ALLOZYME VARIATION IN COREOPSIS NUECENSOIDES AND C. NUECENSIS (COMPOSITAE), A PROGENITOR‐DERIVATIVE SPECIES PAIR

Coreopsis nuecensis Heller and C. nuecensoides E. B. Smith are two plant species occurring on the sandy prairies and plains of southeastern Texas (Fig. 1). Although populations of the two species occur in adjacent areas, plants do not grow intermixed in the same populations (Smith, 1974). The only morphological feature consistently distinguishing the two species in the field is that the inner involucral bracts of C. nuecensis are pubescent whereas those of C. nuecensoides are glabrous (Smith, 1974) (Fig. 2). In addition, the stems of C. nuecensis tend to be glabrous while those of C. nuecensoides tend to be pubescent. When grown in the greenhouse, Coreopsis nuecensis is an annual whereas C. nuecensoides persists for two or more years, flowering each season (Smith, 1974; Crawford, unpubl. observ.). However, under natural conditions, the latter species is strictly annual. Coreopsis nuecensoides has chromosome numbers of N = 9 and 10 whereas N = 6, 7 and 8 (7 bivalents and a pair of B chromosomes) are known for C. nuecensis (Turner, 1960; Smith, 1971, 1974; Crawford, unpubl.). Hybrids resulting from crosses between plants within each species, regardless of chromosome number, are highly fertile and it is possible to produce advanced generation hybrids (Smith, 1974; Crawford, unpubl.). By contrast, interspecific hybridization produces vigorous, but almost completely sterile F1 plants (pollen stainability 2-5%). On the basis of cytogenetic evidence, Smith (1974) proposed that Coreopsis nuecensoides gave rise to C. nuecensis by a process of aneuploid reduction. Morphologically, these two species are much more similar to each other than either is to any other species of Coreopsis; indeed, without the differences between their involucral bracts it would be difficult if not impossible to distinguish them. In addition, the two taxa are annuals whereas nearly all others in Coreopsis Section Coreopsis are long-lived herbaceous perennials (Smith, 1974). The present study examined genetic variation within and among populations of the two species to ascertain the amount of genetic divergence between them. Several investigations (Gottlieb, 1973a, 1974; Gottlieb and Pilz, 1976; Rick et al., 1976) have demonstrated that speciation can occur with little or no divergence at gene loci coding for allozymes. By contrast, much higher levels of differentiation at allozyme loci have been detected for most congeneric species (Gottlieb, 1977). Thus, if Coreopsis nuecensoides and C. nuecensis are related as progenitor and recent derivative, then they are expected to exhibit very high similarity at genes coding for enzymes.