CHROMOSOME BEHAVIOR IN FERTILE TRIPLOID ASTER HYBRIDS

Abstract

HE sterility of triploids and other unbalanced polyploids is generally con-T ceded to be a consequence of irregularities in the distribution of univalents and multivalents at meiosis. Occasional viable gametes are produced, however, because of nonrandom orientation and segregation of chromosome associations. Thus, SATINA and BLAKESLEE (1937a, b) found a higher frequency of n and 2n chromosome numbers in gametes produced by triploid Datura than was expected on the basis of random orientation of trivalents. The Canina roses furnish a striking exception to the general rule of sterility of unbalanced polyploids (FAGERLIND 1940). There are also several reports of fertile triploid and aneuploid individuals in other species. DEMOL (1921) found that triploid Hyacinthus orientalis L. (3n = 24) was fertile, as were various aneuploid strains in this cultivated species. Microspore chromosome numbers of 8 to 14 were found (BELLING 1924) and the fertile progeny contained somatic chromosome numbers of 16 to 30. A triploid form of Crepis artificialis (3n = 36) was obtained in which 17 bivalents and 2 univalents regularly formed (COLLINS et al. 1929). This behavior resulted in 62 to 70 percent pollen fertility in the triploids. I n the course of a biosystematic study of several species of Aster (AVERS 1953), three fertile triploid F1 hybrids were obtained. Their unusual chromosome behavior has furnished the basis for the present report. MATERIALS AND METHODS Twenty separate crosses between Aster shortii Lindl. ssp. shortii (n = 18) and A. cordifolius L. (n = 9) were attempted in 1950, and three of these yielded one F1 hybrid each. One of these three hybrids was successfully back-crossed to the tetraploid parent species in 1951, and nine backcross plants were raised to maturity. In addition to the experimentally obtained individuals, one natural triploid also has been studied cytologically. All plants were grown in the greenhouse at Indiana University. Budms of the 13 plants studied were fixed in chloroform-ethanol-acetic acid, 4: 3: 1, for one week, and squash preparations of pollen mother-cells were made in aceto-orcein. Pollen fertility was determined by staining with cotton blue. 1 P a r t of a thesis submitted to the Faculty of the Graduate School of Indiana University , in partial fulfillment of the requirements for the degree of Doctor of Philosophy.