Blumea 64, 2019: 23–91 ISSN (Online) 2212-1676 www.ingentaconnect.com/content/nhn/blumea RESEARCH ARTICLE https://doi.org/10.3767/blumea.2019.64.01.02 The tuberous epiphytes of the Rubiaceae 7: a revision of the genus Hydnophytum M.H.P. Jebb1, C.R. Huxley2 Key words Abstract Hydnophytum (Psychotrieae Hydnophytinae) is revised. A key is provided, and an informal infrageneric grouping is proposed. Variation in tuber structure is discussed. Fifty-five species are recognised, of which 19 are Fiji described as new. For one species a number of varieties are also described. Forty-four species are found in New Hydnophytum Guinea, of which 41 are endemic. The recent transfer of a number of taxa into an expanded concept of Squamellaria Malesia based on a molecular phylogeny is addressed. Three taxa of Squamellaria are included in this revision to complete Psychotrieae the series of papers on the tuberous epiphytes of the Rubiaceae. revision Rubiaceae Published on 15 March 2019 Squamellaria INTRODUCTION on Hydnophytum from Leiden, which contains many unpub- lished drawings and descriptions of species he had intended Hydnophytum Jack is the largest of the five genera of tuberous publishing. ant-plants in the subtribe Hydnophytinae (Rubiaceae) (Huxley & Jebb 1991a). The other four genera have been revised (Huxley PHYLOGENETIC CONSIDERATIONS & Jebb 1991b, c, 1993, Jebb 1991b, 1993, Chomicki & Renner 2016). During the revision work, substantial new collections Huxley & Jebb (1991a) formed the subtribe Hydnophytinae to were made in Papua New Guinea; however, major problems still include the five epiphytic myrmecophilous genera of the Psy­ remain with the understanding of the genus in the Indonesian chotrieae. Sohmer (1988) had hypothesized that the Hydno­ archipelago. The taxonomic history of the whole group has been phytinae were derived directly from climbing species of Male- described by Beccari (1884) and Huxley (1993). sian Psychotria L. Based on evidence from DNA sequence Of the 111 species names and 14 subspecific names of Hydno­ variation (e.g., Andersson & Rova 1999, Nepokroeff et al. phytum that have been published, 36 are retained here, while a 1999, Andersson 2002) the Hydnophytinae are monophyletic, further 19 species are described as new. Eleven species remain and are nested within the tribe Psychotrieae. Nepokroeff et al. known from their holotype alone; a problem with a number of (1999) suggested the Hydnophytinae would be better ranked species in New Guinea. Six further species remain inadequately as a subgenus of Psychotria. known, either through loss of the type, or paucity of material, The analysis by Andersson (2002), utilising rps16 (cpDNA) data, and these are retained as ‘little known species’. Five names are identified the Hydnophytinae as belonging to a diverse Pacific excluded from the genus. Apart from Beccari (1884–1886) few clade which excluded Psychotria s.str., but included several authors have illustrated new taxa, and therefore all the species other distinctive genera (Amaracarpus, Calycosia, Dolianthus, have been illustrated here. Straussia and Streblosa). Andersson (2002) concluded that the question of how to treat the Pacific clade taxonomically METHODS remained open, however, as recognising strongly supported groups such as the Hydnophytinae taxonomically leads to This paper is based on morphological characters. The authors consequences for the remaining groups. examined all known collections from the herbaria in A, ABD, Andersson’s work (2002) indicated that Hydnophytum was para- BISH, BM, BO, BRI, BSIP, CANB, CGE, E, F, FHO, FI, FU, phyletic, with the endemic Fijian genus Squamellaria, closely G, GH, K, L, LAE, P, PNH, SING, SUVA, UC, UPNG, US and related to the Fijian taxon H. grandiflorum Becc. Morphology WRSL. Extensive field-based studies, including dissection of of the inflorescence and pollen supports this view. Andersson tubers, were conducted by the authors in Papua New Guinea, (2002) has proposed that all species of the Hydnophytinae Indonesia, and Fiji. Of the 55 species recognised, 25 were ex- are united into the single genus Hydnophytum, since this is amined in the field. The arrangement of synonyms is organised the choice that leads to the smallest number of new combi- chronologically in homotypic paragraphs. All cited specimens nations. Razafimandimbison et al. (2014) using gene trees were examined by the authors unless explicitly indicated