Nomenclatural changes in Coleus and Plectranthus (Lamiaceae): a tale of more than two genera

Abstract A synopsis of the genera Coleus Lour, Equilabium A.J.Paton, Mwany. & Culham and Plectranthus L’Hér. (Lamiaceae, Tribe Ocimeae, Subtribe Plecranthinae) is presented. Generic delimitation follows a recently published molecular phylogeny which identified Coleus as the sister of the remaining genera of Subtribe Plectranthinae; Plectranthus as sister to Tetradenia Benth. and Thorncroftia N.E.Br., and a separate phylogenetically distinct genus Equilabium comprising species previously placed in Plectranthus. In this treatment, 294 species of Coleus, 42 of Equilabium, and 72 of Plectranthus are recognized. All but one of the combinations in Equilabium are new as only the genus and type species have been previously published. Two-hundred and twelve names are changed to combinations in Coleus from Plectranthus, Pycnostachys Hook. and Anisochilus Benth.


Introduction
Plectranthus L'Hér. is a widely used horticultural and medicinal plant genus (van Jaarsveld 2007, Lukhoba et al. 2006, Rice et al. 2011. It belongs in tribe Ocimeae, subtribe Plectranthinae, an Old World tropical group of around 450 species. Current taxonomic treatments generally follow Harley et al. (2004) by including the formerly recognized Coleus Lour. and Solenostemon Thonn. within Plectranthus. However, within horticultural literature, Solenostemon is still frequently recognized (e.g. RHS (2017), Shepherd and Maybry (2016)). The commonly cultivated plant shown in Fig. 1 is thus variously known as Plectranthus scutellarioides (L.) R.Br or Solenostemon scutellarioides (L.) Codd., but "coleus" is frequently used as a common name for this species, increasing the confusion.
Recent phylogenetic work has shown that the circumscription of Plectranthus needs to be viewed in the context of related genera in Tribe Ocimeae, subtribe Plectranthinae. Paton et al. (2018) demonstrate that a clade containing the type of Coleus (C. amboinicus Lour. ≡ P. amboinicus (Lour.) Spreng.) is sister to the other genera of the Plectranthinae, and the type species of Plectranthus, P. fruticosus L'Hér., belongs in a clade which is sister to Thorncroftia N.E.Br and Tetradenia Benth. A further clade containing species that were previously placed in Plectranthus was recognized by Paton et al. (2018) as Equilabium Mwany., A.J. Paton & Culham (Fig. 2).
The aim of this paper is to provide the necessary nomenclatural changes consistent with the recognition of Plectranthus, Coleus and Equilabium as distinct genera following phylogenetic results provided by Paton et al. (2018). and Li and Hedge (1994) for Chinese species. Cramer's decisions were in part based upon Sri Lankan Coleus that have imbricate bracts ("floral leaves") and a decurrent upper calyx lip in addition to fused stamens. Cramer also suggested Plectranthus could have a 3-lobed upper calyx lip, but that is misleading since he was considering species of Isodon (Schrad. ex Benth.) Spach as part of Plectranthus: Isodon is now recognized in a separate subtribe and is very distinct from the Plectranthinae (Ryding 1993, Zhong et al. 2010. A common theme in the treatments listed above is that, since Briquet (1897), all have been based on regional accounts of relatively small numbers of species. Morton (1962) considered 28 species of the Coleus /Plectranthus complex and Cramer (1978), 11 species. Morton recognized the problems in this narrow regional approach noting, "hence the genus Plectranthus must for the moment be a repository for Coleus or Plectranthus-like species which cannot satisfactorily be placed in other genera" and Cramer considered his account to be " by no means conclusive, least of all authoritative". Paton in Harley et al. (2004) thus took a broader approach, considering characters used to delimit allied genera in the context of the Plectranthinae as a whole. In Paton's treatment, Coleus and the small segregate genera recognized by Morton were merged into Plectranthus, making Plectranthus s.l. morphologically recognizable from other genera and an easily communicable concept. However, this was a conservative, morphologically based account and did not tackle how best to circumscribe taxa within the large Plectranthus s.l. Paton et al. (2004) provided a preliminary test of the broad concept of Plectranthus by presenting a phylogeny of tribe Ocimeae, demonstrating that the Plectranthinae was a monophyletic group which could be generally diagnosed by having stamens  Paton et al. (2018): the numbers in triangles represent species numbers though the numbers of species in the two subclades of Coleus cannot be estimated (see discussion); dates of branching are given in black.
adnate and contiguous at the base of the anterior corolla lip. The presence of fused stamens, the original defining character of Coleus, was shown to be homoplasious by mapping it onto this phylogeny. However, the study identified two main clades within the Plectranthinae: one containing the type of Coleus and members of Solenostemon, Leocus A.Chev., Pycnostachys Hook. and Anisochilus Benth.; and the other containing the type species of Plectranthus and members of Alvesia Welw., Aeollanthus C. Mart. ex Spreng, Capitanopsis S.Moore, Tetradenia and Thorncroftia.
