| An interspecific cross-incompatibility system in diploid
and tetraploid Aegilops Ichizo NISHIYAMA 18 Hazamacho, Shugakuin, Sakyoku, Kyoto 606, Japan Unsuccessful interspecific crosses are mostly caused by abortion of hybrid seeds. Recently. NISHIYAMA and YABUNO (1978, 1979) have well explained its biological mechanism in the genus Avena in terms of a hypothesis 'polar-nuclei activation'. The hypothesis is based on the facts that the abortion of hybrid seeds is primarily due to abnormal development of the endosperm which has been found by many authors in their histological studies, and this is also supported by successful culture of young embryos excised from abortive crossed ovaries. Development of the endosperm, especially early formation of the endosperm enclosing proembryos, seems to be closely related to an interaction between two polar nuclei and a male nucleus at double fertilization, because it probably initiates mitosis of the primary endosperm nucleus. Then, the intensity of activating action of the male nucleus and reaction of the polar nuclei is indicated 'activating value (AV)' and 'response value (RV)', respectively. The degree of activated polar nuclei is designated 'activation index (AI)=AV/2RV (or x 100)'. In selfed plants the activation index of the polar nuclei (or the primary endosperm nucleus) is always 1/2=0.5 or 50% where AV=RV, and the primary endosperm nucleus could be harmoniously activated for normal development of the endosperm or the seed. AV is a gametophyte character, and may show quantitative differences in different species. Then, in interspecific crossing AI sometimes varies from less to more than 50%, being due to hypo- and hyperactivating action of the male nucleus, respectively. Such unbalanced activations may induce disturbance of the embryogenesis, and results in the formation of deformed or inviable seeds, following the extent of AI deviated from normal 50 per cent. The hypothesis was successfully applied to the cross incompatibility in the genus Triticum (NISHIYAMA 1979). The present paper represents a further application of the hypothesis to interspecific crosses in diploid and tetraploid Aegilops. I owed the crossing data using six diploid (2n= 14) and five tetraploid (2n=28) Aegilops to many earier workers (PERCIVAL 1932. KATAYAMA 1933, KIHARA and LILIENFELD 1932, KIHARA 1937, 1949, KIHARA and TANAKA 1954, BERG 1937, SEARS 1941). The results were simply summarized in a model of the diallel cross (Table 1). No detail of the development and germination (%) of hybrid seeds was reported in most of the crossing experiments. Accordingly relative activating values were arbitrarily estimated only by a comparative analysis of cross-combinations from a standpoint of hybrid-seed viability. They varied from 0.6 for Ae. squarrosa to 2.2 for Ae. cylindrioa and Ae. triuncialis. In Ae. caudata 1.4-1.7 were estimated, but 1.4 was chosen in Table 1. As AV=1 of Triticum boeoticum was employed as a standard the activating values may be available in intergeneric crosses between Aegilops and Triticum which will be reported in the following paper. |
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