| Nuclear DNA content of the amphidiploid, G. hirsutum, and two closely related diploid species, G. herbaceum var. africanum and G. raimondii, was ascertained by the reassociation kinetics of 250 nucleotide DNA fragments. Between diploid species the difference in chromosome size is attributed largely to variation in repetitive sequences, where there has been a change in both frequency and complexity. The evolution of single-copy DNA sequences by cross hybridizations among species reveals: (1) a high degree of sequence conservation between diploid species, showing 78% duplex formation under standard criterion and 6% sequence mismatch upon thermal denaturation; and (2) greater than 95% duplex formation between the diploid species and the amphidiploid with less than 1% single-copy sequence mismatch. The latter findings are consistent with an early Pleistocene origin for the tetraploid cottons. |
