Bananas are one of the most important fruits in the world and are grown in many tropical and subtropical areas where they are used as both a staple food (cooking banana) and dietary supplements (dessert banana) (Pillay et al., 2002). Dessert banana are consumed raw when they are ripe (Pillay et al., 2002). Beside the source of food, other parts of banana, the dry leaves are used for fruit packing and the green leaves used as plates for wrapping food (Pillay et al., 2002).
In most countries, dessert banana production is for export trade, and this is comprised largely of a single cultivar, ‘Cavendish' (Becker et al., 2000). The Cavendish subgroup is the most significant subgroup of edible bananas (Ploetz et al., 2007). They are major export commodities in Central America, South America, the Caribbean, West Africa, and the Philippines (Ploetz et al., 2007). They comprise over 40% of edible bananas that are produced worldwide (Ploetz et al., 2007). In equatorial lowlands where the ambient temperatures are high, fruits turn greenish-yellow when ripe, although where temperatures are a bit cooler or when artificially ripened, they turn bright yellow (Ploetz et al., 2007).
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Therefore, this crop improvement plan would focus on the constraints to production of the Cavendish subgroup.
Taxonomy and Origin
The genus Musa's center of origin is in Indochina and Southeast Asia (Simmonds, 1962). Banana belongs to the Musaceae family, one of the six families of the order Zingiberales (Pillay et al., 2002). The Musaceae comprises of two genera, Musa and Ensete (Pillay et al., 2002; Ploetz et al., 2007). Four sections have been recognized in the genus Musa namely; Australimusa, Callimusa, Rodochlamys and Eumusa (Pillay et al., 2002). The section Australimusa and Callimusa have a basic set of 10 chromosomes whilst the other two sections have 11 chromosomes (Pillay et al., 2002; Ploetz et al., 2007).
Genome Groups in Musa
Domesticated bananas are naturally occurring hybrids between two wild diploid (x = 11) M. acuminata Colla. and M. balbisiana Colla whose genomes are designated as A and B respectively (Ploetz et al., 2007). Hybridisation between subspecies of M. acuminata produced a range of diploid cultivars (AA genomes) (Pillay et al., 2002; Ploetz et al., 2007). Diploid AAs produced triploid AAA types by chromosome restitution (Pillay et al., 2002). Hybridisation between AA diploids and M. balbisiana (BB) gave rise to the many AAB and ABB types (Pillay et al., 2002; Ploetz et al., 2007). The cultivated triploid AAA genome comprises the sweeter dessert banana whereas the AAB and ABB are starchier cooking types (Pillay et al., 2002). Other genome groups mostly products of breeding programs include AAAA, ABBB, AAAB and AABB (Ploetz et al., 2007). The triploid AAA genomes consist of three subgroups namely; Cavendish, Gros Michel and Ibota (Ploetz et al., 2007).
This subgroup is made up of the following clones; ‘Pisang Masak Hijau', ‘Grand Nain', ‘Giant Cavendish', ‘Dwarf Cavendish', ‘Double' and ‘Extra Dwarf Cavendish' in descending order of the height to which they grow in a given location (Ploetz et al., 2007).
The subgroup is resistant to Panama disease in the western tropics, but is susceptible to the Black Sigatoka disease, management of the latter disease is a major expense in commercial production, especially in areas with high rainfall (Pillay et al., 2002). The clones are similar except for their height and characteristics of the bunch and fruit (Ploetz et al., 2007). Except ‘Extra Dwarf Cavendish', all are productive if they are provided with ample fertilizer and water (Ploetz et al., 2007).
This is the most widely distributed clone of edible banana worldwide and it is the shortest used for commercial production since tall clones lodge in high wind and difficult to harvest (Ploetz et al., 2007). It bears good-quality fruit, with a long transport life if picked at the correct maturity and is well suited for home garden, commercial, and agroforestry cultivation (Gubbuk et al., 2004; Ploetz et al., 2007). Short and compact, it is relatively cold tolerant (Ploetz et al., 2007).
Pest and diseases problems are increasing important constrainst (Pillay et al., 2002). Black Sigatoka, caused by Mycosphaerella fijiensis Morelet is the major constraint for banana productions (Pillay et al., 2002). This disease causes from 30 to 50% yield reduction and is usually control with massive application of synthetic fungicides. Yellow Sigatoka (caused by M. Musicola Leach) also causes similar damage (Pillay et al., 2002). Other important diseases include Banana streak virus, Fusarium wilt, bunchy top virus disease, bacterial wilt and banana mosaic disease (Pillay et al., 2002).
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Plant parasites (nematodes) can cause damage to banana resulting in yield losses (Pillay et al., 2002). Other pests, including Banana weevil can cause damage to banana throughout the tropics (Pillay et al., 2002).
Constraints to Improvement
Dwarf Cavendish is a triploid (2n = 3x = 33) and hence present several barriers to conventional improvement methods (Simmonds, 1960; Ssebuliba et al., 2006). Generally characterised by low male and female fertility that results in very low seed yield (less than 1 seed per bunch) and germination rates (less than 1%) (Ortiz and Vuylsteke, 1995; Vulsteke et al., 1993). It takes over a year to go from seed to seed (Pillay et al., 2002).
Ploidy Manipulation, fertility and Seed handling
Conventional crossing methods have been successful in producing inter- and intra-specific hybrids as a result of minimizing infertility barriers (Ssebuliba et al., 2006). Improvement of triploid Musa species has been achieved through crossing 3x landraces with 2x (diploids) wild (Ssebuliba et al., 2006).
