Allozyme Divergence in Coreopsis cyclocarpa (Compositae)

Coreopsis cyclocarpa is a diploid, suffrutescent perennial occurring in the grasslands of high plateau regions of Mexico. The species consists of two allo- patric varieties, var. pinnatisecta and the typical one. They are very similar morpho- logically and appear to differ only by a feature of the leaves. The two infraspecific taxa are highly interfertile. Electrophoresis was utilized to ascertain the amount of divergence between the varieties at 20 genetic loci coding for soluble enzymes. There is high genetic identity among populations of the same variety, with mean values of 0.98 for populations of var. cyclocarpa and 0.95 for var. pinnatisecta. When comparing mean identities for populations of the two varieties, the value is 0.75, indicating genetic divergence between the taxa. It is suggested that genetic differentiation between the two varieties is a reflection of the time they have been isolated. In an earlier study one of us (Crawford 1970) reported on the mor- phology, flavonoid chemistry, chromosome number, and artificial hy- bridizations involving Coreopsis cyclocarpa Blake and C. pinnatisecta Blake. The two taxa are perennial suffructescent plants growing in the moun- tains of Mexico with the latter found primarily in the states of Oaxaca and Guerrero and the former in Jalisco. Both taxa flower at the same time of year and occur in similar open grassy areas of high plateau regions (Crawford 1970). The present known geographic ranges of the two taxa indicate a disjunction of about 700 km, although additional collecting in Guerrero and Michoacan might narrow this hiatus. The only reliable morphological character for distinguishing them is leaf mor- phology, with C. cyclocarpa having simple linear leaves whereas those of C. pinnatisecta are pinnatisect with linear segments (Crawford 1970). Both taxa are diploid with a chromosome number of 2n = 24, and artificial crosses in the greenhouse produced fully fertile F1, F2, and backcross hybrids. Meiosis was regular and seed set was not reduced (Crawford 1970). In addition, the leaf characters used to separate the two taxa segregate in the F2 generation. On the basis of these data, the two taxa were reduced to varietal status. The purpose of the present study was to measure genetic divergence between the two varieties of Coreopsis cyclo- carpa utilizing data from the electrophoretic separation of allozymes. The advantages of enzyme electrophoresis for addressing systematic ques- tions in plants were discussed by Gottlieb (1977a, 1981 a), who also point- ed out that most electrophoretic studies comparing different taxa have been done with annuals, there being a paucity of data for perennial angiosperms. The present study of allozymes in woody, perennial, in- terfertile, morphologically similar plants will provide data for the com-