Cladrastis clade

Introduction

Tribe Sophoreae has been considered a "tribe of convenience" to include the heterogenous assemblage of genera within Papilionoideae that have relatively simple flowers and unspecialized pinnate leaves (Polhill, 1994). Since Polhill's last (1994) treatment of Sophoreae, cladistic analyses of morphology (Chappill, 1995), pollen data (Ferguson et al., 1994), and molecular sequences (e.g. Doyle et al., 1997; Kajita et al., 2001; Pennington et al., 2001; Wojciechowski et al., 2004) have clearly shown Sophoreae to be paraphyletic, its constituent genera distributed as members of disparate, early-branching papilionoid lineages.

One of the more interesting clades to emerge from these analyses is the "Cladrastis" clade (Wojciechowski et al., 2004) containing the genera Cladrastis Raf., Pickeringia Nutt. ex Torrey & A. Gray, and Styphnolobium Schott. Cladrastis and Styphnolobium (segregate of Sophora L.) have been traditionally classified in Sophoreae sens. str. (Polhill, 1981, 1994), whereas Pickeringia has been classified in tribe Thermopsideae (Turner, 1981). The close relationship of Cladrastis and Styphnolobium is notable biogeographically because both genera exhibit East Asian-North American disjunctions.

Cladrastis, with 6 to 7 species of trees found in warm temperate montane evergreen, mixed-deciduous forests, is distributed in east Asia (6 species; China to Japan) and eastern North America (1 sp.; southern Illinois to North Carolina) (Duley and Vincent, 2003; Pennington et al., 2005). Styphnolobium, with 9 species of trees inhabiting temperate to seasonally-dry tropical woodlands to montane forests, is distributed from North and Central America (8 species) to west-central China (1 species) (Pennington et al., 2005). Pickeringia is a monotypic genus of spiny, xerophytic shrub restricted to the sclerophyllous chaparral and mixed evergreen-woodland forest vegetation of California and northern Baja California of Western North America (Raven and Axelrod, 1995).

Consistent with evidence of both Cladrastis (subgenus Cladrastis and subgenus Platycarpus) and Styphnolobium represented in the fossil record from the Middle Eocene (minimum 40 Ma) of Tennessee (Herendeen, 1992), age estimates based on rates analyses of molecular sequences suggest this is one of the oldest clades of papilionoids (Lavin et al., 2005).

Discussion of Phylogenetic Relationships

The "Cladrastis" clade is one of the well-supported, early branching clades of papilionoids to emerge from recent molecular analyses (Wojciechowski et al., 2004). In this clade, the genera Styphnolobium and Pickeringia are nested within a paraphyletic Cladrastis. The close relationship between these three genera is supported by similarities in floral morphology, chromosome number, and absence of quinolizidine alkaloids.

The position of Pickeringia in the Cladrastis clade is consistent with Polhill's (1981) and Sousa and Rudd's (1993) predictions of a close relationship between these three genera based on floral morphology, cytogenetic evidence, and the absence of quinolizidine alkaloids in Pickeringia of the type that is characteristic of other members of Thermopsideae (Turner, 1981). Recent analyses have failed to detect similar alkaloids in Cladrastis and Styphnolobium (Kite and Pennington, 2003), a finding consistent with the molecular data showing a close phylogenetic relationship between these three genera.

References

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Doyle, J. J., J. L. Balinger, E. E. Dickson, T. Kajita, and H. Ohashi. 1997. A phylogeny of the chloroplast gene rbcL in the Leguminosae: taxonomic correlations and insights into the evolution of nodulation. American J. Botany 84: 541-554.

Duley, M. L., and M. A. Vincent. 2003. A synopsis of the genus Cladrastis (Leguminosae). Rhodora 105: 205-239.

Ferguson, I. K., B. D. Schrire, and R. Shepperton. 1994. Pollen morphology of the tribe Sophoreae and relationships between subfamilies Caesalpinioideae and Papilionoideae. Pages 53-96 in Advances in Legume Systematics, part 6, structural botany (I. K. Ferguson and S. C. Tucker, eds.). Royal Botanic Gardens, Kew, UK.

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Kite, G. C., and R. T. Pennington. 2003. Quinolizidine alkaloid status of Styphnolobium and Cladrastis (Leguminosae). Biochemical Systematics and Ecology 31: 1409-1416.

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Raven, R. H., and D. I. Axelrod. 1995. Origin and relationships of the California flora. California Native Plant Society, Sacramento, California, USA.

Sousa, M., and V. E. Rudd. 1993. Revision del genero Styphnolobium (Leguminosae: Papilionoideae: Sophoreae). Annals of the Missouri Botanical Garden 80: 270-283.

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Wojciechowski, M. F., M. Lavin, and M. J. Sanderson. 2004. A phylogeny of legumes (Leguminosae) based on analysis of the plastid matK gene resolves many well-supported subclades within the family. Am. J. Bot. 91: 1846-1862.