| Furthermore, the above mentioned hypothesis will be supported by the following
evidences. 1) F1 hybrids between T. araraticum and T. dicoccoides Korn. from a mixed stand in Northern Iraq showed considerably high chromosome associations. Especially, two hybrid combinations showed very high associations and gave very low frequency of univalents (Table 3. See TANAKA and KAWAHARA 1976). These results clearly show that the two wild tetraploid wheats in Northern Iraq are cytogenetically more closely related to each other than those reported so far. This may give an evidence to the monophyletic origin of the B and the G genome. 2) T. araraticum is known to have various degree of structural differentiations in chromosomes involving several interchanges (SVETOZAROVA 1939, WAGENAAR 1966, TANAKA and ICHIKAWA 1968, 1972, TANAKA and ISHII 1975, KAWAHARA and TANAKA 1977). Recently, the authors found seven reciprocal translocation chromosome types in T. araraticum. Number of strains and localities belonging to each type are summarized in Table 4. As shown in this table, chromosome differentiation in T. araraticum is more abundant in Northern Iraq than in Transcaucasus. In contrast, no distinct structural variations of chromosomes in T. timapheevi occur, and most araraticum and all the timopheevi strains in the Tanscaucasus have chromosomes of the B type (TANAKA and ISHII 1975). It was concluded that cultivated T. timopheevi has been derived from wild T. araraticum having chromosomes of the B type in the Transcaucasus. Distribution of structural differentiation in chromosomes of the Timopheevi wheats would indicate that they first originated in the eastern part of the Fertile Crescent and that later its distribution area was extended northward to the Transcaucasus (KAWAHARA and TANAKA 1977). 3) Two wild tetraploid species, T. dicoccoides and T. araraticum, are distributed in southeastern Turkey, Northern Iraq and Western Iran. Morphological differences between strains of T. dicoccoides and those of T. araraticum collected in these regions are not clear (TANAKA and ISHII 1973). Distribution of some characters in the strains of the two species from these regions are listed in Table 5. In color of ear, glume pubescence and shooting time, a wealth of various forms of both species was encountered in Iraq. Similar results was obtained in the seedling resistance of T. araraticum and T. dicoccoides to brown rust. According to Saito (unpublished), all strains of T. dicoccoides from Palestine, Southern Syria, Iraq and Iran were susceptible. However, two strains collected in Turkey were resistant to moderately resistant. While, all the araraticum strains collected in the Transcaucasus, Turkey and Iran were resistant or moderately resistant. The majority of the strains of T. araraticum in Iraq were resistant or moderately resistant, but six strains were susceptible. As shown in Table 5, almost all the variation of morphological and physiological characters concentrate in the districts around the Zagros Mountains. Considering these evidences, the origin and the evolution of the two wild tetraploid wheats, T. dicoccoides and T. araraticum, is represented in Fig. 1. The two species were originated as the result of disruptive differentiation occured in natural amphidiploid SSAA between Ae. speltoides and T. boeoticum in the Zagros Mountains. |
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