Abstract
1. Differential production of semi-fertile and sterile hybrids when two strains of T. araraticum were crossed with a strain of T. timopheevi has been reported by Tanaka and Ichikawa (1968, 1972). This was reexamined in detail for three years. Results indicate that the difference in fertility observed in the hybrids was genetically controlled. Three possible kinds of factors involved were discussed.
2. Nine strains of T. araraticum and 15 strains of T. timopheevi were hybridized with the above three strains as testers. Both semi-fertile and sterile hybrids were obtained, depending on the hybrid combination involved. Therefore, genetic isolation by hybrid sterility between the two species seems to be incompletely established.
3. Two groups of T. araraticum were found with crosses with the tester strain of T. timopheevi. The first group gave semi-fertile F1 hybrids which showed normal chromosome association at MI. The other group gave sterile hybrids and involved a chromosome interchange at MI. The former had dark green leaves and a procumbent tillering habit, while the latter had light green leaves and a semi-errect tillering habit.
