Maize is the main preferred staple in southern Africa with consumption rate averaging about 100kg per capita per annum (Epka, 2019). However, Striga spp is one of the major biotic factors affecting maize production in Africa (Ejeta and Gressel, 2007) and is considered among the world's worst weeds (Shayanowako et al., 2018) causing up to US$ 7 billion loss annually in Africa (Rubiales et al., 2009). Striga spp is an obligate parasite that draws nutrients and water from its host (Ejeta and Gressel, 2007). There are two types of Striga spp which are Striga asiatica and Striga hermonthica that are prevalent in southern Africa and the rest of Africa, respectively. The widely reported cultural, biological and chemical control options for Striga spp are not feasible for the resource limited farmers in sub-Saharan Africa (Joel et al., 2007). Use of host resistance has been effective in controlling pests, diseases and weeds in various crops.
The International Institute of Tropical Agriculture (IITA) managed to incorporate resistance from wild relatives of maize, Zea diploperennis and Tripsacum dactyloides (Rispail et al., 2007). Resistance was also sourced from maize populations in east Africa where the cereals have co-existed with this parasite for long. Subsequently, a number of inbred lines and hybrids with resistance to Striga hermonthica were developed and were shared across various maize breeding programs in the rest of Africa. Resistant genotypes usually show few Striga root attachments and little Striga germination stimulant (strigolactones) production (Rank et al., 2004). Other resistance mechanisms include reduced flowering and reduced seed set of the Striga species (Awad et al., 2006). These mechanisms of resistance were found to be effective in controlling Striga asiatica in southern Africa (Gasura et al. In Press). Resistance to Striga spp was found to be controlled mainly by additive gene action (Gasura et al. In Press).
Germplasm from IITA has novel sources of resistance to Striga spp while germplasm from the International Maize and Wheat Improvement Centre (CIMMYT) is widely adapted to many regions including southern Africa. In order to maximize heterosis using germplasm from IITA and CIMMYT, there is need to understand the population structure and genetic diversity of these materials (Mengesha et al., 2017). This information is crucial in the identification of potential testers and prediction of potential heterotic groups, which are some key determinants of an effective breeding program in maize (Laborda et al., 2005; Nyombayire, 2016).
Plant breeders need to cross inbred lines that are from different heterotic groups to maximize heterosis. The longest and expensive period during a hybrid production is when selecting parents that when crossed produces superior crosses ( Moose and Mumm, 2008). Genetic distances play a very important role in hybrid vigor (Stinard et al., 2008). According to Acquaah (2012), a heterotic group is a group of related genotypes or distant genotypes from the same or different populations, which have similar combining ability when crossed to complementary germplasm groups. In contrast, a heterotic pattern refers to heterotic groups that complements one another and showing high heterosis when crossed (Jannink et al., 2010). Maize breeders postulated the concept of heterosis, when they observed that, inbred lines from different heterotic groups were producing superior hybrids when crossed (Suwarno et al., 2014). Having information on heterotic groups and patterns, a breeder can fully utilize the available germplasm by exploiting complementary lines to produce better performing hybrids (Sibiya et al., 2013). From some studies conducted it was shown that the intergroup hybrids out yields intragroup hybrids in maize (Mengesha et al., 2017).
To begin a maize hybrid program one has to use a well-documented germplasm with well-known heterotic groups and patterns (Moose and Mumm, 2008). Methods such as geographical information, phenotypic traits, pedigree information, combining ability and the use of molecular markers (Wende et al., 2013) have been widely used to classify genotypes. In the temperate regions, the Reid * Lancaster heterotic pattern has been developed using pedigree analysis and also in Europe, the European flint * Maize Belt dent have been developed based on phenotypic markers (endosperm types) (Paschold et al., 2010; Fischer et al., 2010). In France F2*F6 heterotic pattern derived from open pollinated cultivars was reported while in the tropical regions many patterns have also been developed including the ETO-compote*Tuxpeno and the Suwani*tuxpeno pattern (Acquaah, 2012; Reid et al., 2011). The use of molecular markers especially the single nucleotide polymorphism (SNP) markers has shown advantages over other methods (Acquaah, 2012, Leal et al., 2010, Mammadov et al., 2012, Moose and Mumm, 2008, Xu and Crouch, 2008). The advantages of SNP markers include speed (Leal et al., 2010), locus specificity, codominance, high genomic abundance, high throughput, lower genotyping error rates and low cost per data point (Paschold et al., 2010, Richard et al., 2018, Semagn et al., 2012). The aim of this study was to determine the genetic diversity and population structure of the key germplasm from CIMMYT and IITA using SNP markers. This information could guide in the identification of heterotic groups and potential testers required to kick start a maize breeding program for Striga asiatica in southern Africa.