Transmission of duplication-deficiencies from cotton translocations is unrelated to map lengths of the unbalanced segments.

Adjacent-1 duplication-deficiencies (dp-dfs) are readily recovered from most heterozygous translocations in Gossypium hirsutum L., but frequencies of specific cytotypes differ widely in progenies from heterozygote (female symbol) x standard crosses. Surprisingly, these frequencies seem to be unrelated to the primary (postmeiotic) frequencies predicted by metaphase I configurations or to the proportion of the chromosome arm that is duplicate or deficient. Deficiencies and duplications from different translocations involving the same arm, as well as the two complementary dp-dfs from the same translocation, seldom exhibit similar frequencies. We conclude that the frequency of each of 101 different adjacent-1 cytotypes is largely idiosyncratic and may depend in part on interactions between the specific chromosome regions that are respectively trisegmental and monosegmental. Few, if any, of these interactions can be between homoeologues of the A(h) and D(h) genomes. Adjacent-2 dp-dfs are seldom recovered, even if they involve chromosomes that are readily tolerated in monosomic condition. Comparison of monosomes and telosomes with deficiencies suggests that some chromosomes and chromosome regions may be more dosage-sensitive than others, but their identification is not strongly supported by these data.

Incipient Genome Differentiation in Gossypium. III. Comparison of Chromosomes of G. HIRSUTUM and Asiatic Diploids Using Heterozygous Translocations.

Hybrids between upland cotton (G. hirsutum, genome constitution 2A(h)D(h)) and either A-genome or D-genome diploid species exhibit 26 paired and 13 unpaired chromosomes at metaphase I. The A(h) and D(h) genomes are therefore considered homoeologous with those of the respective diploids. Previous studies, nevertheless, revealed a low level of ("incipient") differentiation between D(h) and various diploid D genomes. The diploid A genomes have been regarded as more closely homologous to A(h) on the basis of low preferential pairing and autotetraploid segregation ratios in allohexaploids.-The present study addressed the following questions: Are the diploid A genomes differentiated from A(h) in meiotic homology? If so, is the differentiation manifested equally by all 13 chromosomes or is it localized in certain chromosomes?-Three diploid A-genome lines representing G. herbaceum and G. arboreum were hybridized by in ovulo culture of embryos (1) with a standard line of G. hirsutum, which differs from G. herbaceum by two and from G. arboreum by three naturally occurring reciprocal translocations involving chromosomes 1-5, and (2) with six lines homozygous for experimental translocations involving chromosomes 6, 7, 10, 11, 12 and 13. Chiasma frequencies in hybrids were compared with those in appropriate G. hirsutum controls. In every comparison overall chiasma frequencies were slightly lower in the hybrids. Therefore A(h) appears to be differentiated from the diploid A genomes. No localized differentiation was detected in chromosomes marked by experimental translocations. The differentiation may be localized mainly in chromosomes 4 and 5.

Incipient Genome Differentiation in Gossypium. II. Comparison of 12 Chromosomes in G. HIRSUTUM, G. MUSTELINUM and G. TOMENTOSUM Using Heterozygous Translocations.

Eight homozygous translocation lines (TT) of G. hirsutum marking 3 chromosomes of the A genome and 9 chromosomes of the D genome were crossed with G. hirsutum, G. mustelinum and G. tomentosum, all homozygous for the standard end arrangements (tt). Chiasma frequencies in the G. hirsutum Tt controls were compared with those in the G. hirsutum x G. mustelinum and the G. hirsutum x G. tomentosum Tt hybrids. Both nucleus-wide and region-specific chiasma frequencies were compared.-Some genome differentiation appears to have arisen between G. hirsutum and G. mustelinum. The G. hirsutum x G. mustelinum hybrids had a 1.8 to 1.9% reduction in the nucleus-wide chiasma frequency. Four of the eight TT lines showed a 3.4 to 10.5% reduction in chiasmata in the hybrid translocation quadrivalents, suggesting that chromosomes 1, 21, 23 and 24 may have undergone localized genome differentiation. The two species may differ naturally in the end arrangement of two chromosomes, since a quadrivalent not due to experimentally introduced translocations was observed in 13% of the PMC's of two G. hirsutum x G. mustelinum hybrids.-Very little genome differentiation has occurred between G. hirsutum and G. tomentosum. In the G. hirsutum x G. tomentosum hybrids, the nucleus-wide estimates showed only a very small (0.1 to 0.2%), though statistically significant, lowering of the chiasma frequency, and there was no reduction in chiasma frequency in the more sensitive readings for specific translocation quadrivalents.

Incipient Genome Differentiation in Gossypium. I. Chromosomes 14, 15, 16, 19 and 20 Assessed in G. HIRSUTUM, G. RAIMONDII and G. LOBATUM by Means of Seven a-D Translocations.

The genus Gossypium is favorable for study of genome divergence at several levels. Early stages of divergence have been studied among four D genomes by comparing chiasma frequencies (reciprocal exchanges) between pairs of genomes and between individual counterpart chromosomes marked by heterozygous translocations. D(5) (G. raimondii) shows barely detectable differentiation from from D(h) (G. hirsutum), whereas D(7) (G. lobatum) is considerably less closely related to D(h) than is D(5). Fragmentary data suggest that D(2-2) (G. harknessii) falls between D(5) and D(7) in its relationship to D(h). Since chiasma frequencies in individual chromosomes and marked regions exhibit the same order of relationships as their corresponding whole genomes, it is concluded that the genome differentiation is generalized (i.e., nucleus-wide) rather than localized in specific chromosomes or chromosome regions. Estimates of relationships based on reciprocal exchange frequencies agree with those based upon preferential synapsis in allohexaploids reported previously. Since preferential synapsis and reciprocal exchange frequencies reveal the same order of relationships, it is concluded that to some extent they reflect common underlying changes in chromosome properties, despite recent evidence that synapsis and crossing over are under independent genetic control.

Genetic Lengths and Break Points in Twelve Chromosomes of GOSSYPIUM HIRSUTUM Involved in Ten Reciprocal Translocations.

Chromosome configurations were recorded in about 5500 pollen mother cells (PMC's) in 2n and 2n-1 (missing the intact A-genome chromosome) heterozygotes of ten reciprocal translocations involving six A-genome chromosomes (H1, H2, H3, H4, H6 and H7) and six D-genome chromosomes (H14, H15, H16, H19, H20 and H21) of Gossypium hirsutum. From these records, chiasma frequencies at each of six positions were determined for nine translocations and at two positions for one. These frequencies were used to calculate recombination frequencies in different chromosome regions, and from these distances the breakpoints in 15 chromosome arms were mapped relative to each other and to their respective centromeres, insofar as the data permitted. The karyotype so derived for twelve chromosomes is in reasonably good agreement with data from genetic mapping, telosome and monosome mapping, and the mitotic idiogram.