| The araraticum-boeoticum amphiploids resemble T. zhukovskyi
very closely with respect to chromosome association and fertility (Table
1). The amphiploids have a rather high frequency of bivalents and a
lower frequency of univalents and multivalents as compared with T. zhetkovskyi.
They are completely male and female fertile. They also resemble T. zhukovskyi
in anther morphology (DHALIWAL and JOHNSON 1976). However the amphiploids
do not resemble T. zhukovskyi with respect to general spike morphology
and leaf coloration. These characteristics are present only in T. timopheevi
but not T. arraticum. High preferential diploid pairing in the synthetic amphidiploid (AA AtAtBtBt), with two A genomes, strongly suggests that the At genome of T. araraticum (contributed by T. boeoticum) might have been modified in relation to the A genome of T. boeoticum. In two araraticum x boeoticum triploid hybrids homology between the T. boeoticum genome and one genome of T. araraticum does not appear to be complete as there is a very low frequency of closed bivalents and a high frequency of multivalents (Table 1). A change m homology between the A genome of T. araraticum or T. timopheevi and A of T. boeoticum presumably was responsible for nearly complete diploidization of the synthetic amphiploids as well as of T. zhukovskyi. A low frequency of univalents and high fertility in the F1 hybrids between the araraticum-boeoticum amphiploid and T. zhukovskyi (Table 1) confirms that the latter indeed is an amphi- ploid between a tetraploid of the timopheevi group and an A genome diploid. T. timopheevi is implicated on morphological grounds, T. monococcum is inplicated on the basis of a pair of chromosomes with small satellites (UPADHYA and SWAMINATHAN 1963). The F1 hybrids have a slightly higher frequency of multivalents and a lower frequency of bivalents as compared with T. zhukovskyi and amphiploids (Table 1). The fertility of the F1 hybrids was not as high as would be expected. The higher frequency of multivalent association and lower fertility of the F1 hybrids may be attributed to reciprocal translocations between the particular T. timopheevi (parent of T. zhukovskyi) and T. araraticum (1767) used in the amphiploids. This is possible because a high frequency of reciprocal translocations was observed in crosses involving T. araraticum and T. timopheevi (unpublished, H.S. DHALIWAL). The F1 hybrids showed only a slight deviation from 21 : 21 disjunction at anaphase I indicating that the univalents occurred mostly due to precocious separation of chromosomes from bivalents and quadrivalents rather than due to lack of their synapsis. The complete diploidization and high fertility of the synthetic hexaploids with two A genomes suggest that completely fertile hexaploids involving cultivated tetraploid wheats such as T. durum and wild A or B genome diploid wheats could also be synthesized and exploited commercially. Literature Cited DHALIWAL, H.S. and B.L. JOHNSON. 1976. Anther morphology and the origin of tetraploid wheats. Amer. J. Bot. (in press). FELDMAN, M. 1966. The mechanism regulating pairing in Triticum timopheevi. Wheat Inf. Ser. 21 : 1- 2. JOHNSON, B.L. 1968. Electrophoretic evidence on the origin of Triticum zhukovshyi. Proc. 3rd Int. Wheat Genet. Symp. Canberra : 105-110. MCFADDEN, E.S. and E.R. SEARS. 1944. The artifical synthesis of Triticium spelta (Abstr.) Rec. Genet. Soc. Amer. 13 : 26-27. UPADHYA, M.D. and M.S. SWAMINATHAN 1963. Genome analysis in T. zhukovskyi, a new hexaploid wheat. Chromosoma 14 : 589-600. UPADHYA, M.D. and M.S. SWAMINATHAN 1965. Studies on the origin of Triticum zhukovskyi and the mechanism regulating chromosome pairing. Ind. J. Genet. Plant Breeding 25 : 1-13. WATANBE, Y.K., K. MUKADE and K. KOKOBUN 1956. Studies on the production of amphiploids as the source of resistance to leaf rust in wheats. II. Cytogenetical studies on the F1 hybrids and the amphidiploids. T. timopheevi ZHUK. x T. monococcum L. Jap. J. Breeding 6 : 23-31. |
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