1. Chromosome pairing has been studied in twenty-two different sterile F1 hybrids involving the genera Aegilops, Agropyron and Triticum, together with their colchicine derived amphidiploids having chromosome numbers of 2n = 42, 56 and 70. Cytological evidence has been correlated with male and female fertility, while chromosome pairing in the parents has been studied in relation to their amphidiploids.
2. Some of the sterile F1 hybrids showed little or no pairing, while in others the pairing was appreciable. There was an association of the amount of pairing with the parental combinations used in the production of the hybrids in that the interspecific hybrids were characterized by a relatively high degree of pairing, particularly those with 28 chromosomes, while the intergeneric hybrids either lacked pairing or showed a low incidence.
3. In the A1 amphidiploid generation, chromosome pairing was in all cases high, and in some cases almost complete. In all cases multivalent formation in the amphidiploid was lower than bivalent formation in its undoubted F1 hybrid. Different amphidiploids showed various degrees of differential affinity. Univalent formation occurred in some amphidiploids, while bivalent formation in some was increased by a loss of chromosomes.
4. In all cases there was a reduction in chiasmata per nucleus and chiasmata per bivalent in the amphidiploid compared with its parent species. Reduction values were not directly associated with any increase in chromosome number of the amphidiploid, nor with the presence of multivalents.
5. No confirmation could be obtained of the view that multivalent formation in amphidiploids is a more generally sensitive index of chromosome homology than bivalent formation in the undoubted F1 hybrid. The absence of multivalents in an amphidiploid does not disprove the existence of structural chromosome homologies between the two parents.