Evolution and differentiation of edaphic races in the Lasthenia californica complex (Asteraceae: Heliantheae)

The Lasthenia californica complex consists of two species, L. californica s.s. and L. gracilis, that include two races differing in flavonoid chemistry and edaphic tolerances. Populations of each race occur in each of the two species and the complex provides an ideal setting in which to examine the evolution of adaptations to soil conditions. Race A plants contain sulfated flavonoids and are found in habitats subjected to ionic stresses. Race C plants lack these sulfated compounds and are restricted to dry yet ionically-benign habitats. Studies described in this thesis show that the races are physiologically differentiated to deal with key environmental variables that are associated with their distinct habitats. Race A is better adapted to deal with ionic stresses, specifically with sodium and magnesium, ions that characterize their edaphic habitat. In contrast, race C is better adapted to drought, a feature that characterizes their edaphic habitat. Since both races achieve higher fitness under conditions that best match their natural environment, it is likely that the unique distribution pattern of the races has been achieved through differential adaptation. The edaphic races are also reproductively isolated via various means and it is likely that ecological selection has contributed to enhance isolation. The population genetic study conducted using RAPD markers strongly agrees with previous studies, supporting the notion of parallel occurrence of edaphic races in both L. californica s.s. and L. gracilis. It appears that race A is ancestral to race C and that edaphic specialization is an ancestral trait in the complex. Flavonoid features and traits associated with tolerance to ionic stresses appear to have been lost in race C populations of both species found under ionically-benign habitats. The L. californica complex provides an ideal model to further test both the hypotheses of adaptive differentiation and parallel evolution and conduct much-needed studies on the genetics of ecological speciation in plants.