taxonomy The status of Sophora With Sophora japonica now moved to Styphnolobium, To b y Pe n n i n g t o n and Ma r t i n Wo j c i e c h o w s k i discuss the splitting of the genus
o p h o r a is a g e n u s of deciduous or evergreen small Strees, shrubs, and perennial herbs. The species vary considerably in their vegetative, ecological and cytogenetic characteristics. Sophora belongs to the papilionoid subfamily of the legumes (Leguminosae), some times recognised as a family in its own right (Papilionaceae). Most Sophora species grow in warm temperate to tropical regions of the northern hemisphere but the genus is present on all continents and many oceanic islands. Characterized by odd-pinnately compound leaves and separate stamens, many species are of horticultural value due to their attractive, relatively large, yellow or white to purple flowers clustered in often pendulous terminal or axillary racemes. The delimitation of Sophora has always been somewhat controversial, but recent morphological, phyto chemical and DNA sequence-based studies have converged on a stable, more workable classification that has become widely accepted by the legume systematics community. The aim of this article is to outline Images World Garden Previously Sophora japonica, the Japanese pagoda tree is now Styphnolobium japonicum how these diverse sources of evidence indicate that the traditional The taxonomy of Sophora tendency towards a broad circum broad circumscription of Sophora is sensu lato: morphological data scription of Sophora (Sophora sensu no longer tenable, and that it There has always been a degree of lato) comprising about 60 species of comprises three genera, Sophora sensu controversy about the exact limits of trees, shrubs, and herbs recognised stricto (s.s., ie in the narrowest sense), the genus Sophora, with various both by legume taxonomists (eg Calia and Styphnolobium, which are workers (eg Yakovlev 1967) arguing Polhill 1981), and in the horticultural remarkably distantly related for the recognition of series of community (eg Brickell 2003). phylogenetically. distinct smaller genera. However, In 1993, Mario Sousa of the until recently, there was a general Universidad Nacional Autónoma
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Herbarium (Mexico) and Velva Rudd of the Smithsonian Institution (USA) suggested that Sophora s.l. should be split into three genera on the basis of a series of morphological and cytological criteria (Sousa and Rudd 1993). The key morphological characters were the presence of bracteolate flowers in Styphnolobium and Calia, and the fact that the legumes of these genera were indeh iscent. In contrast, Sophora s.s. has flowers lacking bracteoles, and fruits that dehisce. They also pointed out that Styphnolobium has a higher chromosome number (2n = 28) than Calia and Sophora (both 2n = 18). Styphnolobium could be further dist inguished from Calia on the basis of other morphological characters, as well as this difference in chromo some number. For example, the fruits of Styphnolobium have a fleshy mesocarp and endocarp, whereas the fruit wall of Calia is dry. Approp riately, the name Styphnolobium Schematic cladogram highlighting the placement of Sophora s.s., Styphnolobium and Calia in the derives from this key taxonomic papilionoid legumes as resolved by molecular phylogenetic studies character, referring to the astringent taste of the pulp: stryphno (Greek 1994), the segregation and subseq What has been most surprising for sour, astringent); lobion (Greek uent recognition of these two genera about the results of these molecular- for pod). The other main diagnostic from Sophora s.l. did not quickly based studies is how distantly related character separating the two genera enter the botanical mainstream, Sophora s.s., Calia and Styphnolobium is more technical and concerns the perhaps because their paper was are from each other within the morphology of the calyx: it is bilab published in Spanish. papilionoids (Fig. 1). Sophora s.s. is iate in Calia, with the upper (vexilar) placed in the ‘Genistoid clade’ (in the lobe longer than the lower (carinal) The taxonomy of Sophora sense of Wojciechowski et al. 2004), lobe compared to Styphnolobium sensu lato: DNA sequence data a group that includes familiar garden where the calyx is more truncate, In the past decade, a series of mole plants such as Lupinus and Genista. and if bilabiate, then any upper lobe cular phylogenetic studies have prov Calia is found outside this Genistoid is shorter than the lower lobe(s). ided strong support that Sophora s.l. group but closely related to it, Calia can also be