Segregation in New Allopolyploids of Gossypium. II. Tetraploid Combinations.

N two other papers in this series (GERSTEL 1956; GERSTEL and PHILLIPS in prep.) I the segregation of artificial hexaploid combinations was described. These had been obtained by crossing tetraploid New World cottons with diploid Wild American species and subsequent doubling of the chromosome number. For certain characters these hexaploids were of duplex genotype (i.e. of the type ZZzz). Segregation occurred in the crosses with recessive testers, yet the resulting ratios were wider than the 5:l ratio expected from random chromosome association. The results were interpreted as due to weakly preferential associations at the prophase of meiosis between chromosomes derived from the same parent species. The present paper deals with allotetraploids synthesized from various diploid members of the genus. These were either taken from the same section as in the cases of 4n(G. arboreum X herbaceum) and 4n(G. thurberi X raimondii) or from different sections as in 4n (G. arboreum X anomalum) and 4n(G. arbmeum X thurberi). In several instances, it was possible to compare the segregation ratios for factors located on different chromosomes.