| Results and Discussion No plump seed was obtained from the crosses of Ae. squarrosa as female parents with any species of Aegilops and Triticum used (Table 1). More than 50% of the crossed florets in these crosses produced the residues of undeveloped seeds, which were categorized as shrivelled seed in this paper. In fact, fertilization and early embryogenesis were recognized by observing disected young seeds at 10-14 days after pollination. These young seeds were filled with fluid, and lost the moisture along the plants matured, resulting in dry residues of about 0.1-0.2 cm in size. On the other hand, plump seeds were obtained from the crosses of the colchicine-treated Ae. squarrosa as female parents with Emmer or Dinkel wheats although they produced shrivelled seeds in certain proportion (Table 1). The reciprocal crosses (Emmer or Dinkel x Ae. squarrosa) set plump seeds. These facts suggest that the compatible balances of D genome dosage between embryo and endosperm may determine the seed plumpness with some other modifying factor(s). It was reported that the crosses of Ae. squarrosa (female) x Emmer wheat (male) resulted in zygotic lethality, and that an induced autotetraploid line of Ae. squarrosa set seeds in the cross with Emmer wheats, of which progenies were utilized for breeding alloplasmic wheat lines having cytoplasm of Ae. squarrosa (KIHARA 1973, 1979). Seven seedlings were obtained from germinating embryos in the test tubes out of 62 embryos cultured, and five were grown till mature plants. All five plants had 21 chromosomes in somatic tissues, and their morphological appearrances were of wheat type with long awns, although they were weak in vigor in comparison with the parental T. durum Sel. 56-1 (Fig. 1-a). All five F1 hybrids produced fertile pollen grains and set F2 seeds on bagged spikes. The F2 progenies had 42 chromosomes of 2.58I+19.70II+0.02III on average in PMC (Table 2). These facts indicated that the natural amphiploidization had occured in the F1 hybrids of Ae. squarrosa (femal) x T. durum (male). In general, natural amphiploids might originate by the spontaneous doubling of chromosomes in somati ctissues of interspecific hybrids, producing fertile sectors, or by the production of the unreduced functional gametes and self-fertilization. In Triticinae, the latter event has been experimentally observed (KIHARA 1949). MAAN et al. (1980) suggested a pairing suppression system of homoeologous chromosomes involved in the induction of the unreduced gamete formation. Cytological observations on PMC's of the F1 hybrids in the present experiment indicate the following meiotic process and microsporogenesis; (i) complete asynapsis at MI without any chromosome pairing (Fig. 1-b), (ii) univalent chromosomes showing x-shape, gathered in the center of the cells (Fig. 1-c), (iii) equational division of univalent chromosomes at the first division of PMC's (Fig. 1-d), (iv) synmetrical dyad formation as product of 'meiosis', which is characterized to have large obscure nuclei (Fig. 1-e), and (v) production of fertile pollen grains with three nuclei filled with starch granules. This meiotic process is characteristic for the meiotic non-reduction and the unreduced gamete formation in interspecific hybrids, resulting in amphiploid production observed by KIHARA (1949), MAAN et al.(1980) and others. Out of 46 plants obtained from the fertile F1's, one plant had 41 chromosomes and the others had 42 with relatively stable chromosome pairing (Table 2). The plants obtained from the crosses of the F1's with T. aestivum cv Chinese Spring and T. durum Sel. 56-1 showed pairing conformations of 3.80I+19.10II, and 7.82I+13.59II, respectively, which indicated that there was no significant reduction of pairing stability in these hybrids. Many lines of synthetic hexaploid wheat (AABBDD) have been obtained by KIHARA and his coworkers (KIHARA et al. 1957; TABUSHI 1957; TANAKA 1961) and KERBER & DYCK (1978). All of them were from the crosses between Emmer wheats as female and Ae. squarrosa as male parents. Some of them were natural and others are colchicine-induced. The newly induced AABBDD amphiploid in this experiment is unique because this synthesized common wheat have squarrosa cytoplasm. In the present experiment, auto-tetraploid lines of Ae. squarrosa including four subspecies (typica, strangulata, meyeri, and anathera) were induced. They showed partially reduced fertility probably because of multivalent formations and irregular segregations of the duplicated chromosomes. Various F1 hybrids were obtained from the crosses of these auto-tetraploids with T. durum Sel. 56-1 and T. aestivum cv Chinese Spring. Their paring conformations on over-all average were 16.26I+5.87II in ABDD and 20.75I+6.56II+0.35III in ABDDD, respectively. They were highly sterile but a few seeds were obtained from the crosses of each F1 line with T. durum. The lines obtained in the present experiments will be used as the maternal plants for introducing cytoplasmic variability in NC-heterosis breeding program. |
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