Through embryo rescue and somatic embryogenesis technique banana plants can be regenerated (Assani et al., 2001). Embryo culture is reported to enhance seed germination and hybrid recovery (Pillay et al., 2002). It is possible to obtain embryo germination rates of up to 30% on phytohormone free medium compared to 1% from direct sowing of seeds in soil (Vuylsteke and Swennen, 1992; Ortiz and Vuylsteke, 1995). Thus, in vitro embryo culture is considered as an integral component of banana-breeding programs (Pillay et al., 2002). However, the success in banana breeding relies on the identification of female fertile landraces (Ssebuliba et al., 2006).
Improvement Plan for Dwarf Cavendish
To improve resistance of Dwarf Cavendish to Black Sigatoka disease.
According to Ssebuliba et al., 2006 adequate levels of resistance against this disease have not been found in the cultivated bananas but have been identified in wild species. Of particular interest is a fertile wild diploid species M. acuminata spp. burmannicoides ‘Calcutta 4'(Vuylsteke et al., 1993), which is resistant to black sigatoka and a number of other diseases and pests. ‘Calcutta 4' with an AA genome composition is one of the progenitors of edible bananas and has been used in many breeding programs to transfer black sigatoka resistance to cultivated bananas (Swennen and Vuylsteke, 1993; Vuylsteke et al., 1993).
Use Asymmetric Somatic Hybridisation to transfer Black Sigatoka resistance trait from Calcutta 4 to Dwarf Cavendish. Since conventional crossing will take a long time, labour intensive and problems with low female fertility, this technique will help in overcome this barriers (Ssebuliba et al., 2006).
Plant materials and cultures
Disease free embryogenic cell suspensions of the triploid banana (Musa sp. AAA group) and non-embryogenic calli of the diploid banana (Musa sp. AA Calcutta 4) can be used as the source of protoplasts (Matsumoto et al., 2002). The embryogenic cell suspension can be obtained from a male inflorescence and maintained in a modified MS (Murashige and Skoog, 1962) liquid medium (Matsumoto et al., 2002). The non-embryogenic calli can be induced from a male inflorescence tip on a medium consisting of MS (Matsumoto et al., 2002). The induced calli can be maintained on the modified MS medium without the growth regulators (Matsumoto et al., 2002).
Isolation of protoplasts and Cultures
Protoplasts from the cells suspension of Dwarf Cavendish and calli of Calcutta 4 can be isolated as described previously (Matsumoto and Oka, 1998). Protoplasts of Dwarf Cavendish can be treated with Indoacetoamide (IOA) to prevent cell divisions prior to fusion whilst protoplasts of Calcutta 4 can be treated with soft X-rays to inactivate the nucleus (Akagi et al., 1989). Resistance to Black Sigatoka is contributed through cytoplasmic genomes and this will help in the restoration of the seedless trait of Dwarf Cavendish while maintaining it yield and fruit quality (Pillay et al., 2002).
Flow diagram showing Improvement Scheme for Dwarf Cavendish using Marker-assisted selection (MAS) and QTLs associated with general and specific combining ability (GCA and SCA).
Asymmetric fusion of Protoplasts and plant regeneration
Fusion of the two parental protoplasts can be achieved with chemical (polyethylene glycol PEG) or electrofusion technologies. Methods for protoplast culture, colony development and plant regeneration as described by Matsumoto and Oka, 1998 can be followed to regenerated plants.
Identification of Asymmetric Somatic Hybrids
Early identification of asymmetric somatic hybrids at R1 stage using molecular markers (RAPD, SSR, ISSR and RFLP) have been developed for the two parents and ploidy can be determine to confirm asymmetric hybrids using flow cytometric (Matsumoto et al., 2002).
In vitro Selection for Resistance
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Harelimana et al. (1997) have demonstrated that it is possible to use Mycosphaerella fijiensis toxins for the selection of banana cultivars resistant to Black Leaf Streak in vitro. Therefore, it can be considered as an option for evaluation at an early stage.
Host Pant Pathogen Studies
This can be done by planting asymmetric hybrids that will be identified at stage R1 in an experimental plot surrounded by highly susceptible Cavendish cultivar to create an environment that provided sufficient inoculums potential for assessment of Black sigatoka resistance (Ortiz and Vuylsteke, 1994). All crop management practices can be used except fungicide application (Craenen and Ortiz, 1998).
Quantitative Traits for Black Sigatoka resistance
Resistance of individual plant trait may include: youngest leaf with symptoms, youngest leaf spotted at flowering, because no leave develops after flowering (Craenen and Ortiz, 1998). Individual leaf traits may include: incubation time, evolution time, disease development and life span of the leaf (Craenen and Ortiz, 1998). These traits can be used to develop quantitative trait loci for Black sigatoka resistance (Ortiz et al., 1997).
General Quantitative traits
This may include: days to flowering, plant height at flowering, fruit filling time (days from flowering to harvest), growth cycle, plant girth, total number of leaves, bunch weight per plant, fruit weight, fruit length, fruit girth, hands per bunch, fruits per bunch, and fruit parthenocarpy traits (Ortiz et al., 1997).
Field testing and Selection
Multilocational testing of resistant varieties and subsequent release of resistant hybrids.
Evaluation will be base on the heritable quantitative traits involve in disease resistance (Ortiz et al., 1997).
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Crop Improvement Plan IPage 7