Chromosome Behavior in Fertile Triploid Aster Hybrids.

HE sterility of triploids and other unbalanced polyploids is generally conT 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. In 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.