Phylogenetic Relationships and Evolution in the Strelitziaceae (Zingiberales)

Evolutionary trends and phylogenetic relationships in the Strelitziaceae (Zingiberales) were investigated using sequence data from ten plastid and two nuclear regions and a morphological dataset. The status of species of Strelitzia were evaluated in terms of the phylogenetic species concept. Relationships among the genera remain equivocal with two hypotheses emerging: (i) Strelitzia sister to a clade comprising Ravenala and Phenakospermum when indels are included, or (ii) Ravenala sister to the remainder of the Strelitziaceae when indels are excluded in/from the combined molecular and ‘total evidence’ analyses. Within Strelitzia, S. nicolai is sister to the rest of the genus, with S. alba sister to S. caudata. Strelitzia reginae is shown to be paraphyletic as S. juncea is nested within it, but more sampling at the population level is needed to confirm the taxonomic status of S. juncea. The highly localized and endangered Strelitzia alba is confirmed as a distinct species, as are S. caudata and S. nicolai, despite few morphological differences. Evolutionary trends are linked to changes in habitat and coevolution with pollinators. Climate change in southern Africa is thought to have restricted Strelitzia nicolai (or its ancestor) to the eastern coastal region, with subsequent allopatric speciation of S. alba and S. caudata, and relatively recent parapatric divergence of S. juncea from S. reginae. Keywords—Biogeography, molecular phylogenetics, morphology, pollination, Strelitzia. The Strelitziaceae is one of eight families in the Zingiberales, placed loosely within a basal paraphyly known as the ‘banana group’ (Kress 1990; Kress et al. 2001). The family shares with other members of the order an herbaceous habit, large petiolate distichous leaves with transverse venation, and colorful bracteate inflorescences. Features characterising the Strelitziaceae include its woody stem, boat-shaped coriaceous bract enclosing a cincinnus of flowers, three free sepals, two fused petals, and a loculicidal woody capsule containing seeds with brightly colored arils (Wright 1913; Dyer 1976; Kress 1990; Heywood et al. 2007). Despite numerous phylogenetic studies at various levels in the Zingiberales that included members of the Strelitziaceae (e.g. Kress 1990; Kress 1995; Kress et al. 2001; Givnish et al. 2006), to date there has been no thorough investigation of relationships within the Streltiziaceae. The Strelitziaceae are a charismatic family with two of the genera (Ravenala Adans. and Strelitzia Aiton) widely cultivated as ornamentals and well known throughout the world. Strelitzia reginae is emblematic of the South African National Biodiversity Institute (SANBI) and Aloha Airlines, and is currently on the KwaZulu-Natal coats of arms and on coinage in South Africa (50 cent coin), while Ravenala symbolizes Air Madagascar. Ravenala, the ‘traveller’s palm’ (Fig. 1A), grows naturally in primary rainforests of Madagascar (Kress et al. 1994), while Strelitzia (the ‘crane flower’ or ‘bird of paradise’) occurs along the east coast and eastern mountains of southern Africa (Dyer 1976; Goldblatt and Manning 2000; Coates Palgrave and Coates Palgrave 2002). The third genus in the family, Phenakospermum Endl. (Fig. 1B), grows in transitional or secondary rainforest in tropical northern and central South America (Kress and Stone 1993). Dating of phylogenies and fossils suggest a post-Gondwanan dispersal of the Strelitziaceae (Kress and Specht 2006), but relationships within the family need to be clarified to make meaningful inferences of speciation processes and/or ancestral distributions, as well as evolutionary trends associated with habitat changes and pollination shifts. Ravenala and Phenakospermum are both monotypic, while Strelitzia has five currently recognized species (Appendix 1). Confusion regarding the identity and distribution of some of the caulescent (tree-like) species of Strelitzia has occurred in the past, e.g. between the widespread S. nicolai (Fig. 1C) and restricted S. alba (Wright 1913; Dyer 1946a; Moore and Hyypio 1970) and between S. nicolai and S. caudata, the most northerly species occurring in the Soutpansberg and Barberton mountains of South Africa and the eastern highlands of Zimbabwe (Dyer 1946b, c; van Wyk and van Wyk 1997; Coates Palgrave and Coates Palgrave 2002). These species are difficult to distinguish when not flowering. In addition, there has been considerable discussion regarding the taxonomic status of the two rhizomatous herbaceous species, S. reginae (Fig. 1D, E) and S. juncea (Moore and Hyypio 1970; Dyer 1975; Dyer 1979), with S. juncea sometimes reduced to varietal level within S. reginae (Moore and Hyypio 1970). Strelitzia reginae has had no less than 22 species or varieties placed in synonymy with it since it was described by Aiton in 1789 (Moore and Hyypio 1970; Archer 2003). Currently two subspecies are recognized in S. reginae: S. reginae subsp. mzimvubuensis, known only from the lower Mzimvubu River in the Eastern Cape, differs from the widespread S. reginae subsp. reginae in having white petals and a much shorter stigma, as well as leaves with a minutely corrugated texture (van Jaarsveld and Loedolff 2007). As distributions of both S. alba and S. juncea are restricted and their populations are small, it is important to examine relationships within the genus and to establish whether the currently recognized species are valid in terms of phylogenetic species concepts (PSCs). Our aim in this study was to construct a phylogeny of the Strelitziaceae using ten plastid DNA regions, two nuclear regions, and morphological data to examine relationships and evolutionary trends within the family. In addition, we aimed to confirm or elucidate the specific status of certain of the Strelitzia species, crucial for accurate conservation assessment. The phylogeny also serves as a framework on which to hypothesize the biogeographic history of the group.