CHROMOSOME-PAIRING IN HYBRIDS BETWEEN HEXAPLOID BREAD WHEAT AND TETRAPLOID CRESTED WHEATGRASS (AGROPYRON-CRISTATUM)

To transfer drought and cold tolerance from crested wheatgrass, Agropyron cristatum (2n = 4x = 28; PPPP genomes) to bread wheat, Triticum aestivum (2n = 6x = 42; AABBDD), intergeneric hybrids (2n = 5x = 35; ABDPP) were synthesized. The F1 hybrids were perennial like the male wheatgrass parent. Their mor-phology was almost intermediate between the two parents. A low-pairing (LP) hybrid and a high-pairing (HP) hybrid were studied. The LP hybrid, with apparently functional Ph1 (the pairing regulator that suppresses homoeologous pairing), had a mean of 0.14 III + 1.16 ring II + 3.17 rod II + 25.91 I, with a c (the mean arm-pairing frequency) of 0.206. If A. cristatum were an autotetraploid, its haploid complement (PP) in the hybrid should form approximately 7 II. The mean of only 4.33 II (of which about 1.0 bivalent probably involved the A, B and D genomes of wheat) would suggest a certain degree of divergence between the two P genomes. The HP hybrid had 0.03 chain IV + 0.47 III + 2.98 ring II + 5.89 rod II + 15.73 I, with c = 0.460. Such a high pairing probably involved both autosyndesis (pairing within the ABD component and within the PP component of the ABDPP hybrid) and allosyndesis (pairing between the parental complements), and could have occurred only if PhI was partially inactivated. Homoeologous pairing of chromosomes of P with wheat chromosomes seems sufficient to facilitate the transfer of desirable traits of A. cristatum into wheat.