Tribal Relationships of Sphinctospermum (Leguminosae): Integration of Traditional and Chloroplast DNA Data

Abstract

By using both consensus and combination approaches, new chloroplast DNA data are integrated into a primarily morphological data set that bears on the tribal affinities of Sphinc- tospermum. Both approaches are useful in testing hypotheses of homology among traditional char- acters but the combination approach is more useful than the consensus approach in testing hy- potheses of reticulate evolution. This is because discrete chloroplast DNA markers can be tracked on the topology of alternative cladograms. Such discrete DNA markers are derived from a chloroplast DNA phylogeny using the bootstrap resampling procedure; a chloroplast DNA lineage with a 95- 100% confidence level is scored as a discrete character in an integrated data set. The results suggest that Sphinctospermum is related to genera of Robinieae by common descent and that introgressive hybridization has not been involved in the evolution of this morphologically distinctive genus. Sphinctospermum is a monotypic genus dis- tributed along the Pacific Slope from Oaxaca, Mexico north to southeastern Arizona, U.S.A. The genus is morphologically very unusual for a member of the subfam. Papilionoideae; its leaves are entire and linear-lanceolate, its in- florescence consists of a solitary ebracteolate flower, and its seeds are squarish and constrict- ed in the middle. It is also intermediate in mor- phology between two tribes of Papilionoideae, Robinieae and Millettieae. With the former it shares a base chromosome number of x = 8, canavanine accumulated in seeds, and legumes with seemingly similar constrictions between seeds. With Millettieae it shares falcate foliar cotyledons, parallel secondary leaflet venation, a well-developed endodermis of the legume, and others enumerated by Lavin (1987). A recent cladistic analysis of primarily mor- phological data was equivocal in determining the tribal relationships of Sphinctospermum (Lav-