Allozyme Variation in Coreopsis Sect. Coreopsis (Compositae)

Abstract

An electrophoretic study of the nine species of Coreopsis sect. Coreopsis (C. auriculata, C. basalis, C. grandiflora, C. intermedia, C. lanceolata, C. nuecensis, C. nuecensoides, C. pubescens, and C. wrightii) revealed high genetic identities (above 0.90 for most comparisons) for all species. Uniformly high identities among all taxa made it difficult to discern patterns of relationships among the species in sect. Coreopsis. Similar levels of gene diversity were found in perennial and annual species in the section, and between geographically widespread vs. more restricted species. It is suggested that the high identities result from the recent divergence among the species in sect. Coreopsis, which is viewed as a highly evolved section within North American Coreopsis. The genus Coreopsis in North America consists of 11 sections containing a total of about 44 species. The genus is distributed throughout Mexico, California, and the eastern and southeastern United States. The sections are welldefined with regard to chromosome number, morphology, and geographic distribution (Smith 1975). Attempts to produce artificial hybrids between plants of different sections have, for the most part, failed, while there is some intercompatibility and interfertility between species within sections (Smith 1975, 1976). Section Coreopsis is the largest of the North American sections. Smith (1975) viewed it as one of the most advanced groups of North American Coreopsis. The section is native to and centered within the eastern and southeastern United States, with certain species widely distributed throughout this range while others are restricted in distribution. It consists of nine species, four of which are annuals and five are perennials. Perennial species include C. pubescens Elliott, C. auriculata L., C. intermedia Sherff, C. lanceolata L., and C. grandiflora Hogg ex Sweet. The annual species are C. basalis (Dietr.) Blake, C. wrightii (A. Gray) H. M. Parker, C. nuecensoides E. B. Smith, and C. nuecensis A. Heller. The base chromosome number for the section is n = 13. Two species, C. nuecensis and C. nuecensoides, exhibit aneuploidy (n = 6, 7, 8 and n = 9, 10, 11, respectively), and one taxon, C. grandiflora var. longipes (Hook.) Torrey & A. Gray, is hexaploid (n = 39). All species are self-incompatible and presumably strongly outcrossing (Smith 1975, 1976). Discerning evolutionary relationships within the section has proven problematical. Although the section itself is a well-defined, monophyletic group, relationships among the perennial species and origin of the annual species within the section remain unclear. However, available evidence suggests that the four annual species consist of two progenitorderivative species pairs, C. basalis-C. wrightii and C. nuecensoides-C. nuecensis, the former species in each pair representing the progenitor species (Crawford and Smith 1982a, 1982b; Smith 1974; Smith and Crawford 1981). Smith (1982) constructed a phylogenetic hypothesis for the section based largely on morphology (fig. la). He suggested that C. pubescens may represent the basal taxon within the section, and that the two progenitor-derivative annual species pairs are relatively advanced, but their origin from the perennials is obscure. His phylogeny also indicates a close relationship between C. lanceolata and C. auriculata (fig. la). The cladistic analysis of North American Coreopsis by Jansen et al. (1987) agrees in many respects with Smith's proposed phylogeny, including the separation of annuals and perennials into two distinct lines (fig. lb). Annuals are composed of two pairs, namely C. basalis-C. wrightii and C. nuecensoidesC. nuecensis, in their treatment as well. However, the cladistic study differs in that C. auriculata shows closer affinities with C. pubescens rather than with C. lanceolata as suggested by Smith (1982). Preliminary chloroplast DNA data (Crawford et al. 1988; unpubl. data) suggest that C. pubescens and C. auriculata are closely allied, and that one of these taxa (or a common ancestor) could