used to assist plant breeders who realize wide crosses to be able to nurture young embryos into full plants. Plant germplasm of vegetatively propagated species may be maintained in germplasm banks using the tissue culture technique (see Box 7.1).
David Okeh Igwe, George Nkem Ude, and George Acquaah
Department of Natural Sciences, Bowie State University, Bowie, Maryland, USA
Introduction
Bananas and plantains (Musa spp.) are perennial crops with rapid growth rate. They belong to the family Musaceae and are cultivated all year round in the tropics and sub‐tropics. Considered to be the favorite fruit crops of the world, they also are globally distributed in more than 120 countries, with a total production of approximately 106–140 million tonnes per year (Molina and Kudagamage 2002; FAOSTAT 2014). Bananas and plantains are the highest export fruit crops (FAO 2011) and are ranked fourth‐most important in sub‐Saharan Africa (SSA) after other vital crops including cassava, maize, and yam (FAO 2009). They are rich sources of carbohydrates, vitamin C, potassium, and sodium (International Banana Association 2007) in addition to their abundant uses in medicines, industrial raw materials, and other domestic applications (Abiodun‐Solanke and Falade 2010). The different ploidy constitutions of Musa species were derived from Musa acuminata (AA) and Musa balbisiana (BB) and are categorized into different groups including diploids (AA, AB, and BB), triploids (AAA, AAB, ABB, and BBB), and tetraploids (AAAA AAAB, AABB, and ABBB) (INIBAP 2003; Pollefeys et al. 2004). Also, East African bananas (mainly dessert types) (AA, AAB, AAA, ABB, and AB), and the African plantains (AAB) are grown mainly in Central and West Africa, while the East African Highland Banana (AAA), are for cooking and beer brewing (Karamura et al. 1998). Variations are evident in the phenotypic expressions of different ploidy constitutions including triploid plantain (ABB), diploid banana (AA), and triploid banana (AAA) (Figure B7.1) (IITA 2018). Phenotypically, plantains are known to be taller than banana trees (Figure B7.2). Bananas, especially the dessert ones (M. Cavendish, AAA) have thinner fleshy texture, and when in ripened form, more compact fruits than those of plantains, and change to yellow color (Figure B7.3). They contain sugar, have a sweeter taste, and are usually consumed raw. Plantains on the other hand, have thicker, fleshy skin due to high starchy composition, and turn yellow first and then black when ripened. In addition, plantain fruits have rough surfaces and more vitamins A and C and potassium, and can be cooked before consumption unlike banana fruits (Valmayor et al. 2000). The identified cultivated banana and plantain fruits vary with wild relatives by exhibiting seedless condition and parthenocarpy (development of fruit without seed or pollination and fertilization).
Figure B7.1 Variations in phenotypic expressions of different ploidy constitutions of plantain and banana fruits. (a) = triploid plantain (ABB); (b) = diploid banana (AA); (c) = triploid banana (AAA) (IITA+banana&fr = tightropetb&imgurl=http%3A%2F).
Figure B7.2 Different accessions of banana and plantain being maintained in the greenhouse at Natural Science Department, Bowie State University: (a) = plantain (Poteau Geant, ABB); (b) = Plantain (Pata, ABB); (c) = banana (Gros Michel, AAA); and (d) = plantain (Balonkawe, ABB).
Source: Picture – Natural Science Department Greenhouse, Bowie State University.
Figure B7.3 Phenotypic comparison of plantains and bananas in their fruits form. (a) = bunch of Agbagba plantain fruits on plantain tree; (b) = bunch of plantain hybrid PITA 2 with fruits; and (c) = bunch of banana fruits (IITA+banana&fr = tightropetb&imgurl = http%3A%2F).
Production of these vital crops is challenged by pathogenic and abiotic factors. With rising global temperatures, which are expected to have drastic effects including altered patterns of drought, salinity, emergence of new pests, and diseases, plant growth and yield will be adversely impacted (Tester and Langridge 2010). Using informative molecular markers for assessment of genetic diversity to harness allelic richness, and selecting those with unique and desired traits for rapid production, maintenance, and germplasm conservation via plant tissue culture techniques, would be the right combination of tools to obtain favorable genotypes and to ensure periodic availability of good planting materials for farmers. This is vital since the world population is growing fast and is expected to reach over 9 billion by the year 2050 (FAO 2015). Feeding this overwhelming population level is generating much pressure on agricultural crop production (Dempewolf et al. 2014; Khoury et al. 2014).
Genetic diversity and population structure of Musa species using CDDP, ISSR, and SCoT markers
To increase food supply, especially Musa species, harnessing genetic diversity and novel traits can result in developing new genotypes that are capable of withstanding changing environmental factors, since populations with narrower range may fail to survive climatic extremes. There are now very informative and cost‐effective molecular markers that target conserved domains and can effectively exploit the genetic indices or gene pools inherent in banana and plantain plants as well as their wild relatives for crop genetic improvement. It has been reported that structural variant genes possessing presence or absence of variants contribute to diversity of the gene pool (Golicz et al. 2016). Identification of Musa accessions (wild and elite ones) that can be adopted and optimized to perform in diverse environmental conditions based on abundant allelic diversity is very important, since the optimal development of these accessions is dependent on the allelic/genetic diversity (Montenegro et al. 2017). Knowledge of genetic diversity within accessions of Musa species with diverse genomic groups can facilitate the efficiency of breeding programs. Comparatively, assessing the effectiveness of different molecular markers is essential for identification of appropriate ones for genetic improvement and germplasm conservation.
Conserved DNA‐derived polymorphism (CDDP), inter‐simple sequence repeat (ISSR) and start codon targeted (SCoT) markers
Conserved DNA‐derived polymorphism (CDDP) markers comprising transcriptional factors (TFs) – MYB, ERF, WRKY, and APB – are cost‐effective, target‐conserved sequences of plant functional genes, and possibly produce candidate markers that may be partly or completely
