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545

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542

Abstract

The genus Datisca comprises two species and has an intercontinentally disjunct distribution: D. cannabina is native to southwest and central Asia, whereas D. glomerata is distributed from northern California to northern Baja California. In 1975, Axelrod proposed a geohistorical scenario to account for such "Madrean-Tethyan links," suggesting that these disjunctions resulted from migration across the mid-Atlantic from the Paleogene up to the Neogene, approximately 23 to 65 m.y.a. The two species are quite similar in most phenotypic traits which have been studied to date. The major difference between the two involves their breeding system: D. cannabina is dioecious while D. glomerata is apparently androdioecious. Despite these similarities, Nei's mean genetic identity between the two species is I = 0.142. This is one of the lowest values yet reported for congeneric flowering plants and provides evidence for an ancient origin of the disjunction. Furthermore, the fact that the western populations of D. cannabina have a much higher genetic identity value with D. glomerata than does the eastern population supports the idea that dispersal occurred across the Atlantic. In addition, the population genetic structure of D. glomerata is consistent with an androdioecious breeding system.

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© 1990 Aaron Liston, Loren H. Rieseberg, Thomas S. Elias

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