distinguished from must be regarded as three separate whereas Styphnolobium is a member Styphnolobium by the polysaccharide genera exactly as suggested by Sousa of one of the earliest branching composition of its seeds; Calia, like and Rudd. The studies have been lineages of the entire Papilionoideae, Sophora (Bailey 1974), lacks galacto based upon cladistic (character alongside Cladrastis and Pickeringia. mannans (polymers consisting of a based) analyses of a diverse array of mannose backbone with galactose different regions from the chloro The taxonomy of Sophora residues as side groups). plast genome including the genes sensu lato: chemical data Although Sousa and Rudd’s matK (Wojciechowski et al. 2004) The Genistoid clade, where Sophora careful work was assimilated, and in and rbcL (Kajita et al. 2001) and the s.l. belongs, was a novel taxonomic some cases followed, by the legume non-coding trnL intron (Pennington grouping when it first emerged from systematics community (eg Polhill et al. 2001). molecular studies in the 1990s, ➤
September 2008 187 taxonomy and was not previously defined or Calia uniquely characterised by any morph Calia is a genus of four species of ological characters. However, it was small trees and shrubs named after quickly realised that virtually all of S D Antonio Cal, a professor of the 65 genera that accumulate quin botany in Puebla, Mexico. The olizidine alkaloids, well known for species are distributed in the south their toxicity to humans and live western USA (Arizona, New Mexico stock, belonged to the Genistoid and Texas) and south to central clade, and that the presence of these Mexico. They are characteristic of alkaloids might therefore constitute arid to seasonally dry habitats, such a defining character for the group in as woodlands, chaparral and desert. having systematic significance at a Calia species are distinguished by higher taxonomic level in the family their often long, highly decorative, (eg Crisp et al. 2000, Pennington et pendulous racemes of purplish al. 2001). flowers, and coriaceous (leathery) Given the distant placement of leaves. Calia secundiflora (Texas Styphnolobium from the Genistoid mountain laurel, mescal bean) is clade in the DNA phylogenies, widely cultivated in arid regions of Geoffrey Kite, a phytochemist from the USA as an ornamental and its Royal Botanic Gardens, Kew, set out large red seeds are used to make to determine whether species of this jewellery. MF Wojciechowski genus, and Calia, contained quinol Flowers and fruit of Calia secundiflora, an izidine alkaloids. He tested extracts Styphnolobium attractive tree widely cultivated in warmer of leaves, flowers, and seeds. The Styphnolobium is a genus of nine parts of the USA results indicate that quinolizidine species, of which the best known is alkaloids are absent in Styphnolobium the widely cultivated S. japonicum the genus Pickeringia (chaparral pea) species, but present in Calia secund (Japanese pagoda tree), which is is the third member of the ‘Cladrastis iflora and several Sophora s.s. species native to west and central China. clade’ defined by Wojciechowski that were tested (Kite & Pennington Interestingly, the remaining eight and co-workers (2004). Like Styphno 2003). These results provided species of the genus are found in arid lobium, Pickeringia and Cladrastis are further corroboration that Styphno and seasonally dry habitats of the known to have chromosome numbers lobium should be regarded as separate southern USA, Mexico, and Central of 2n = 28 (Sousa and Rudd 1993). from Sophora s.s. and Calia, just as America, with one species reaching Pickeringia, a monotypic genus Sousa and Rudd had suggested on Colombia. This disjunct eastern restricted to the sclerophyllous the basis of morphological evidence. Asian – American distribution may chaparral vegetation of the Pacific seem odd, but it is shared with coastal ranges and foothills of the The genera as currently Cladrastis, to which Styphnolobium Sierra Nevada in the California recognised is closely related, other legumes Floristic Province, has been The most authoritative and up-to- (eg Gleditsia), and a series of other considered one of the relictual date account of legume classification, angiosperm genera including lineages of the North American which reflects the consensus view Aesculus, Anemone and Liriodendron. extension of tribe Sophoreae (along of the world’s legume systematists, These eastern Asian – American with Cladrastis) from the Sino- is given in the beautifully illustrated disjunct distributions of warm Himalayan region of Asia (Polhill volume Legumes of the World, temperate/tropical taxa are cons 1981), and an example of the classic published by the Royal Botanic idered to be vestiges of a more ‘palaeo-endemics’ which are Gardens, Kew, in 2005 (Lewis et al. widespread ‘paratropical forest’ common in the flora of California 2005). In this book, based upon all vegetation found in the northern (Stebbins and Major 1965). the evidence outlined above, three hemisphere during the early genera are recognised: Sophora, and Tertiary, which became fragment- Sophora the two segregates Calia and ed as global climates cooled. The good news for horticulturists Styphnolobium. Closely related to Styphnolobium, who do not like too many changes of
