Allozyme Divergence and the Mode of Speciation for Coreopsis gigantea and C. maritima (Compositae)

Abstract

Coreopsis gigantea and C. maritima consist of herbaceous perennial, diploid plants oc- cupying maritime habitats in California and Baja California. They are distinct from all other members of Coreopsis and comprise the only two members of section Tuckermannia. The two species differ by a number of morphological features but their synthetic hybrids are highly fertile, and their allopatric distributions appear to be largely responsible for lack of hybridization under natural conditions. An electrophoretic study of 10 enzymes revealed a lowered mean genetic identity for populations of the two species as compared to conspecific populations. The lowered identity ostensibly results in part from mutations that have accumulated at isozyme genes during the gradual divergence of the two taxa via a geographic mode of speciation. This pattern is contrasted with that found in a number of annual plants where presumed rapid speciation is accompanied by little or no divergence at genes specifying soluble enzymes. Greater gene diversity was found in C. maritima than in C. gigantea, and this suggests possible alternatives within the geographic mode of divergence. Apparent gene duplications for several enzymes are reported and discussed briefly.