Differentiation and Migration of Cakile (Cruciferae): Seed Glucosinolate Evidence

Abstract

Paperand gas-chromatographic analyses for seed glucosinolates are reported for 20 natural and two botanical garden samples of Cakile, thus completing coverage of all 14 taxa in the genus throughout its native and naturalized range. Sixteen glucosinolates, deriving from three major biosynthetic pathways, were identified. Plants with distinctive and characteristic arrays of seed glucosinolates (chemical races) generally correspond to morphologically recognizable species or subspecies. Chemical differentiation in the genus involved diversification in Mediterranean strand populations, follozwed by simplification in glucosinolate composition coincident with further evolutionary migration in the Old World and to the New TWVorld. Glucosinolates (mustard oil glucosides; Ettlinger 8c Kjar, 1968), a class of thioglucosides restricted to approximately 5,000 species of dicotyledons, have been the focus of numerous interesting ecological investigations (Whittaker & Feeny, 1971; Feeny, 1975, for a partial listing). Less convincingly have they been demonstrated to be of systematic interest and utility. Merxmiiller and Leins (1967) advocated the separation of the Rhoeadales into the orders Capparales and Papaverales by citing, among other characters, the presence of glucosinolates (and of the associated enzyme myrosinase in specialized myrosin cells) in the former and apparent absence in the latter. Kjar (1974) suggested that the occurrence of glucosinolates in the Limnanthaceae and Tropaeolaceae strengthened Cronquist's view (1968) that these families are phylogenetically related. Few comprehensive studies have been undertaken at lower taxonomic ranks, however, and one of the foremost investigators of these secondary plant products could write recently (Kjar, 1974): At the genus and species level knowledge of the glucosinolate pattern has, in a few cases, been put to good use as an auxiliary character in analyzing delimitations. The data available are, however, insufficient to define their specific merits for such purpose. Recent monographs of the cruciferous genera Thelypodium (Al-Shehbaz, 1673) and Cakile (Rodman, 1974) now clearly demonstrate the taxonomic utility of these compounds and reveal the potential of glucosinolate analyses for studies of evolutionary differentiation, migration, hybridization and introgression, and coevolution. The first report on Cakile (Rodman, 1974) presented data on seed 1 I thank Ihsan Al-Shehbaz, Hansjoerg Eichler, James Mandaville, Jr., and especially Martin G. Ettlinger for their generous gifts of seed samples. This study was supported in part by National Science Foundation Grant BMS-75-0331 1. 2 Biology, Yale University, New Haven CT 06520. This content downloaded from 207.46.13.181 on Thu, 14 Apr 2016 06:57:26 UTC All use subject to http://about.jstor.org/terms 138 SYSTEMATIC BOTANY [Volume 1