| Reproductive barriers in wild diploid wheats
Yoshitada YAMAGISHI and Masatake TANAKA Plant Germ-plasm Institute, Kyoto University, Kyoto, Japan In diploid wheats two wild species, Triticum boeoticum Boiss. and T. urartu Tum., have been recognized. In order to clarify reproductive barriers involved in interspecific F1 hybrids which might be occurred in several developmental stages, crossability, viable seed formation, germinability, hybrid weakness, and pollen and embryo sac fertility were observed. In this experiment 120 strains of T. boeoticum collected from Armenia, Turkey, Iraq, Iran, Lebanon, and Greece, and 16 strains of T. urartu from Armenia, Turkey, Iran, and Levanon were used. The intrapsecific F1 hybrid plants among five tester strains of T. boeoticum from Armenia, Turkey, Iraq, Iran, and Greece had high pollen fertility (more than 90%), that was estimated from the percentage of stainable good pollens, and high seed fertility (more than 80%), that was estimated from the percentage of seed set in the lowest floret of the spikelets. Similarly, F1 plants among 16 strains of T. urartu from four different geographic areas had more than 60% pollen fertility and more than 40% seed fertility (Table 1). These results suggest that each diploid wheat throughout its distribution area was homogeneous with respect to genic factors affecting the fertility of F1 plants, and that, therefore, no significant reproductive barriers existed within each species. On the contrary, all interspecific crosses used T. boeoticum as a female parent gave small, plumped, but viable seeds. However, among 82 reciprocal crosses used T. urartu as a female parent, 66 cross combinations gave slender and shrivelled inviable seeds, and only 16 cross combinations gave some shirvelled viable seeds. Crossability between two species did not differ significantly in the reciprocal crosses using T. boeoticum or T. urartu as a female parent, but germinability differed significantly. In these interspecific crosses the F1 hybrids derived from certain cross combinations showed hybrid weakness, indicating yellow coloration at the tip of leaves that begins at about tenth day after germination and poor adventitious root development. At 25C in the greenhouse, those plants stopped their growth at three-leaf stage and died soon. However, in the experiment field most of them survived, and a clear distinction between them and normal plants could not be recognized during the winter season. But at tillering stage necrosis started at the tip of leaves and progressed to the whole plant finally. All of them came to die before the maturing stage. The reciprocal crosses also gave the same symptons. This results suggests that hybrid necrosis is not caused by cytoplasmic factors, but by two complementary genes designated as Ned1 and Ned2. Of 120 strains of T. boeoticum examined, 48 and Ned1 gene and all 16 strains of T. urartu had Ned2 gene. The pollen and seed fertility in the F1 hybrids between the remaining 72 strains of T. boeoticwa and T. urartu from Lebanon are shown in Table 2. The table indicates that pollen and seed fertility are generally very low, with the exceptions of hybrids involved one Armenian, eleven Turkish, and three lraqi strains. The pollen fertility was 20-30%, 20-80% and 20-40%, respectively, and the seed fertility was all 10-50%. The embryo sac fertility of F1 hybrids used T. boeotioum and T. urartu as female parents is shown in Table 3. Embryo sac fertility was estimated from both the percentage of the seed set in pollinated florets and the germination rate of obtained seeds. In all cases the seed set was about 30% and the seed germination rate was about 80%. All data indicate that the reproductive barriers, i.e., inviable seed formation, hybrid weakness, reduced pollen and embryo sac fertility, play as significant isolation mechanisms between two wild diploid wheat species. |