Englerarum (Araceae, Aroideae): a new genus supported by plastid and nuclear phylogenies

The number of recognized genera in the family Araceae has increased in the last few years from 105 (Mayo et al. 1997) to 118. Reasons for this, besides for the inclusion of the duckweeds (as Lemnoideae) in the family, are the discovery of new species of e.g., Pichinia P.C.Boyce & S.Y.Wong (Wong and Boyce 2010b) and Lorenzia E.G.Gonç (Gonçalves 2012) and new taxonomic work on the highly speciose Schismatoglottideae group (Boyce and Wong 2008; Wong and Boyce 2010a, c). In addition, new molecular phylogenetic studies have shown the non-monophyly of already described genera and resulted into new combinations such as in Croatiella E.G.Gonc¸, Incarum E.G.Gonç (Gonçalves 2005), and Typhonium Schott (Cusimano et al. 2010). Molecular analyses of four plastid regions (trnL, trnL-F, trnK/matK, and rpl20-rps12) and one nuclear gene (phyC) revealed that the species described as Alocasia hypnosa J.T.Yin, Y.H.Wang & Z.F.Xu has to be excluded of Alocasia (Schott) G.Don to maintain the monophyly of Alocasia (Nauheimer et al. 2012a). Plastid and nuclear phylogenies both corroborated that A. hypnosa is not a part of any closely related genera, but represents its own and separate lineage next to the four major clades in the Pistiaclade (sensu Cusimano et al. 2011), the Colocasia-clade (comprising Ariopsis J.Graham, Colocasia Schott, Remusatia Schott, and Steudnera K.Koch), the Areae-clade (comprising the tribes Areae and Arisaemateae), and the Alocasia-clade (Alocasia and Leucocasia Schott).
Alocasia hypnosa is a lithophytic or occasionally terrestrial herb up to 1 m in height (Fig. 1). It occurs on humid ledges, exposed areas of forest regrowth, and deciduous, bamboo-dominated forest on karst limestone at an altitude of approximately 800–1,000 m (Fig. 1a). Alocasia hypnosa ranges from SW China (type locality) to the Lao PDR, and N and SE Thailand. It has prominent stolons for vegetative reproduction that enable colonization on almost vertical limestone cliffs. Wang et al. (2005) described this species based on a collection from southern Yunnan, China. The species was assigned to Alocasia based on overall similarity to Alocasia odora K.Koch. Two morphological characters distinguish A. hypnosa from all other Alocasia species; the absence of wax glands in the axils of the primary veins on the abaxial surface of the leaf blade, and the production of long stolons with tubercles for vegetative reproduction.
Further A. hypnosa has a fully deciduous habit, which is unusual for Alocasia, and a protracted leafless resting phase, which is unknown in Alocasia. Alocasia hypnosa was also compared to species of the morphologically similar Colocasia; however, the two genera differed in the type of placentation and their fruits (Hay 1998).
Here, we use nuclear and plastid data to show that A. hypnosa does not group with Alocasia or with any other closely related genus, but instead represents an isolated lineage in the Pistia-clade. We propose a new genus for the family Araceae in the subfamily Aroideae: Englerarum Nauheimer & P.C.Boyce, a new combination for this species, Englerarum hypnosum (J.T.Yin, Y.H.Wang & Z.F.Xu) Nauheimer & P.C.Boyce.