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Cytotaxonomy of Helianthus ciliaris and related species of the Southwestern U. S. and Mexico

In and around cultivated fields, along roadsides, and in various drainage habitats of the southwestern United States, central and northeastern Mexico, one frequently encounters plants of Helianthus ciliaris DC., commonly called blue weed. In a comprehensive submonographic treatment of Ilelianthus, Watson (1929) considered H. ciliaris as a single, exceedingly variable species, pointing out, in particular, variation in leaf shape, size and pubescence. As a result of cytological, morphological, and population studies in Mexico and the southwestern United States, I have come to the conclusion that several species were included in this taxon as delimited by Watson. The several species are generally similar to H. ciliaris and share with it the ability of rapid vegetative propagation by slender rhizomes, slightly or heavily glaucous stems, and certain types of pubescence. Cytological study has ellucidated the situation in some respects. Heiser and Smith (1955 and unpubl.) have shown that the chromosome number of H. ciliaris is n = 34. They also have one plant from western Texas with X 51 which probably arose by chromosome doubling in a natural hybrid between H. citiaris and one of the related species with n 17 discussed later. Counts from several additional populations of H. ctliaris have all been n 34 (Table 1). leliantivus has a basic number of x -_ 17, and H. citiaris (n 34) would be considered a tetraploid on this base. However, the closely related genus Viguiera has taxa with n -8 and 17, and Heiser and Smith (1955) have suggested that Helianthus may be of polyploid origin. Microsporocytes at diakinesis in Heltianthus ctlitaris from several localities had several quadivalents per cell (Fig. 1). This suggests that H. ctliaris is a segmental allotetraploid, derived from taxa with segments of chromosomes homologous enough for heterogenetic pairing to occur, and the genomic formula C1C1C2C2 is appropriate. The predicted result from intergenomal crossing over is segregation for characteristics by which the parental types differ (Stebbins 1950). At one extreme in H. ciliaris, one finds leaf types that are somewhat pinnately lobed, pilose or hispid, and stems that are pilose or hispid. At the other extreme are plants with entire or almost entire leaves with both leaves and stems glabrous. This suggests two possible ancestral types, both of which can be matched by existing diploids. One probable ancestral type of H. ciliaris is H. arizonensis, a diploid (Table 1), found in west-central New Mexico and east-cenrtal Arizona. Plants of this species are generally shorter than those of H. ctliaris and have smaller heads, but the only qualitative character that has been found to separate the two thus far is the completely yellow disc corollas in H. arizonensis. The disc corollas of H. ciliaris are invariably red-tipped or sometimes completely red. The populations of H. arizonensis tend to be smaller and more scattered than those of H-. cittaris. The other possible ancestor of H. ciliaris, assuming that the tetraploid has