Coreopsis

Abstract: 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.

Abstract: An intraspecific chloroplast DNA (cpDNA) restriction site survey within Coreopsis grandzjlora was used to evaluate the correspondence of genetic variation with patterns of morphological differentiation and geographical separation. Eight frequent-cutting restriction enzymes were used with a sample of 273 individuals representing 14 populations from two geographical regions. Thirteen distinct cpDNA haplotypes were detected, differing by as many as 22 restriction site changes, or an estimated 0.674% nucleotide sequence divergence. The survey detected sufficient cpDNA variation for analyses of intraspecific and intrapopulational genetic structure and yielded evidence for interpopulational gene flow, a cpDNA polymorphism that transcends the species boundary, and possible cpDNA introgression. Patterns of genetic diversity do not strictly correspond to either morphological variation or geographical distribution; this may be a result of either cpDNA introgression or the maintenance of a cpDNA polymorphism through the speciation event that gave rise to C. grandiflora.

Abstract: Coreopsis grandiflora is an herbaceous perennial occurring primarily in the southeast- ern United States. It is morphologically variable and the variation is accommodated in three diploid (var. grandiflora, var. harveyana, var. saxicola) and one hexaploid (var. longipes) varieties. Populations of the three diploid varieties are usually highly interfertile. An electrophoretic study of 14 soluble enzymes presumably coded by 26 genes revealed no differentiation between any of the varieties. Comparable levels of genetic variation were found in populations of each variety and within each variety as a whole. The high genetic identity between the hexaploid var. longipes and the three diploid varieties suggests that the former is an autopolyploid in the sense that its origin lies exclusively within C. grandiflora. All alleles detected in the hexaploid were also found in the diploids, and no fixed heterozygosity was found at any gene in any population of var. longipes. The electrophoretic evidence is concordant with the high morphological similarity between var. longipes and diploid varieties of C. grandiflora in suggesting an autoploid origin for the former.