Cytology and Cytogenetics

The cotton genus Gossypium is composed of about 45 diploid (2n = 2x = 26) and 5 allotetraploid (2n = 4x = 52) species. Two allotetraploid species, Gossypium hirsutum L. and G. barbadense L., consisted of A and D subgenomes, were domesticated after polyploidization and are cultivated worldwide. The identification, morphology, and number of the chromosome; the genomic differentiation and origin of the allotetraploid; and the structure and number variations of chromosome in cotton are reviewed. Gossypium herbaceum L. and G. raimondii Ulbr. are generally regarded as the best explants of the A and D subgenome progenitors. Diploid Gossypium species fall into eight different genome types designated A to G, and K. Cotton chromosomes are numbered in the order of their distinguishing features related to translocations and aneuploids. On the basis of the present molecular marker, the 13 homeologous chromosome pairs have been established. The 62 translocations identify 25 of the 26 chromosomes in the allotetraploid. D12 or chromosome 26 has not been isolated in a translocation, but it is identified as a telocentric chromosome. Therefore, the translocation stocks constitute the only complete set of cytogenetic markers for the allotetraploid cottons. Monosomes for 15 of the 26 chromosomes of G. hirsutum have been identified. They have been used extensively to assign genetic factors to specific chromosomes.

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