A Cytogenetic Study of a Three‐paired Race of Haplopappus Gracilis

Haplopappus gracilis (Nutt.) Gray is an aneuploid species of Compositae of wide distribution in the southwestern U.S.A. and northern Mexico. Except for 2 types of supernumeraries, cytological samples from throughout its range have regularly shown 2 pairs of chromosomes. However, recent collections from a small area in south‐central Arizona had 2 n = 4, 2 n = 5, and 2 n = 6 in the same population. The 2 n = 5 plants were hybrids between the 2 n = 4 and 2 n = 6 types. Both natural and artificial F 1 hybrids show preferential disjunction from a trivalent and produce genetically balanced gametes with n = 2 and n = 3. In backcrosses of the F 1 to n = 2 plants, a 1:1 ratio of 2 n = 4 and 2 n = 5 plants was obtained. The possible mechanisms for the evolution of n = 2 from n = 3 and vice versa are discussed.