| Suggestive information on an interspecific cross-incompatibility
system in Triticum Ichizo NISHIYAMA 18 Hazamacho shugakuin, Sakyoku, Kyoto, 606, Japan Failure of interspecific crosses in plants is usually induced by sexual barriers in pre-fertilization, resulting in non-fusion of the male and female nucleus or in post-fertilization, resulting in ceasing of the development of embryo and endosperm. Recently, Nishiyama and Yabuno (1978) have well interpreted a mechanism of hybrid seed failure in Avena in terms of a new hypothesis 'polar-nuclei activation' which is based on an idea that the triple fusion of the polar nuclei can play a major role in the abortion of hybrid seeds. That is, the intensity of activating action of the male nucleus and the reaction of the female nucleus at double fertilization are indicated as 'activating value (AV)' and 'response value (RV)', respectively. The seed development is closely related to the activation index (AI) of the polar nuclei, AV/2RV (or x 100) but not to that of the egg nucleus, AV/RV (or x 100). In a selfed plant the activation index of the polar nuclei is always 1/2=0.5 (or 50%) where the polar nuclei could be harmoniously activated for the normal development of the endosperm or the seed. The activating value which would be governed by a gene or genes may vary according to different species and roughly show additive effects in doubling the genome number. Then the activation index often deviates from 50% in distant crosses and the balanced activation may be disturbed, probably because of hyper- or hypoactivating action of the male nucleus. Such unbalanced activation may cause disturbance of the physiology of embryogeneis and result in the formation of deformed seeds or inviable seeds, following mainly the extent of defective endosperm development. The present paper deals with a further application of the hypothesis to the interspecific hybridizations in Triticum which were previously reported by certain authors (Kihara, Wakakuwa and NISHIYAMA 1929, KATAYAMA 1934, LILIENFELD and KIHARA 1934, WAKAKUWA 1934, DHALIWAL 1977). The results are arranged in a model of the diallel cross using three diploid, five tetraploid and two hexaploid species (Table 1). Based on the degree of abnormality of the hybrid seed relative activating values of the 10 species are arbitrarily assigned from 0.5 in T. urartu (2x) to 3 in T. aestivum (6x) where a standard value of 1 is adopted for T. boeoticum (2x) (Table 1). Table 1 further lists four seed development types, seed germination (%) and activation index of the polar nuclei (%) in most of 58 reciprocal crosses. According to the degree of seed development the hybrid seeds are grouped into four types, N-type, nearly normal seed, W-type, weakly developed seed, sometimes slightly wrinkled, Ps-type, partially shrivelled seed, and E-type, shrivelled- empty seed. The activation indices widely vary from 15 to 150% compared with a standard of 50% in selfed plants. |
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