COMPARATIVE MORPHOLOGY OF THE CARPEL IN THE ROSACEAE VI. POMOIDEAE: ERIOBOTRYA, HETEROMELES, PHOTINIA, POURTHIAEA, RAPHIOLEPIS, STRANVAESIA

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 2n = 4, 2n = 5, and 2n = 6 in the same population. The 2n = 5 plants were hybrids between the 2n = 4 and 2n = 6 types. Both natural and artificial F1 hybrids show preferential disjunction from a trivalent and produce genetically balanced gametes with n = 2 and n = 3. In backerosses of the F1 to n = 2 plants, a 1:1 ratio of 2n = 4 and 2n = 5 plants was obtained. The possible mechanisms for the evolution of n = 2 from n = 3 and vice versa are discussed. Haplopappus gracilis is anl annual composite of wide distribution in arid, semi-arid, and somewhat more mesic habitats of the southwestern United 1 Received for publication December 24, 1964. This study has been supported by Grant G 20864 from the National Science Foundation. States and northern Mexico. Cytogenetic and morphological studies (Jackson, 1962) have demonstrated that H. gracilis (n = 2) evolved from the spatially more restricted H. ravenii (n = 4) by a series of unequal reciprocal translocations and loss This content downloaded from 157.55.39.211 on Fri, 24 Jun 2016 06:38:50 UTC All use subject to http://about.jstor.org/terms