Appendix
8.3 Tree Crop Development
8.3.2 Participatory selection
Genetic gain and diversity in the domestication of miombo fruit trees
The high genetic diversity of selected Uapacapopulations in Malawi compared with the mother source and their progenies confirmed that the extensive participatory selection used to capture superior individuals was adequate
(Hamisy, 2004). Knowledge of phenotypic and genetic variation is a prerequisite for the domestication and improvement of indigenous fruits from the wild. Within-stand genetic variation of U. kirkiana and Sclerocarya birrea trees was determined (Fig. 8.1), and implications for developing germplasm collection and conservation guidelines were suggested (Agufa, 2002). S. birrea had more diversity than U. kirkiana in the study. The role of geography and structure in subspecies status in S. birrea in contributing to the pattern of variation was affirmed. S. birrea from Tanzania contained more chloroplast variation, supporting the hypothesis that this country is the centre of diversity and origin of S. birrea(Agufa, 2002).
In genetic terms, human-induced domestication results in either gain or loss of genes, altered gene frequencies or modifications of the way genes are packed, i.e. gene complexes (Simons, 1996). The domestication of wild fruit trees must be seen as a continuum between the undisturbed wild state, through cultivation of semi-domesticates, to monocultural plantation or orchards of advanced
B Chyulu (Kenya) (Sbc) J Kalimbeza (Namibia) (Sbc) M Pandamatenga (Botswana) (Sbc) K Choma (Zambia) (Sbc)
I Oshikondilongo (Namibia) (Sbc) L Siavonga (Zambia) (Sbc) G Mangochi (Malawi) (Sbc) H Ntcheu (Malawi) (Sbc) P Kalanga (Swaziland) (Sbc) R Manyonyaneni (Swaziland) (Sbc) N Tutume (Botswana) (Sbc) E Magamba (Tanzania) (Sbc) F Makadaga (Tanzania) ( Sbm) D Mialo (Tanzania) (Sbb) C Mandimu (Tanzania) (Sbb) A Missira (Mali) (Sbb)
Genetic distance R
S U T Chloroplast haplotypes
0.02 0.04
0.06 0.08
0.10
0.12 0.00
Fig. 8.1. Phenogram-based genetic distance from 80 RAPD markers for 16 populations of Sclerocarya birreasampled from eight countries in sub-Saharan Africa. Codes in
parentheses indicate subspecies designations made during collection. Sbc, ssp. caffra; Sbb, ssp.birrea; Sbm, ssp. multifoliolata(Agufa, 2002).
generation lines. The extent to which domesticated trees differ from their wild progenitors depends on population size, the heritability of the desired trait under selection, the mating system, the intensity of selection and the inherent variability of the traits (Nyland, 1996; Simons, 1996; Cornelius et al., 2006).
Domestication requires not just breeding, but also selection and management.
Simons (1996) has asserted that qualitative traits, such as fruit size, shape and taste and tree form and precocity, are more strongly inherited from the selected mother trees, such that 60% of the progeny might be similar to their parents.
Therefore cloning, after identifying and screening for a large number of superior traits, is the best way to capture such genetic variation because it eliminates the recombination or segregation of genes. This is the basis of participatory clonal selection in the domestication programme of ICRAF.
Hamisy (2004) assessed the efficiency of tree selection using the technique of random amplification of polymorphic DNA (RAPD). Two populations were used, and data on 109 RAPD loci in 181 Uapaca kirkianaindividuals showed a lack of clear differentiation between populations and subpopulations, showing a high level of genetic identity and thus indicating close relationships and the possibility of gene flow between the populations. Farmer-mediated short-, medium- and long-distance seed movements among populations may also explain the lack of distinct genetic subdivisions. A similar, low amount of genetic variation was observed in peach palm in Brazil (Cornelius et al., 2006). However, Hamisy’s study confirmed that the elite trees selected from the wild using the participatory approach showed more diversity than their wild counterpart sources and their progenies. This was attributed to the extensive sampling used in the phenotypic selection from natural forests, communal lands, homesteads and farms, which enabled the capture of superior trees from wild populations (Akinnifesi et al., 2006a) (Fig. 8.1). Large amounts of diversity between mother trees and their progenies indicate the potential for using vegetative propagation techniques for improvement to ensure genetic similarity as well as genetic diversity.