The phylogenetic study by Paton et al. (2018) increased the sampling of the Plectranthinae and added another plastid marker to increase tree resolution, and similarly found that Plectranthus s. l. was polyphyletic (see summary of analysis in Fig. 2). As a result of this work, Paton et al. (2018) suggested recognition of Coleus for the clade containing the Coleus type, Plectranthus s.s. for the clade sister to Thorncroftia and Tetradenia, and a new genus, Equilabium, for the clade formerly containing Plectranthus species mainly from tropical Africa, but with one in India. Paton et al. (2018) pointed out that these clades were morphologically diagnosable by characters that had been used to define informal morphological groups by Paton et al. (2009Paton et al. ( , 2013. These characters are the position of attachment of the pedicel to the calyx, the relative lengths of the corolla lobes and the shape of the corolla tube (Fig. 3). The phylogenetic results indicate that three genera recognized by Harley et al. (2004), (Pycnostachys, Anisochilus and Leocus) are nested within Coleus, and this close relationship is corroborated by species of intermediate morphology between Coleus and the former taxa Pycnostachys (Paton et al. , 2013, Anisochilus (Suddee et al. 2014), and Leocus (Pollard and Paton 2009). Paton et al. (2018) also showed that Solenostemon is not monophyletic, and that recognition of Solenostemon would render Coleus paraphyletic by its exclusion, therefore we argue that Solenostemon should not be recognised.

Methods
The phylogenetic study presented by Paton et al. (2018) was based on a representative sample of species of the Plectranthinae. Recent floristic accounts and the World Checklist Of Selected Plant Families were consulted (Govaerts et al. 2019, Codd 1985, Forster 1992, 1994, 2011. IPNI (www.ipni.org) was consulted to trace any recently described species of Coleus or Plectranthus and the protologues of these were examined. A survey of herbarium collections (AAU, BK, BKF, BM, BOL; BR, C, CALI, COI, EA, ETH, FT, G, HBG, K, L, LE, LMA, LISC, LMU, MEL, MH, MO, NHT, P, PRE, RNG, S, STU, TCD, UPS, W, WAG; Abbreviations following Thiers (2017)), was undertaken also using JSTOR Global Plants (https://plants.jstor.org/) to access type specimens not otherwise seen. Cited types have been seen unless otherwise stated. The attribution of species to the genera Coleus, Plectranthus and Equilabium was guided by the characters given in Paton et al. (2018) and in the key at the beginning of the taxonomic account below. . Calyx and corolla morphology. Calyces in Coleus with pedicel attached excentrically above midline of calyx: 1 C. betonicifolius 2 C. platyphyllus showing the rounded lateral calyx lobe in some species of the formally recognized Solenostemon. Calyx in Equilabium with pedicel attached ± at centre of calyx base with the lateral lobe held midway between the anterior and posterior lobes: 3 E. megafolium 4 E. masukensis. Calyces in Plectranthus with pedicel attached ± at centre of calyx base with the lateral lobe closer to the anterior than the posterior lobes: 5 P. alboviolaceus. Corolla shape in Coleus showing unequal anterior and posterior corolla lips: 6 C. schizophyllus 7 C. hockii. Corolla shape of Equilabium showing equal anterior and posterior corolla lips: 8 P. parvus 10 P. laxiflorus. Corolla shape of Plectranthus showing equal anterior and posterior corolla lips and gibbous-based corolla: 9 P. strangulatus;. Drawn by Emmanuel Papadopoulos. Reproduced from Flora of Tropical East Africa ).

Results and discussion
The changes in generic delimitation proposed by Paton et al. (2018) have a large impact on nomenclature. An alternative nomenclatural treatment would be to consider the monophyletic Plectranthinae to be components of a single genus, Plectranthus. Most recent taxonomic papers have included Coleus within Plectranthus. However, taking 2005 as a starting point, following publication of Harley et al. (2004) as the most comprehensive baseline taxonomy for the Lamiaceae, 15,700 papers cite Coleus in the title, but only 9320 cite Plectranthus (source Google Scholar). This suggests that Coleus is still a broadly used concept. Tetradenia with mainly dioecious, ± actinomorphic flowers is part of the sister group of the Plectranthus in the strict sense. Thus there are no easily observed morphological apomorphies that could be used to easily diagnose an enlarged Plectranthus; whereas the generic delimitation proposed in Paton et al. (2018), results in monophyletic genera which are relatively easy to diagnose morphologically.
