Pak. J. Bot., 39(1): 57-66, 2007. GENETIC DIVERSITY OF AEGILOPS VARIABILIS (2n=4x=28; UUSS) FOR WHEAT IMPROVEMENT: MORPHO-CYTOGENETIC CHARACTERIZATION OF SOME DERIVED AMPHIPLOIDS AND THEIR PRACTICAL SIGNIFICANCE A. MUJEEB-KAZI 1,2 , ALVINA GUL 2 , MUHAMMAD FAROOQ 1 , SUMAIRA RIZWAN 1 and JAVED I. MIRZA 1 1 National Agricultural Research Center (NARC), Islamabad, Pakistan. 2 National Institute of Biotechnology & Genetic Engineering (NIBGE), Faisalabad, Pakistan Abstract The tertiary gene tetraploid Aegilops variabilis (2n = 4x = 28; UUSS) is an alien germplasm resource that provides much needed genetic diversity for resistance to Cochliobolus sativus (spot blotch) and Tilletia indica (Karnal bunt). This resource has been hybridized with several durum and bread wheat cultivars yielding cytologically normal F 1 hybrids (2n = 4x = 28, ABUS or 2n = 5x = 35, ABDUS) from which fertile amphiploid progenies of 56 (2n=8x=56, AABBUUSS) and 70 chromosomes (2n=10x=70, AABBDDUUSS) were derived. The morphology and cytogenetics of these cross combinations plus their amphiploids, screening data for spot blotch and karnal bunt response under field conditions is reported to elucidate some probable strategies that would permit genetic transfers from Ae. variabilis into the recipient durum and bread wheat germplasms. Introduction Tertiary gene pool resources, even though complex to utilize are a potent means of enriching wheat germplasm (Mujeeb-Kazi, 2006). Aegilops variabilis (2n=4x=28, UUSS) possesses this diversity for providing resistances to at least two important biotic stresses that limit wheat production globally i.e., spot blotch of wheat (Triticum aestivum L.) caused by Cochliobolus sativus (Ito & Kuribayashi) Drechs. ex Dastur (syn.: Bipolaris sorokiniana (Sacc.) Shoemaker, Helminthosporium sativum Pammel, C.M. King & Bakke) and karnal bunt caused by Tilletia indica (Mitra). Spot blotch is an important pathogen that limits production in many nontraditional hot, humid wheat producing areas of Asia, Africa, and South America. C. sativus can attack seedlings, roots, leaves, nodes, spikes, and grains during various stages of plant development (Villareal et al., 1995). Yield loss estimates due to spot blotch on wheat vary widely. Losses of 85% were reported from Zambia and 40% from field trials in the Philippines. In addition, yield losses with the highly susceptible cultivar ‘Mitacore’ in an experiment conducted at Londrina, Brazil ranged from 79 to 87%, and the disease severely affected grain quality. De Milliano & Zadoks (1985) found a 38% yield loss using African wheat cultivars in growth-chamber studies in the Netherlands. Because of the importance of this disease, chemical control is applied in order to obtain crop production stability in many parts of the world. Emphasis is also being given to an integrated pest management approach utilizing resistant cultivars, healthy seed, cultural practices and chemical sprays. Though breeding for resistance is a high priority, it is hampered by scarcity of adequate resistance within T. aestivum. Sources of resistance to C. sativus in species other than T. aestivum (i.e., alien gene pools) are of special interest in breeding programs. We at the International Wheat and Maize Improvement Center (CIMMYT) and NARC (National Agricultural Research Center) have been making some effort to incorporate and exploit