CHROMOSOMAL INHERITANCE IN A BACKCROSS PROGRAM BETWEEN A HEXAPLOID COTTON LINE AND TETRAPLOID COTTON (CYTOGENETICS).

A breeding program was begun to transfer the caducous bract trait from the wild cotton diploid species Gossypium armourianum Kearney (D genome, 2n = 26) to the cultivated tetraploid species G. hirsutum (AD genomes, 2n = 4x = 52). The sterile triploids were then doubled with colchicine to obtain fertile hexaploid plants. These plants and their open pollinated progeny varied in their chromosome number from 73 to 82 chromosomes, the majority being the expected 78 chromosomes. Chromosome associations included bivalents, trivalents, quadrivalents, and hexavalents. The caducous bract trait varied from being similar to each parent species to intermediate expression. Backcrossed to G. hirsutum, progeny with 61 to 67 chromosomes were obtained with associations including frequent trivalents, quadrivalents, and one hexavalent. The caducous bract trait was not expressed in most plants and only variable in others. Progeny from open pollination or backcrossing these plants gave chromosome numbers closer to the tetraploid parent, with ranges of 56 to 64 chromosomes in open pollinated progeny and 52 to 58 for backcrossed plants. Again, the caducous bract trait was variable, possibly due to the influence of the A and D genomes of the New World cottons. Tetraploids recovered from the progeny had bivalent pairing and chiasma frequencies similar to G. hirsutum. Further backcrossing is hoped to increase the expression of the caducous bract trait