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Fourteen North American members of the ' Xanthocephalum group' were studied by classical and molecular cytogenetics. Location and number of rDNA sites were determined by FISH. For the 5S rDNA, a probe was obtained from Prionopsis ciliata. Most species were diploid (2 n = 12), although Isocoma menziesii, Grindelia hirsutula, G. robusta, both varieties of G. stricta, and one population of G. camporum were tetraploid (2 n = 24). Diploid Grindelia and Prionopsis ciliata were 5 m + 1 sm, tetraploids 10 m + 2 sm, except G. hirsutula (8 m + 4 sm), and Isocoma and Olivaea 6 m + 2 sm and 3 m + 3 sm, respectively. Most species had satellites on the short arms of m pairs: two in tetraploids and P. ciliata and one in diploids. Satellites were associated with two CMA/DAPI bands in diploid species and four bands in tetraploids and in P. ciliata. rDNA loci (two in diploids to four in tetraploids) may be indicative of ploidy level. Grindelia tetraploids could have originated recently by autopolyploidy. Chromosome duplication was followed by modifications in the genome structure, resulting in higher heterochromatin amounts not associated with NORs. There is only one 5S site per basic genome in para or pericentromeric regions. Although not always large, chromosome variation has accompanied the evolutionary divergence of the taxa studied. [ABSTRACT FROM AUTHOR]

Grindelia is among the most taxonomically challenging groups of North American composites. The genus as a whole has an amphitropical distribution, with approximately half of the species native to North America and Mexico and the remainder native to South America. We used DNA sequence data from the nuclear ribosomal ITS and ETS and chloroplast psaI-accD regions to revisit hypotheses on biogeographic history across the genus. Grindelia as a whole is well-supported and is composed of two sister clades, one native to South America and the other native to North America including Mexico. The North American taxa constitute two clades that largely occur on different sides of the Continental Divide. The diverse radiation of Grindelia in the California Floristic Province appears to be most closely related to species from the Great Basin and Colorado Plateau and evidently descended from drought-adapted ancestors. Although Steyermark's hypotheses about the relationships of North American Grindelia are not all supported, we did recover a clade corresponding to his Pacific radiation and many of the Mexican and Texan species that he hypothesized to be basal in the genus represent early diverging lineages in our trees. Dunford's cytogenetic data on the North American species of Grindelia were also examined in a phylogenetic context. [ABSTRACT FROM AUTHOR]