**Abstract**

Maize (*Zea mays* L.) is the most important staple cereal cultivated in sub-Saharan Africa but its productivity is considerable low due to several factors. Development and deployment of maize hybrids have been reported as one of the crucial options in achieving sustainable maize production in sub-Saharan Africa. Information on the heterotic response among available genetic materials in a breeding program is valuable before commencement of any hybrid development program. Unlike the temperate germplasm, maize tropical germplasm is characterized with wide genetic base and genetic complexities and thus, proper organization of the pools, populations, varieties and inbreds that can serve as parental materials for hybrid development through identification of a distinct heterotic groups and patterns among tropical germplasm becomes very essential. This paper reviewed past research efforts at characterizing heterotic response among tropical maize genetic materials with a view to point out merits and demerits in the methods used and future direction towards achieving sustainable hybrid cultivation and enhancing food security in the sub-region.

**Keywords:** combining ability, gene action, heterotic grouping, hybrids, tropical maize

### **1. Introduction**

The term 'heterosis', as first introduced by Shull in 1909, was used to describe the phenomenon when the mean of any character or characters in a hybrid exceeds the mean of its descendants obtained by any system of close inbreeding. Four hypotheses were proposed to explain this; the dominance hypothesis which postulates that the increase in vigor after crossing results from the combination of different dominant alleles contributed by each parent [1]. The heterozygosis hypothesis attributes the increase in vigor to the existence of loci at which the heterozygous state is superior to either homozygotes [2, 3]; the pseudo-overdominance hypothesis that attributes the hybrid vigor to the effect of tightly linked genes with favorable dominant alleles in repulsion phase in the parental lines resulting in an apparent overdominance when combined in the hybrid [4] and epistasis hypothesis which explains the increased vigor in the light of the interaction of favorable alleles from two parents at different loci that show additive, dominant and/or overdominant action [5]. Among these hypotheses, heterozygosis gained prominence. Milborrow [6] asserted, from physiology view point, that even though the growth of a plant may be limited by the genes that regulate certain metabolic pathway down to a lower level than the maximum possible, heterozygous plants may partially escape the growth regulation, thereby giving them advantage over the homozygous

individuals. Brieger [7] explained that heterosis is easily obtained when the parents from which the hybrids are produced are inbreds or purelines and that heterosis does not affect the individual plant as a whole, but the expression of each of the traits that are heterotic. For instance, characters in maize that are affected by heterosis include plant and ear heights, size of leaves, intensity, size and strength of root system, amount of pollen shed, number and size of kernels and response to biotic and abiotic stresses [7]. Characters such as earliness to maturity, row number of the ear, plant and kernel color are not heterotic characters.

Two types of heterosis have been described in literatures. Falconer and Mackay [8] defined mid-parent heterosis as the difference between the hybrid and the mean of the two parents. They also defined high- or best-parent heterosis as the difference between the hybrid mean and the mean of either of the parent. Mid-parent heterosis value has been of more importance because it provides the basis for the identification of heterotic patterns among a fixed set of populations/inbred lines [9]. Melani and Carena [10] asserted that the utilization of mid-parent values is an effective practical method to identify heterotic responses among parents.

A heterotic group has been defined as a collection of germplasm that, when crossed with germplasm from an external group, tends to exhibit a higher degree of heterosis (on the average) than when crossed with a member of its own group [11]. Melchinger and Gumber [12] also defined heterotic group as a collection of related or unrelated genotypes from the same or different populations, which display similar combining ability and heterotic response when crossed with genotypes from other genetically distinct germplasm groups. Heterotic pattern refers to a specific pair of two heterotic groups, which express high heterosis and consequently high hybrid performance in their cross. Melchinger and Gumbler [12] recommended the following criteria for the choice of heterotic pattern in hybrid breeding; (i) high mean performance and large genetic variance in the hybrid population; (ii) high *per se* performance and good adaption of the parent population to the target regions; and (iii) low inbreeding depression, if hybrids are produced from inbred lines. Establishing heterotic pattern is of prime importance in the development of a successful maize hybrid program [13].

## **2. Heterotic grouping methods for maize germplasm.**

After establishing significant genetic variability among parental materials to use, plant breeders employ several methods for classifying the parents into heterotic groups. The methods include morphological traits, pedigree method, multivariate technique, genetic methods involving mating designs and the use of molecular markers. At advanced stage of breeding, genetic and molecular methods are preferred because of their high level of precision since their results are minimally influenced by environmental factors. Among several mating designs in plant breeding, three are prominent for classifying parents into heterotic groups. Where proven testers exist in a breeding program, a line x tester mating design is embraced in which each tester represent a heterotic group. Where there is no proven testers, diallel method and North Carolina Design II become better alternatives. In studies where such designs are employed, information on heterotic groups as well as identification of testers are usually the prime objectives. The advent of molecular markers has offered a less-stressful, faster, smarter and somewhat cheaper alternative through the use of genetic distance. Examples of markers for popularly used this purpose are Amplified Fragment Length Polymorphism (AFLPs), Simple Sequence Repeats (SSRs) and Single Nucleotide Polymorphism (SNPs) markers. The qualities of these markers that make them suitable for this purpose include the following: high throughput, they are highly reproducible and they are relatively easy to assay. In more recent times, SNPs markers has become the most popular and marker technologies include Microarray and DarT and DarTSeq have been developed on the basis of SNPs.