In practice, the use of seeds collected from isolated trees on the farm may cause inbreeding (through parent–offspring or half-sib mating), hence the need for proper selection guidelines for participatory domestication. A comprehensive framework for breeding programmes has been described which distinguishes four conceptually distinct breeding population components. First, the ‘base population’ is chosen from which individuals are selected to be carried forward to the next generation (Cornelius et al., 2006). Secondly, the
‘selected population’ is the subset of the initial population that is to be carried forward to the next generation. Thirdly, the ‘breeding population’ is composed of trees that are used to produce the next generation, and may consist of all of the selected population. Fourthly, the ‘production population’ is composed of the trees used to produce propagules for commercial planting, or on-farm planting in this case.
Cornelius et al. (2006) has articulated several ways in which these breeding programmes could lead to genetic erosion and loss of genetic diversity: (i) Because genetic variation depends on population size, intensive selection in the breeding programme will lead to loss of diversity in the base population, and possibly to cumulative genetic erosion if repeated in succeeding generations.
Ensuring that adequate numbers of trees are selected can become a challenge in small populations with desirable tree traits. (ii) Establishing a production population by intensive selection within the selected population will also lead to loss of genetic diversity, although it will not lead to long-term erosion. (iii) The use of seed sources from commercial plantations or orchards (farmers in our case) would lead in the short term to reduced diversity and inbreeding depression. Ways in which recurrent selection can achieve genetic diversity are well known, but arduous and long-term breeding investment is required.
Fortunately, most of the miombo indigenous fruit trees are amenable to vegetative propagation, and efficient clonal propagation techniques that might be used for accelerated impact have been described (Akinnifesi et al., 2006;
Leakey and Akinnifesi, Chapter 2, this volume). Cornelius et al. (2006) has outlined how the propagation of many selected clones within the network of the farming community, using village cultivars and the propagation of only a few selected clones (five to ten) from each village, can be used for the rapid production of superior propagules. This participatory clonal propagation approach has been used by ICRAF in West Africa (Tchoundjeu et al., 2006) and southern Africa for Uapaca kirkianaandStrychnos cocculoides(Akinnifesiet al., 2006), and Latin America has applied clonal seed selection in the peach palm (Cornelius et al., 2006).
Participatory identification of elite trees for clonal selection
Tree domestication is a paradigm shift, from a focus on tree improvement based on breeding and conventional forest tree selection to horticultural approaches focused on quality germplasm production for wider cultivation to serve the needs of smallholder farmers. It is an iterative process that includes a wide range of activities. The processes involved include the exploration of wild populations and the identification of superior tree species and provenances from natural variability; the evaluation and selection of suitable trees and clonal propagation to develop superior cultivars; macro- and micropropagation techniques for multiplication; scaling up to the dissemination of germplasm;
and acquiring knowledge of tree management (Akinnifesi et al., 2006). The domestication approaches and strategies deployed for individual species vary according to their functional use, ecology and biology, niches and biophysical limits (Simons and Leakey, 2004; Akinnifesi et al., 2006; Tchoundjeu et al., 2006; Leakey and Akinnifesi, Chapter 2, this volume).
Tree domestication in ICRAF Southern Africa has evolved from multipurpose tree screening in the 1980s to a more participatory domestication programme involving pomological and market-led approaches, starting from 1996. The concept of ‘ideotype’ is a first step towards developing an improved plant by combining characters which provide a guide to the selection of potential breeding stock in a wild population (Dickman, 1985). Its application to fruit trees, such as mango (Mangifera indica) (Dickman et al., 1994), and the domestication of priority indigenous fruit trees, such as Irvingia gabonensis (bush mango) in Nigeria, has been described by Leakey and Page (2006) as an aid to the multiple-trait selection of superior trees for cultivar development, and offers an
opportunity for developing a hierarchy of different ideotypes to meet different market opportunities.
