Gene number in species of Astereae that have different chromosome numbers.

Differences in the gametic chromosome numbers (n = 4, 5, 9) of species in the Astereae tribe of the Compositae have been variously interpreted. One hypothesis proposes that n = 9 was the original base number of the group and that the lower numbers resulted from aneuploid reduction. The alternative hypothesis asserts that the ancestral base number was n = 4 or n = 5 and that species in which n = 9 are allotetraploids derived by hybridization between taxa with the lower numbers. Electrophoretic analysis of 17 enzyme systems in five species of Machaeranthera, in which n = 4, 5, and 9, and two species of Aster in which n = 5 and 9, demonstrates that all of these species have the same number of gene loci specifying the tested enzymes. The absence of isozyme multiplicity in the species in which n = 9 suggests that they did not arise by polyploidy.