Oxalis simplicifolia (Oxalidaceae), an unusual new unifoliolate species from the Marquesas Islands (French Polynesia)

Abstract Oxalis simplicifolia Lorence & W. L. Wagner sp. nov., a new species from the Marquesas Islands (Ua Huka) is described and illustrated. It differs from the other Marquesas species, Oxalis gagneorum, in having simple, unifoliolate glabrous leaves, minutely glandular-puberulent calyx lobes, shorter corolla lobes, erect capsules, and smaller seeds. As its habitat is under serious threat from human impact, feral animals, and weeds, we conclude this new species should be added to the IUCN Red List as critically endangered (CR).


Introduction
Intensive botanical exploration of the Marquesas Islands (French Polynesia) for the Vascular Flora of the Marquesas Islands and Flore de la Polynésie française projects has resulted in numerous additional new collections from these islands. During the collecting expeditions for the current Vascular Flora of the Marquesas Islands project under the direction of David H. Lorence and Warren L. Wagner (Wagner and Lorence, 1997) a unique unifoliolate species of Oxalis was collected on the island of Ua Huka by Steve Perlman and Ken Wood of the National Tropical Botanical Garden. An analysis of the conservation status of this new species reveals it should be included as a critically endangered (CR) species in the IUCN Red List.
Oxalis is a cosmopolitan genus of approximately 500 to 700 species with its greatest diversity in South America and the African Cape region (Mabberley 2008). Previously only a single native species of Oxalis was known from the Marquesas: O. gagneorum Fosberg & Sachet, a small shrub 30-40 cm tall endemic to the islands of Eiao, Hiva Oa, Tahuata, and Fatu Hiva. The naturalized herbaceous species O. corniculata L. also occurs in suitable habitat on most of the main islands.

Oxalis simplicifolia
Distribution. Known only from Ua Huka, Marquesas Islands. Ecology. Known only from two localities on Ua Huka, this new species occurs in shrubby and herbaceous vegetation on vertical basalts cliff above a mesic to wet lowland forest zone with Freycinetia impavida (Gaudich. ex Hombr.) B.C. Stone, Hibiscus tiliaceus L., Pandanus tectorius Parkinson, and Pisonia grandis R. Br. Plants grow scattered on cliffs rooting in rock crevices (Figure 2A Discussion. Although the majority of Oxalis species have palmately compound leaves with three (rarely to nine) leaflets, several taxa with unifoliolate leaves occur in South America in subgenus Thamnoxys (Lourteig 1994). At least three South African taxa are also unifoliolate: Oxalis monophylla L., Oxalis salteri L.Bolus, and Oxalis flava L. var. unifoliolata Dreyer & Oberl. (Dreyer et al. 2010). These are small, bulbous, acaulescent plants with white or yellow flowers apparently unrelated to O. simplicifolia. Certain species, e.g. O. renifolia Kunth and a few other South American taxa can have one and three leaflets on the same branch (E. Emshwiller, pers. comm. 2009). Following Lourteig's (1994Lourteig's ( , 2000 monograph O. simplicifolia keys to subgenus Monoxalis (leaves simple, 1-foliolate, stigmas linguiform) which consists of two herbaceous species from the southwestern United States and Mexico (O. dichondrifolia A. Gray and O. robusta Kunth), neither of which bears any morphological similarity to O. simplicifolia.
This new species may be related to the Marquesas endemic O. gagneorum, from which it differs by its glabrous simple, unifoliolate leaves with ovate to broadly ovate blades having subpalmate venation of one to three basal vein pairs, minutely glandular puberulent calyx lobes, shorter corolla lobes 8-12 mm long, shorter stamens 5-8 mm long, capsules not twisting at maturity (they often twist in O. gagneorum), and smaller seeds 0.8-0.9 × 0.5 mm (Table 1). Fosberg and Sachet (1981: 3-5, Fig. 1 Kunth & Schlechter, a species belonging to section Caledonicae (= section Neocalidonicae), but that mature seeds were needed for more accurate placement. Lourteig (2000) placed O. gagneorum in section Rhombifoliae along with several neotropical species characterized by "lianoid" transversely striate stems [a character not apparent in material of either Marquesan species studied by us] and trifoliolate leaves with oblong to rhomboidal leaflets and lacking stipules, but expressed doubt as to its relationships due to the poor material available for study (i.e., lacking stigmas and seeds). Unfortunately, it has not been possible to obtain DNA sequences from samples of either Marquesan species thus far (E. Emshwiller, pers. comm. 2009), and consequently their phylogenetic relationships remain unclear.
Oxalis species often have different floral morphs, frequently tristylous or sometimes distylous (Weller et al. 2007). Examination of material of O. gagneorum revealed most flowers appear to be homostylous with styles about equaling the stamens. Due to a paucity of flowering collections of O. simplicifolia, only the long-styled floral morph with both whorls of stamens shorter than the style is known (illustrated in Fig. 1). There may be several possible explanations for this: either O. simplicifolia might be a clonally reproducing species with a single morph (as are several Oxalis species in Mexico), or it could be an autogamous species that reproduces sexually but has only a single morph (S. Weller, pers. comm. 2009). Further collections and field studied are clearly necessary to resolve this question.