Genetic and Morphological Divergence among Varieties of Aphanostephus skirrhobasis (Asteraceae-Astereae) and Related Species with Different Chromosome Numbers

Thirty-seven populations representing Aphanostephus riddellii (n = 5), A. ramosissimus (n = 4), and all three varieties of A. skirrhobasis (n = 3), were examined for allozyme differentiation at 17 loci to test hypotheses of hybridization between A. skirrhobasis and A. ramosissimus, and to assess genetic divergence within A. skirrhobasis and among congeneric species with different chro- mosome base numbers. Thirteen populations of A. skirrhobasis were examined also for variation in achene and pappus characters. The species are differentiated easily using isozymes, chromosome numbers, and achene hair types. In contrast to relationships suggested by a hypothesized aneuploid reduction series, A. riddellii is more similar genetically to A. skirrhobasis than to A. ramosissimus. Within A. skirrhobasis, populations of vars. skirrhobasis, thalassius, and kidderi are intermixed in analyses based on allozyme variation, but var. kidderi is differentiated readily using achene length, awn/achene length ratio, and awn number. Average genetic identities within and among varieties are similar, reflecting an isozyme pattern characterized by minor frequency differences among shared alleles. Varieties of A. skirrhobasis apparently have incomplete reproductive barriers in areas of sympatry, and may be in the early stages of regional differentiation initiated during the Pleistocene. Isozyme data do not support purported hybridization between A. skirrhobasis and A. ramosissimus. Varieties of A. skirrhobasis are partially autogamous, but diversity levels suggest a mixed to outcrossing mating system.