CHROMOSOME NUMBERS IN THE COMPOSITAE. IV. NORTH AMERICAN SPECIES, WITH PHYLETIC INTERPRETATIONS

TURNER, B. L. , W. L. ELLISON, and R. M. KING. (U. Texas, Austin.) Chromosome numbers in the Compositae. IV. North American species, with phyletic interpretations. Amer. Jour. Bot. 48(3): 216-223. Illus. 1961. -Chromosome counts from 116 different plant populations representing 75 taxa (72 species in 39 genera) are reported. These include the first species counts for the following genera: Actinospermum (x _ 19), Baltimora (x =15), Calea (x ca. 17, 18), Calyptocarpus (x = 12), Hecubaea (x = 17), Lagascea (x = 17), Schistocarpha (x 8), Melanthera (x = 15), Pectis (x = 12), Perymenium (x = 15), Sanvitalia (x = 8), and Trigonospermum (x = 15). Chromosome counts for Chrysopsis trichophylla (n = 5), Cirsium horridulum (n = 16), Hidalgoa ternata (n _ 16,) Tridax balbisioides (n 10), Tridax trilobata (n = 10), and Verbesina crocata (n =18) differ from the reported basic numbers as determined from other species in these genera. Taxa closely related to Tridax procumbens were found to have the diploid number n -_ 9, thus establishing the polyploid nature (n = 18) of this widespread polymorphic species. When appropriate, the chromosomal information has been related to systematic problems. THE PRESENT contribution is essentially a continuation of several papers, the latest of which (Turner and Johnston, 1961) dealt with chromosome counts from species of northeastern Miexico; except for reports for 3 Florida taxa, the counts reported in the present paper were obtained, for the most part, from species of southern Mexico and Guatemala. Chromosome counts were made from pollenmother-cell squashes as outlined by Turner and Johnston (1961). Voucher specimens (table 1) are deposited at The University of Texas Herbarium; these all were collected during the year 1960. The tribal and subtribal arrangements listed in table 1 follow those of Hoffmann (1894). The identifications are our own, except for those of Erigeron spp. and Senecio praecox which were made by Dr. Arthur Cronquist and Dr. Theodore Barkley of the New York Botanical Garden. EUPATORIEAE. -Ager-atum (x 10) . -A. hoUstonianum (n 10) has been reported previously (Darlington and Wylie, 1956). Including the 6 taxa listed in table 1, 7 of the approximately 30 species in the genus have counts reported for them. Eupatorium (x 10, 17). -This is the largest genus in the Eupatorieae, variously estimated as containing 400-800 species. Chromosome counts are available for only about 40 species of the genus, but it is evident that 2 basic chromosome numbers exist for the taxon, x 10 and x 17 (Grant, 1953). Ten species, including the present E. glabratum, are diploid with n -17, and all of these are members of the section Eximbricata; only a few species of this section, e.g., E. cannabinum and E. incarnatum, are reported to have counts of 2n 20. While Hoffmann (1894) and others have placed the latter species in this section, it would seem 'Received for publication July 13, 1960. This study has been supported by National Science Foundation Grant G-9025. prudent, in view of the apparent chromosomal discrepancy, to reinvestigate their phyletic position. It is difficult to evaluate the significance of the chromosomal hiatus in such a large and varied genus as Eupatorium, unless one assumes that the x 10 and x 17 groups are parallel ancestral lines from some unknown prototype with a yet lower chromosome base. In fact, it seems likely that the number x 5 is the ancestral basic number for the Eupatorieae, much as Grant (1953) has suggested. The lowest basic count, to date, for a species of this tribe is n 5 (Adenostemma brazilianum, Turner and Irwin, 1960). The next lowest diploid number known for any species of the approximately 52 genera in the Eupatorieae is n 9. Only 12 genera in this tribe have counts reported for them, but the gaps between x 5 and x 9, 10, 11 and x 17 seem significant. ASTEREAE. -Erigeron (x 9). The chromosome counts reported for the 2 species listed in table 1 are consistent with the reported basic numbers for other species (Montgomery and Yang, 1960). The collection of E. karwinskianus (King 2345), with 27 univalents at meiotic metaphase, is probably apomictic. E. karwinskianis is a widespread, variable weed found throughout tropical America. Aster (x 5: 8, 9). A. subulatus is a highly variable, weedy, annual species of tropical and subtropical America. It belongs to a closely related group. of North American species in the section Oxytripolium, which contains species with n numbers of 5 and polyploids derived from these. Huziwara (1958) and more recently Raven et al. (1960) have argued that the number n 5, which is known to occur in at least 10 species of the genus Aster, has been derived through aneuploid loss from an ancestral generic base of x 9, much as Stebbins et al. (1953) have postulated for the tribe