linkage (Stuber et al., 1992; Crow, 1999). Epistasis, particularly between linked loci, may also be an explanation Heterotic groups and patterns are of fundamental importance in for heterosis in maize (Cockerham and Zeng, 1996). No hybrid breeding of maize (Zea mays L.). The major goal of this study data exclude the possibility of all three mechanisms was to investigate the relationship between heterosis and genetic distance determined with simple sequence repeat (SSR) markers. The contributing to heterosis, albeit in different proportions. objectives of our research were to (i) compare the genetic diversity Lamkey and Edwards (1999) coined the term panmicwithin and between seven tropical maize populations, (ii) test alterna-Breeding, Seed Sci.,

The paper describes the sequence of breeding stages that led to the development of acceptable quality protein maize (QPM) germplasm at the Centro Internacional de Mejoramiento de Maiz y Trigo (CIMMYT). Initial emphasis was on the development of soft opaque-2 (o2) maize varieties, but these had undesirable consumer characteristics, among other problems. Research then shifted to the development of hard-textured QPM germplasm. Several breeding approaches were explored and tested in early 1970. The combined use of two genetic systems involving the o2 gene and the genetic modifiers of the o2 locus appeared to be the most promising approach. This strategy first involved the development of donor stocks from which a large volume of QPM germplasm was generated through a modified backcross programme and various recurrent selection procedures. Later merging and reorganization of QPM germplasm was undertaken, which resulted in a definite number of QPM populations and pools to meet germplasm needs for various agroclimatic conditions. This was a turning point in the breeding strategy that permitted work with homozygous o2 genetic backgrounds. In the mid-1980s, a hybrid development initiative was started. Basic information on combining ability and heterotic pattern(s) of QPM germplasm was generated. Later emphasis was shifted to development efforts for inbred and hybrid QPM. Several superior QPM germplasm products are now spreading commercially in several developing countries of Asia, Central and South America, and Africa

Heterotic groups and patterns are of fundamental importance in hybrid breeding. The objectives of our research were to: (1) investigate the relationship of simple sequence repeats (SSR) based genetic distances between populations and panmictic midparent heterosis (PMPH) in a broad range of CIMMYT maize germplasm, (2) evaluate the usefulness of SSR markers for defining heterotic groups and patterns in subtropical germplasm, and (3) examine applications of SSR markers for broadening heterotic groups by systematic introgression of other germplasm. Published data of two diallels and one factorial evaluated for grain yield were re-analyzed to calculate the PMPH in population hybrids. Additionally, 20 pools and populations widely used in CIMMYT's breeding program were assayed with 83 SSR markers covering the entire maize genome. Correlations of squared modified Roger's distance (MRD(2)) and PMPH were mostly positive and significant, but adaption problems caused deviations in some cases. For intermediate- and early-maturity subtropical germplasm, two heterotic groups could be suggested consisting of a flint and dent composite. We concluded that the relationships between the populations obtained by SSR analyses are in excellent agreement with pedigree information. SSR markers are a valuable complementation to field trials for identifying heterotic groups and can be used to introgress exotic germplasm systematically.

No abstract

Information about combining ability and heterotic patterns among CIMMYT's maize (Zea mays L.) gene pools and populations should assist research programs in their hybrid development activities. A nine‐parent diallel was formed to (i) study the combining ability of CIMMYT's subtropical and temperate intermediate‐maturity maize gene pools and populations and (ii) evaluate the potential of CIMMYT's subtropical and temperate materials as exotic germplasm sources for temperate breeding programs. Parents and crosses were evaluated in 5 environments in Mexico and 11 in the USA. Highly significant, positive general combining ability (GCA) effects for yield were observed with Populations 42, 47, and 34 in Mexico. The Population 42 × Population 47 cross ranked first in yield (7.87 Mg ha−1, with 9.1% high‐parent heterosis). Population 33 x Population 45 was the only cross with a significant, positive specific combining ability (SCA) effect for yield. Average yields in U.S. environments were low (3.49 Mg ha‐I), due partially to adaptation problems and late plantings. Only Pool 41 had a significant, positive GCA effect for yield in the USA. The four highest yielding combinations included Pool 41 with Populations 42, 47, 34, and 45. In Mexican subtropical environments, superior crosses included Population 42 × Population 47 (white grain) and Population 33 × Population 45 (yellow grain). Though the CIMMYT materials did not generally perform well in U.S. environments, Pool 41 and Population 42 showed the best potential as exotic germplasm sources for temperate breeding program