Molecular phylogeny of the helianthus genus, based on nuclear restriction-fragment-length polymorphism (RFLP)

Abstract

The systematics of 44 species of the Helianthus genus were studied by digestion of total DNAs by three six-base restriction endonucleases (BglII, EcoRV. and HindIII) and by probing with 10 nuclear probes, resulting in a mean number of 63 hybridization signals per sample. First, the number of substitutions, per nucleotide site, between each of the genomes was estimated on the basis of the distribution of 345 fragments between samples, according to the "fragment method" of Nei and Li. These pairwise distances were used to construct evolutionary trees by means of the unweighted pair grouping with arithmetic means and by the neighbor-joining method. The phylogenies produced by these two methods, where all the sections and series defined by morphological classification are separated, mirror the botanical classification of the genus. However, the integration of polyploid species (2n = 4.v or 2n = 6x) leads to their artificial separation from the diploids. The separation of H. petiolaris and H. neglectus from the Helianthus section could provide evidence for two distinct lineages of annual species within the Helianthus genus. Second, varimax factor analysis, based on the F matrix, allows separation of species according to the similarity index and gives a plane representation that is close to the morphological classification given by the weighted pair grouping with arithmetic means. Third, the changes in the accuracy of distance estimates according to the number of probe-enzyme combinations used show that 30 combinations produce a general dispersion of <5% on both sides of the mean value. Model fitting to the decreasing dispersion indicates that 60 probe-enzyme combinations would ensure a dispersion of distance estimation of *1%, even in the case of distantly related species.