as based upon two nuclear and five plastid regions, have recently ‘not seen’ or ‘presumed lost’. We have also had the benefit of made the generic combinations making all the members of the using Theodoric Valeton’s (1855–1929) manuscript material tribe Psychotrieae synonymous with the genus Psychotria. Whilst this creates a monophyletic grouping (with some 4 000 1 National Botanic Gardens, Glasnevin, Dublin 9, Ireland; formerly Depart- taxa), the authors admit that the phylogenetic relationships ment of Plant Sciences, Oxford University, South Parks Road, Oxford, OX1 3RB, UK; corresponding author e-mail: [email protected]. are only partly resolved and continue to be a major challenge 2 Picketts Heath, Ridgeway, Boars Hill, Oxford OX1 5EZ, UK.. 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Nothing in this license impairs or restricts the author’s moral rights. 24 Blumea – Volume 64 / 1, 2019 Recently, Chomicki & Renner (2016) have recognised an ex- THE GENUS HYDNOPHYTUM panded concept of Squamellaria, based upon a molecular phylo- geny. Those Hydnophytum species related to the former Squa­ Amongst the Hydnophytinae, the genus Hydnophytum is here mellaria s.str. (Beccari 1885, Jebb 1991b) have been included distinguished on the basis of its 4-merous flowers lacking squa- in a sister clade to all other taxa of the Hydnophytinae (Cho- mules within the corolla, and having an unmodified stem with micki & Renner 2016). The authors acknowledged that there interpetiolar stipules (with the exception of H. mamberamoense is unfortunately no useful morphological character that can Jebb & C.R.Huxley and H. trichomanes Jebb & C.R.Huxley). be used to distinguish the recircumscribed genus other than The remaining genera each have derived characters unique the geographic distribution of these species in the Solomon, to their genus, whereas Hydnophytum is largely devoid of Vanuatu and Fijian Islands (Chomicki & Renner 2016). Whilst apomorphies and is a plesiomorphic, paraphyletic grouping, as removing the predictive value of morphology, this satisfies a confirmed by all molecular studies to date (see above). monophyletic approach to genus recognition in the subtribe. On the criteria of monophyly, morphological diagnosability, and INFRAGENERIC GROUPINGS geographic coherence, the Hydnophytinae and the included genera are of utility, providing a category whereby species In the absence of a complete phylogeny, besides the Squamel­ can be discussed from an ecological, biological or evolutionary laria clade (Chomicki & Renner 2016), an informal infrageneric point of view. Whether this is recognised as a subgenus, or grouping of species was developed because of the number of even an informal category in the future concept of Psychotria species within the genus. Natural divisions are limited in that remains to be seen. they only separate a few groups of species, leaving the remain- Morphological studies of tuber structure have considerably im- der of the genus unresolved. Ten groupings have developed proved generic delimitation within the subtribe. Using characters during the revision, and these provide a useful framework for provided by the study of fresh material, a morphological cladistic examining the genus, and are practical for identifying specimens analysis of the subtribe has been undertaken (Jebb 1985). that prove difficult to key out, since in many cases we have been Morphological insights from tuber and inflorescence structure obliged to use combinations of flower, fruit and pyrene char- in particular suggest that whilst all the remaining genera of acters, which may well be absent from a specimen. Although the Hydnophytinae are derived, Hydnophytum is paraphyletic the majority of these groups can be considered to be more (Jebb 1985, 1991a). The derived genera, namely Anthorrhiza or less natural (1, 2, 3, 4, 7, 9, 10), two groupings are purely C.R.Huxley & Jebb, Myrmecodia Jack, Myrmephytum Becc. and morphological-geographic (5, 6), whilst ten species remain in Squamellaria Becc., appear to be monophyletic themselves, but a mixed bag grouping (8). share no particular affinities to one another. This conclusion is 1) Formicarum group based upon the fact that, unlike Hydnophytum, the four latter 2) Ovatum group genera each embody a characteristic tuber-cavity architecture 3) Petiolatum group and have various derived morphological features of the stems 4) Moseleyanum group which appear to be