CHROMOSOME NUMBERS IN COMPOSITAE. I. ASTEREAE

Raven , Peter H. (U. California, Los Angeles.), Otto T. Solbrig , Donald W. Kyhos , and Richard Snow . Chromosome numbers in Compositae. I. Astereae. Amer. Jour. Bot. 47(2) : 124—132. Illus. 1960.–Ninety-two new counts are reported for the tribe Astereae of Compositae, mostly based on determinations of meiotic material. These include the first counts reported for the genera Acamptopappus, Amphipappus, Benitoa, Chrysothamnus, Corethrogyne, Lessingia, Monoptilon, and Xanthocephalum, as well as for many species. The original counts are discussed in relation to those previously reported for the tribe; together these constitute a total of 39 genera examined cytologically out of the approximately 100 known. Because of its widespread occurrence in diverse phylogenetic lines within the tribe and the family, and because of its high degree of correlation with the woody habit, which is thought to be primitive, x=9 is regarded as the original basic number for Astereae. Within the Haplopappus alliance there is a strong secondary mode of chromosome numbers centering around x=5. The hiatus between these two modes in number is explained on the basis of ancient phylogenetic reduction in chromosome number followed by the extinction of less successful intermediate types, and is compared with similar trends that have been reported for Cichorieae. It is suggested that the family is not of polyploid origin but may have had an original diploid basic number.