ABSTRACT
Tetraploid durum wheat (Triticum turgidum L., 2n=4x=28, AABB) and hexaploid bread wheat (Triticum aestivum L., 2n=6x=42, AABBDD) are central to world agriculture. Although bread wheat came on the world scene much later than its predecessor durum wheat, it has become the most important cereal crop for feeding humankind (see Chapter 1 by Jauhar in this volume). Genetic diversity and wide hybridization in relation to bread wheat improvement have been studied for almost two centuries. According to Ciferri (1955), the first interspecific cross was made in 1806 by Barelle. Subsequently, the first wheat-rye hybrid was reported by Wilson (1876). The first fertile Triticale came on the scene 5 years later (Rimpau 1881), followed by a mention of the complex wheat-barley hybrid (Farrer 1904), although of questionable authenticity (Shepherd and Islam 1981). Greater attention was paid to such divergent crosses for several reasons (McFadden and Sears 1946; Gupta and Priyadarshan 1982), primarily to enhance the genetic diversity in wheat that had a narrow genetic base because of adherence to the requirements of uniformity (Kronstad 1998), even though extensive genetic variation is present and stored in ex situ gene banks (Valkoun 2001). The holdings were estimated some years ago to be around 8,000,000 accessions (FAO 1996). Only 3% of these gene bank holdings are comprised of wild wheat relatives distributed within three gene pools (Harlan and de Wet 1971) possessing annual and perennial species (Dewey 1984). These are a valuable source of genes for biotic and abiotic stresses (Kimber and Feldman 1987). It is well recognized (Mujeeb-Kazi and Rajaram 2002) that genetic diversity is a prerequisite for ensuring durability of resistance and a major factor for sustainable agricultural systems that are challenged with the responsibility of producing a target of 1 billion tons of wheat over the next two decades (Braun et al. 1998). This output figure means a shift from the current wheat global average yield of 2.5 K/ha to roughly 4 K/ha (Rajaram 2001) to adequately address the projected world population needs of about 8.2 billion people.
