PHYLOGENETIC RELATIONSHIPS AMONG TEOSINTE, MAIZE AND ITS HYBRIDS

Annual teosinte (Zea mays ssp. mexicana) was introduced to Egypt in the last century but never gained wide importance as a summer fodder because of difficult in seed production and relatively slow early growth (Radwan et al., 2000). Teosinte has special merits over fodder maize including multiple cutting, high nutritive value and ease of production. Teosinte differs from corn by abundant tillering which results in tufted plants, and the ability to recover and produce new growth from the crown buds after cutting (Kellogy and Birchler, 1993; Rammah, 1995). Hand-crossing studies demonstrated that Z. mays ssp. mexicana and maize exhibit genetically based cross-incompatibility (Baltazar et al., 2005). Unusually, the flow of genes has occurred in both directions (reciprocal introgression) (Wilkes, 1977) although a number of factors tend to favor gene flow from teosinte to maize rather than from maize to teosinte (Baltazar et al., 2005). There is also evidence of a restriction to cross ability in some populations of maize x teosinte when teosinte is the female and maize the male parent and this has been linked to a teosinte gene or gene cluster known as teosinte crossing barrier1 (Tcb1) (Evans and Kermicle, 2001). The incompatibility is asymmetric, being very strong when maize is the pollen parent, but weaker when teosinte is the pollen parent (Baltazar et al., 2005; Kermicle and Evans, 2005). Maize-Teosinte hybrids have been of considerable interest to both maize and teosinte breeders. The close genetic relationship between the two subspecies has stimulated interest in enriching the gene pool of maize with useful genes from maize. Likewise, maize-teosinte or teosinte-maize hybrids have also received attention for enhancing the fodder production potential of teosinte by taking advantage of hybrid vigor shown by the hybrids. Hybrids of ssp. mays x ssp. mexicana have statistically significant heterosis compared to the wild teosinte but not when compared to the cultivated parent (Guadagnuolo et al., 2006). Genetic distance GD among the germplasm lines has been quantified by means of morphological, biochemical and molecular analyses and by means of heterosis (Menkir et al., 2004; Laborda et al., 2005). The degree of heterotic effect of F1 populations correlated with GD of the parental lines, as parents are more divergent, the heterosis is higher and vice-versa (Prasad and Singh, 1986). Cultivated maize derived from teosinte and their morphological differences resulted from human selection in the process of domestication (Matsuoka et al., 2002; Doebley, 2004). Despite being one of the cultivated species with greater genetic diversity, molecular analysis of the maize genome suggests that a single domestication event reduced diversity when compared with teosinte (Vigouroux et al., 2002; Warburton et al., 2008). Most maize commercial varieties in the world has limited genetic diversity, whereas today the germplasm base in maize breeding programs is relatively narrow (Tarter et al., 2004). With the development of molecular marker techniques, DNA polymorphisms have been used as markers to measure genetic diversity in many plant species. Some scientists have been trying to predict yield heterosis on the molecular level. The relationship between molecular marker distance and heterosis remains unclear. Some of the reports state significant association (Lanza et al., 1997; Amorim et al., 2006; Srdic et al., 2007) whereas, the others state non-significant or no association between markers based GD and heterosis (Shieh and Thseng, 2002; Legesse et al., 2008; Devi and Singh, 2011). Molecular markers allow a direct comparison of the similarity of genotypes at the DNA level. Restriction fragment length polymorphisms (RFLPs; Botstein et al., 1980) have been used quite extensively for this purpose. However, RFLP assays are labor intensive and time consuming and, therefore, increasingly substituted by other marker techniques such as randomly amplified polymorphic DNA (RAPDs; Williams et al., 1990), Amplified fragment length polymorphism, (AFLPs; Zabeau and Vos, 1993), and simple sequence repeats (SSRs; Tautz, 1989). RAPDs marker has been used to investigate GD across the diverse species including segregating lines of maize (Ajmone-Marsan et al., 1993), to predict the best crosses among lines for hybrid development (Lanza et al., 1997) and to assess genetic diversity among maize collections (Moeller and Schaal, 1999). Study of genetic diversity is the process by which variation among individuals or group of individuals or populations is analyzed by a specific method or a combination of methods. Maize breeders frequently use genetic diversity evaluation as an alternative method for germplasm selection. The objectives of this study were to estimate the variation of teosinte, maize parents and its hybrids for mean performance and degree of divergence, to assess the correlation of morphological genetic distance (GDmor) with mean performance and to predict the best crosses among most distant hybrids selected from morphological clusters by RAPD molecular marker.