In this treatment, 294 species of Coleus, 42 of Equilabium, and 72 Plectranthus are recognized. All the combinations in Equilabium are new as only the genus and type species E. laxiflorum have been previously published (Paton et al. 2018). 212 names need to be changed to combinations in Coleus from combinations in Plectranthus, Pycnostachys and Anisochilus. However, 82 of these combinations in Coleus already exist, reflecting previous confusion. No new names are necessary for species in Plectranthus.
Of the characters cited by Cramer (1978) as supporting the delimitation of Coleus and Plectranthus, the presence of a decurrent calyx tube is variable and found in Coleus, Plectranthus and Equilabium. Imbricate bracts do only occur in Coleus, but this character is only obvious in some species. Generally the species are very easy to attribute to Coleus, Plectranthus or Equilabium using the pedicel attachment, corolla lobe length and tube shape characters described in the key below. The presence of persistent bracts and slightly flattened nutlets also supports the division of Coleus (often cauducus bracts and slightly flattened nutlets), from Plectranthus and Equilabium (usually persistent bracts and ovoid nutlets), though those characters have more exceptions than those characters used in the key. Both Equilabium and Plectranthus have rarely more than 3-flowered cymes, whereas Coleus can have one to very many flowers per cyme. Although species can be misplaced in some instances due to atypical corolla structure, the asymetrical pedicel attachment at the base of the calyx gives the most clear indication of the species' placement in Coleus. Generic identification can be harder with small-flowered Coleus species that show asymetrical pedicel attachment but have more or less equal corolla lips; however, the number of these cases is small. In Equilabium the upper lip can be slightly shorter than the lower lip in some cases but at the extreme it is about 0.8 of the length of the lower lip, whereas in Coleus the upper lip is usually much shorter. The southern African Coleus zombensis (Baker) A.J.Paton, C. gibbosus A.J.Paton (Plectranthus rehmannii Gürke) and Coleus sessilifolius (A.J.Paton) A.J.Paton can also provide confusion as the corolla is basally gibbous, similar to that of many species of Plectranthus; however, they have a Coleus-like calyx and the corolla tube is sigmoid rather than straight as in true Plectranthus.
We present the list of species in alphabetical order, rather than according to the previously proposed infrageneric classification by Codd (1975), which is unworkable. Although the smaller infrageneric groups he recognized in the infrageneric classification of Plectranthus s.l. of Southern Africa are natural groups of related Southern African species, Codd's classification does not cover the extent of character variation and combinations seen outside the Flora of Southern Africa area, making it difficult to apply globally. As pointed out by Paton et al. (2018), groups previously thought to be easily delimited by morphological characters can be paraphyletic. For example, subgenus Calceolanthus Codd was delimited by Codd by having a dense ring of hairs in the calyx throat, but the phylogeny presented by Paton et al. (2018) suggests that the clade containing these species also contains species which lack this character. Larger infrageneric groupings recognized by Codd contain a mixture of lineages, for example, Codd's section Plectranthus while containing mostly species placed in Plectranthus here, also includes P. dolichopodus Briq. (here placed in Coleus) and P. laxiflorus Benth. and P. petiolaris Benth. (here both are placed in Equilabium).
Further study is required to provide a more robust formal infrageneric classification of Coleus and Plectranthus, although within Coleus there seems to be a continuity of form making further morphological diagnoses of clades difficult. There are two well supported clades within Coleus (clades A and B sensu Paton et al. (2018)), but no clear morphological apomorphies to diagnose these clades have been identified. Further phylogenetic work is underway by the authors: fifty species are now described for Australia and a current phylogenetic analysis focusing on these species is being prepared from a comprehensively sampled data set (T.C. Wilson, unpubl.); and an analysis of species in India is also being prepared (K. Smitha, unpubl.).

Taxonomic account
A key to the genera of the Plectranthinae is provided and followed by treaments of the three genera now containing species of Plectranthus s.l. sensu Harley et al. (2004): Coleus, Equilabium and Plectranthus. New combinations of Capitanopsis and the generic protologue of Equilabium can be found in Paton et al. (2018). No changes to other genera of the Plectranthinae are proposed. The generic descriptions are followed by a listing of the species in alphabetical order with details of synonymy, types and distribution. The account ends with a list of unplaced names, followed by a list of excluded names now placed in other genera. Readers should also consult the online treatment in the World Checklist of Selected Plant Families (Govaerts et al. 2019, www.kew.org/ wcsp) as this is continually updated and includes TDWG level 3 distributions and Raunkiaer life form descriptors. A short description of the Plectranthinae and key to the genera recognized is provided below.
