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.

Highlights

  • One of the primary goals of biogeography is the interpretation of disjunct distributions (Brown and Gibson 1983)

  • Tpi-3 was determined to be compartmentalized in the plastid (Fig. 8-9)

  • Tpi-1 behaved as a monomeric enzyme, while Tpi-2 behaved as a dimer

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Summary

Introduction

One of the primary goals of biogeography is the interpretation of disjunct distributions (Brown and Gibson 1983). The results ofsuch studies, may be ambiguous, since rates of morphological change are not readily correlated· with time of divergence. Comparative molecular studies provide new data sets which can contribute to our understanding ofbiogeographical problems. Disjuncts can serve as a model for evolutionary biologists who are interested in comparative rates of evolution. By definition, disjunctly distributed (Brown and Gibson 1983). Intercontinental disjunctions present the most extreme manifestation of this. As such they allow us to observe evolution in the absence of gene flow, and thereby provide an ideal situation for the study of evolutionary rates

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