A reexamination of the diploidlike meiotic behavior of polyploid cotton.

Chromosome associations at pachytene, diakinesis, and metaphase I were analyzed in haploids of the allotetraploid G.hirsutwn, in the F1 hybrid between G.arboreum (A2) and G. raimondii (D5), and in the doubled hybrid 2(A2D2) in an effort to define more clearly the mechanism responsible for the diploidlike behavior of the natural allotetraploids. The mean number of bivalents per cell at pachytene, diakinesis, and MI were respectively 10.00, 7.40, and 0.80 for the haploids and 11.00, 9.50, and 5.82 for the A2D5 hybrid. The two pachytene means were not significantly different, but the two diakinesis means and the two MI means were significantly different.At early pachytene members of paired chromosomes were not equal in length, but at late pachytene both members were equal in length in most bivalents. It was particularly evident in the unpaired regions that one partner was much thicker or more deeply stained than its counterpart. Since A chromosomes are twice the size, have twice the amount of DNA, and contain greater amounts of repetitive DNA than D chromosomes, it was concluded that the bivalents consisted of A and D homoeologs. In order for the paired homoeologs to attain equality in length at late pachytene, it is presumed that the A chromosomes either started contracting before the D chromosomes or contracted at a faster rate. During the stages following late pachytene, the D chromosomes contracted at a faster rate than the A chromosomes resulting finally in a two-fold difference in the size of the A and D chromosomes at MI. It is assumed that this differential rate of contraction would limit intimate pairing attraction and chiasma formation between homoeologous chromosomes.We concluded that a gene control system analogous to that wheat and oats does not determine the diploidlike meiotic behavior of the natural allotetraploids of Gossypium. This is based on the high frequency of homoelogous pairs at pachytene in the haploids, on the strict homologous pairing in the synthetic 2(A2D5) hybrid, and on the roles repetitive DNA is assumed to have in regulating synapsis and chiasma formation and in structurally differentiating the A and D homoeologs.It was proposed that differences in genome chromosome size may determine whether a gene or non-gene control mechanism is required for regulating homologous pairing in allotetraploids.