In southern Africa, local knowledge of the rural communities was captured by brainstorming at village workshops about the objectives of selection with 20–30 people in each group. Tree-to-tree variation was measured, with the communities, in wild populations of Uapaca kirkianaandStrychnos cocculoides, and selection of superior trees was based on market-oriented ideotype products (Akinnifesi et al., 2006). Together with villagers, the authors identified the superior trees on the basis of superior traits, and these were were systematically named and tagged in situaccording to year of collection, location and ownership. Site descriptors were documented and fruits sampled for detailed assessment of the qualitative and quantitative characteristics, including chemical and organoleptic analysis. Seeds and scions were also collected for growth and multiplication in the nursery. In some cases, duplicate materials were collected by farmers and raised in individual or group nurseries in their own communities. The superior germplasm was subsequently evaluated in clonal orchards on-station and on-farm and fruits were characterized and analysed for their chemical characteristics. This evaluation identified the trees for subsequent vegetative propagation and clonal testing, so that high-quality planting materials could be made available to farmers as soon as possible.
Through the selection and propagation of elite genotypes from the wild, new cultivars with superior or better marketable products – fruit size, sweetness and fruit load with improved uniformity – have been obtained. A total of 429 trees with superior phenotypes of priority indigenous fruits (Uapaca kirkiana, Strychnos cocculoides, Sclerocarya birrea, Vitex mombassae) was selected in the region from the wild, on the basis of criteria determined jointly with rural community dwellers (farmers, marketers, traditional chiefs, schoolchildren) in Malawi, Zambia, Zimbabwe and Tanzania. These include 107 superior Uapaca kirkiana and 20 Strychnos cocculoides phenotypes in Malawi; 108 Uapaca kirkiana and 34 Strychnos cocculoidestrees in Zimbabwe; 78 Uapaca kirkiana and Strychnos cocculoidestrees in Zambia; and 30 trees of Vitex mombassae and 20 superior Sclerocarya birreatrees in Tanzania.
In Malawi, the natural population of female Uapaca kirkianatrees was 40 trees per hectare in Dedza, 81 trees per hectare in Kasumbu and 490 trees per hectare at Phalombe. During the participatory workshops with farmers, a few outstanding trees, ideal for processing, were identified. To supplement the information received from the participatory village workshops, elite trees were identified with communities and measurements were made on fruit size, fruit sweetness (sugar content) and pulp content (Akinnifesi et al., 2006). The selection was based on superior fruit characteristics from different populations and land uses. For example, in Dedza, Malawi, among the elite Uapaca kirkianatrees identified as having high fruit yields, one tree in particular had more than 6000 fruits per tree, with large fruit size (3–4 cm diameter) and sweet taste (Akinnifesi et al., 2006). The big-fruit ideotype was locally described by expressions such as ‘gundete okolera’ in Dedza and ‘mapunbu amutiye’ in Phalombe, Malawi, depicting its outstanding size, appeal and taste. These same expressions were used to describe the most beautiful unmarried girl or potential village beauty queens in the communities.
In Zimbabwe, using PRA approaches, 416 participants (farmers, fruit vendors and schoolchildren, 65% of them female) identified 108 superior trees of Uapaca kirkiana in seven districts across the country. In addition, 200 participants (50% women) from four districts identified 34 superior phenotypes of Strychnos cocculoides. Total soluble sugar from Strychnos cocculoidestrees ranged from 7 to 23% and fruit size ranged from 75 to 514 g. ICR04MapanzureZW8 had the biggest fruits (514 g) and ICR04MafungautsiZW17 had the highest total soluble sugar (23%). The variability of fruit characteristics was clearly illustrated in Zimbabwe; trees with very contrasting fruits were identified as ICR02Chimani ZW9, ICR02UrandaZW29 and ICR02MafaZW40. ICR02MafaZW40 had the highest pulp content and greatest fruit weight, but a low sugar content. In contrast, ICR02UrandaZW29 had small fruits that were very sweet and had high pulp content (Akinnifesi et al., 2006). On the other hand, although ICR02ChimaniZW9 had large fruits, it was rejected in the selection process as it had low pulp content, high relative shell weight and low sugar content. It also had the highest seed weight. A consequence of domestication and cultivar development is that trees propagated clonally from mature tissues will flower and fruit earlier. They will therefore be smaller in stature and produce fewer fruits, especially in the climate of miombo woodlands. To compensate for this, trees can be grown at higher density. In our experience, marcots are bigger than grafted trees, and trees established from seedlings tend to be taller but with a smaller crown size compared with marcots. However, trees established from seedlings have a longer juvenile phase than trees established from either grafted or marcottage stocks. In high-rainfall areas of West Africa, marcots have been shown to be bigger than plants established from seedlings (Z. Tchoundjeu, personal communication).