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taxonomic names is that virtually all of the widely cultivated species of Sophora are still in the genus in the strict sense. Sophora, as narrowly defined, comprises about 50 species, distributed principally in Eurasia (southeast Europe, west, central and east Asia) and south through tropical regions to Australasia and the Pacific. There are three to four species in temperate South America that belong to section Edwardsia, which contains the most widely cultivated ornamentals such as S. tetraptera, S. microphylla and S. davidii. The yellow-flowered species, and hybrids derived from them such as Sophora Sun King (‘Hilsop’), will remain as Sophora
Conclusion RHS Herbarium
There is now overwhelming references evidence from comparative analyses Bailey, RW (1974) Galactomannans H, Klitgaard, BB & Preston, J of DNA sequences, chromosome and other soluble polysaccharides in (2001) Phylogenetic relationships of numbers and morphology that Sophora seeds. New Zealand J. Bot. 12: primitive papilionoid legumes based Sophora s.l. should be considered 131–136 upon sequences of the chloroplast three separate, and distinct genera: Brickell, C (2003) The Royal intron trnL. Syst. Bot. 26(3): 537–566 Calia, Styphnolobium and Sophora s.s. Horticultural Society Encyclopedia Polhill, RM (1981) Sophoreae. In Although their morphological of Garden Plants. RHS / Dorling Polhill, RM & Raven, PH (eds.) Kindersley, London Advances in Legume Systematics, part 1. differences may appear somewhat Crisp, MD, Gilmore, S & Van Royal Botanic Gardens, Kew subtle, these three genera occupy Wyk, B-E (2000) Molecular Polhill, RM (1994) Classification of phylogenetically disparate, long phylogeny of the genistoid tribes of the Leguminosae. In Bisby, FA, separated positions in the papilionoid legumes. In: Herendeen, P Buckingham, J & Harborne, JB (eds.) papilionoid legume evolutionary & Bruneau, A (eds.) Advances in Phytochemical Dictionary of the tree, which reinforce their Legume Systematics, part 9. Royal Leguminosae 1: plants and their systematic distinction. In fact, a Botanic Gardens, Kew constituents. Chapman and Hall, recent study of the timing of legume Kajita, T, Ohashi, H, Tateshi, Y, London Bailey, CD & Doyle, JJ (2001) rbcL Sousa, SM & Rudd, VE (1993) evolution using techniques that and legume phylogeny, with particular Revisión del género Styphnolobium estimate divergence times (Lavin et reference to Phaseoleae, Millettieae and (Leguminosae: Papilionoideae: al. 2005) indicates that the lineages allies. Syst. Bot. 26: 515–536 Sophoreae). Ann. Missouri Bot. Gard. leading to these three genera Kite, GC & Pennington, RT (2003) 80(1): 270–283 diverged from each other, or last Quinolizidine alkaloid status of Stebbins, G L & Major, J (1965) shared a common ancestor, more Styphnolobium and Cladrastis Endemism and speciation in the than 50 million years ago – almost (Leguminosae). Biochem. Syst. Ecol. 31: California flora. Ecol. Monogr. 35: 1–35 1409–1416 Wojciechowski MF, Lavin, M & back to the age of the dinosaurs! Lavin, M, Herendeen, PS & Sanderson, MJ (2004) A phylogeny Wojciechowski, MF (2005) of legumes (Leguminosae) based on To b y Pe n n i n g t o n is Head of Evolutionary rates analysis of analyses of the plastid matK gene the Tropical Diversity Section at the Leguminosae implicates a rapid resolves many well-supported Royal Botanic Garden Edinburgh diversification of lineages during the subclades within the family. Amer. Ma r t i n Wo j c i e c h o w s k i is a Tertiary. Syst. Biol. 54: 530–549 J. Bot. 91: 1846–1862 Professor in the School of Life Lewis, G, Schrire, B, Mackinder, B Yakovlev, GP (1967) Zametki po Sciences at Arizona State University & Lock, M (2005) Legumes of the sistematike i geografii roda Sophora L. World. Royal Botanic Gardens, Kew i blizkikh rodov. Vopr. Farmakogn. 4: in Tempe, Arizona, USA Pennington, RT, Lavin, M, Ireland, 42–62
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