Type species. C. amboinicus Lour. Description. Annual or perennial herbs or shrubs, sometimes succulent, sometimes with a fleshy or tuberous rootstock. Leaves opposite, sometimes whorled. Inflorescence thyrsoid, strongly condensed to lax, with cymes sessile or pedunculate, sometimes with long cincinnate branches, 1-many-flowered; bracts subtending cymes usually not persistent beyond flowering, sometimes erect forming a coma at apex of flowering stem. Flowers sessile or pedicellate, sometimes spirally arranged on inflorescence axis. Calyx funnel-shaped to tubular, straight or curved, often ventrally gibbous, with pedicel attaching asymmetrically at calyx base opposite the posterior lip, two lipped to regularly 5-lobed, rarely circumscissile at base (C. coeruleus); throat glabrous, pubescent or bearded, open or closed by scales or converging lips; if regularly lobed the lobes triangular, lanceolate or spinescent; if two lipped, posterior lip 1-lobed, larger or similar size to other lobes, decurrent or not; anterior lip 4-lobed with lobes variously shaped; lateral lobes lanceolate, triangular to oblong or rounded; held midway between the posterior lobe and median lobes of the anterior lip; median lobes lanceolate to triangular, sometimes separated by a sinus or sometimes fused at base for a longer distance than the lateral and median lobes. Corolla two-lipped with posterior lip shorter than anterior lip, rarely subequal; tube sigmoid to almost straight, narrow and parallelsided at base, expanding towards throat, very rarely gibbous, rarely annulate; posterior lip 4-lobed, erect or ascending; anterior lip horizontal cucullate to concave. Stamens 4; filaments fused together or free, held within the anterior lip or exserted. Style bifid with subulate lobes, rarely entire. Nutlets ovoid, often dorso-ventrally flattened.

Distribution. Cameroon.
Notes. Paton et al. (2009Paton et al. ( , 2013 considered this as a synonym of subsp edulis, but the prominent indumentum and disjunction warrant separate recognition.   Description. Annual or perennial herbs or soft wooded shrubs, rarely woody shrubs, sometimes with a persistent woody or fleshy rootstock. Leaves opposite. Inflorescence thrysoid, usually lax; cymes sessile or pedunculate, 1-3(-5, very rarely -7); bracts subtending cymes persistent. Flowers pedicelate. Calyx funnel-shaped, straight with pedicel attaching symmetrically at the calyx base, not opposite posterior lip, twolipped; throat glabrous within, open; posterior lip 1-lobed, usually broader than an-terior lobes, shortly decurrent on tube or not; anterior lip 4-lobed; lobes triangular or lancolate; lateral lobes broader or equal to median lobes and held midway between the posterior lobe and the median lobes of the anterior lip; median lobes triangular to lanceolate. Corolla two-lipped with lips equal in length; tube sigmoid, sometimes strongly so, narrow and parallel-sided at base and expanding towards throat; posterior lip 4-lobed erect or ascending; anterior lobe horizontal, cucullate, sometimes frilled at apex. Stamens 4; filaments not fused together, held within the anterior lip. Style bifid with lobes subulate. Nutlets ovoid.
42 species mainly in Tropical Africa but including two in India. Distribution. S. India, Sri Lanka. Occurrence in Sri Lanka is questionable and it is likely to be extinct there (Cramer 1981 Description. Perennial, rarely annual herbs or soft-wooded shrubs, sometimes succulent; sometimes with a tuberous base. Leaves opposite. Inflorescence thrysoid, usually lax; cymes sessile, 1-3-flowered; bracts subtending cymes persistent. Flowers pedicelate. Calyx funnel-shaped straight, with pedicel attaching symmetrically at calyx base, not opposite posterior lip, two-lipped; throat glabrous within, open; posterior lip 1-lobed, broader than lobes of anterior lip, shortly decurrent on tube or not; anterior lip 4-lobed; lobes lanceolate; lateral lobes broader at base than medial lobes and held closer to the median lobes than the posterior lip; median lobes usually fused for a greater distance than the lateral and median lobe. Corolla two-lipped with lips equal in length; tube straight or gently curved downwards, often gibbous, saccate or almost spurred a base, more rarely dilating gradually towards throat; posterior lip 4-lobed erect, often becoming reflexed; anterior lip cucullate or concave straight or becoming reflexed. Stamens 4, very rarely only 2 fertile (P. zuluensis); filaments not fused together (anterior stamen and adjacent posterior stamen sometimes fused in P. alboviolaceus), held within the anterior lip or becoming free from it and sometimes coiling downwards away from style after anthesis. Style bifid with lobes subulate. Nutlets ovoid. 72 species in Southern and Tropical Africa and Madagascar and including one in Sri Lanka.