruction and inferred node ages, and subsequent dispersal and diversification proceeding eastward per the Progression Rule, i.e., hypotheses of phylogeographic congruence among codistributed taxa that track the ages of the islands [279]. This scenario receives further support from the facts that Kauai is home to the only species that has retained the ancestral herbaceous morphology (V. kauaensis) and that the average branch length is higher for taxa on Kauai than for taxa on any other Hawaiian island.