Molecular cytogenetic studies of the “Xanthocephalum group” (Asteraceae)

Abstract

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 (2n = 12), although Isocoma menziesii, Grindelia hirsutula, G. robusta, both varieties of G. stricta, and one population of G. camporum were tetraploid (2n = 24). Diploid Grindelia and Prionopsis ciliata were 5m + 1sm, tetraploids 10m + 2sm, except G. hirsutula (8m + 4sm), and Isocoma and Olivaea 6m + 2sm and 3m + 3sm, 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.