THE EXPERIMENTAL FORMATION OF POLYPLOIDY AND ITS EFFECT IN THE GENUS MUSA

Polyploidy was induced in two Musa species and their hybrid by means of immersing newly germinated diploid seedlings in 0.5% aq. colchicine solution. Tetraploid M. balbisiana plants were taller and more robust but had slower growth rate, droopier leaves, fewer suckers, and scantier root systems than the diploids. Tetraploidy affected fruit size and shape in M. balbisiana and in M. acuminata. Tetraploidy did not affect bunch size (the number of fruit‐bearing nodes) in M. acuminata banksii but caused great reduction in bunch size of M. acuminata microcarpa ‘Zebrina.’ Doubling of the chromosome number resulted in doubling of the anthocyanin concentration in the leaves of the pigmented banana plants. Colchicine‐induced female sterility was detected in a treated diploid plant of M. acuminata subsp. microcarpa. A number of either female or male, or both female and male sterile plants were encountered in colchicine‐induced tetraploid M. acuminata species. Only one secondary M. balbisiana tetraploid plant was found to be partially female sterile. Colchicine treatment of the seedlings resulted in chromosomal irregularities at mitosis. Chromosomes appeared as open chains, rings and multiple configurations at metaphase. In the majority of cases doubled chromsomal complements reverted to their original number. However, there were cases in which diploids were induced to tetraploidy and later reverted to triploidy instead of diploidy. Also, there were cases in which triploids were doubled to hexaploidy and later reverted to tetraploidy instead of triploidy. From the close morphological similarity between a number of triploid and tetraploid M. balbisiana plants and some edible triploid cultigens belonging to the “ABB Group,” it is suggested that the karyotype of some of these cultigens could be “BBB” rather than “ABB” which contains one “A” M. acuminata genome. New pathways and additional evolutionary possibilities are incorporated into Simmonds’ scheme on the evolution of edible bananas.