Abstract
Two cases of natural interspecific hybridization in Japanese Agropyron were observed in Misima. One is a pentaploid hybrid between Ag. ciliare and Ag. tsukushiense, and the other is a hexaploid hybrid between Ag. humidum and Ag. tsukushiense.
The pentaploid hybrid was highly sterile and no seed setting was observed. It is assumed that in this hybrid combination introgressive hybridization would not occur in natural conditions.
As to the hexaploid natural hybrid, two cases were observed; i.e., one allopatric and the other sympatric. In the allopatric case Ag. humidum and the common type of Ag. tsukushiense were involved, while in the sympatric case Ag. humidum and the early ecotype of Ag. tsukushiense were the parental species. In both cases low seed setting was found from the examination of a considerable number of hybrid clones in natural conditions. From progeny tests of backcrossed plants obtained from the sympatric natural hybrids between Ag. humidum and the early ecotype of Ag. tsukushiense, the following conclusions are drawn: It is quite possible that introgression of characters, such as waxiness, from one species to the other was taking place in the natural populations through backcrossing to the parental species of the F1 hybrids followed by segregation of the characters concerned. Rather quick restoration of fertility in the hybrid progenies made possible the establishment of hybrid swarms in a state of nature having intermingled characteristics of the parental species. However, so far no introgressants have been found yet in the natural sympatric populations of these two species. High sterility of F1 hybrids might be a major cause of restricted introgression.