However, it is important to recognize that, as in Uapaca kirkiana in Zambia, some fruit traits, including tree fruit load and pulp content per fruit, can be manipulated to a limited extent by management practices such as thinning (Mwamba, 1995b). The challenge is not only of practical improvement per se, but also of reconciling the potential genetic improvement with the practical realities of farmers’ needs and perceptions and the delivery of germplasms (Simons, 1996), considering also intellectual property right (IPR) environments.
Clonal propagation
Seedling propagation is not a desired approach for commercial fruit production because of the high variability of progenies from mother trees; asexual means are therefore preferred. Both macropropagation (conventional vegetative propagation) and micropropagation (in vitro culture) techniques have been applied to some priority miombo indigenous fruits in southern Africa. Vegetative propagation is needed to rapidly test, select from, multiply and use the large genetic diversity in wild tree species on stations and farms. From our experience, some of the miombo fruit trees are not amenable to propagation by juvenile stem cuttings; examples are Uapaca kirkiana,Parinari curatellifoliaandSclerocarya birrea.
MACROPROPAGATION Grafting is the most efficient way to rapidly effect improvements in these fruit trees: Adansonia digitata (85–100% graft success), Mangifera indica (97%), Uapaca kirkiana (80%), Strychnos cocculoides (40–79%), Sclerocarya birrea (52–80%), Vangueria infausta, (100%) and Parinari curatellifolia (71%) (Mhango and Akinnifesi, 2001) compared favourably with exotics such as mango (90%) in the same trial. Air-layers were promising for Uapaca kirkiana (63%) but were not successful for Parinari and Strychnos species (Mhango and Akinnifesi, 2001). Interestingly, rooting hormone did not improve the rooting of Uapaca air-layers. Both grafting and air-layering set during November–December gave the best results. Top-wedge and whip methods were the most successful for grafting. The results showed that the factors determining grafting success are the skill of the person, the time of the year the scion is collected and the interval between scion collection and grafting (Akinnifesi et al., 2004a, 2006).
MICROPROPAGATION As the awareness of the potential of indigenous fruit trees increases, demand will inevitably increase, and, considering the massive tree- planting initiatives in the many countries in the miombo ecoregion, explosive demands for high-value trees, including indigenous fruit trees, cannot be accommodated in the short run. All known approaches of vegetative propagation are inherently slow and are attractive for only a few thousand farmers. For instance, to produce rootstocks for grafting Uapaca kirkiana, at least 1 year of seedling growth is required. In order to be able to deliver high-quality propagules of superior indigenous fruits in sufficient quantities for wider adoption, it is important to explore biotechnological methods, such as tissue culture. Currently, research on tissue culture has been conducted in the ICRAF programme in southern Africa on some priority indigenous fruit trees, especially Uapaca kirkiana,U. nitidaandPappea capensis(plum), with the objective of developing a reproducible clonal protocol for rapid regeneration and multiplication, and to determine early graft compatibility using in vitrotechniques (Mng’omba, 2007).
Incompatibility between stock and scion in the fruit orchard could constitute a major bottleneck to production. Simons (1987) estimated that half a million grafted peach trees had died in the southeast of the USA as a result of scion/stock incompatibility. Selection of scion and stock for compatibility is important for profitable orchard establishment and management.
Phenolic compounds and p-cumaric acids have been implicated in early graft incompatibility in Uapaca kirkiana. Plants normally release phenolics to heal wounds and as a defensive mechanism against pathogen attack, lignification and protein biding. Multiplication was much easier in P. capensis than in Uapaca kirkiana. Preconditioning grafted Uapaca kirkiana trees and decontaminating explants in 0.1% mercuric chloride for 8 min was found to be effective in achieving a high level of in vitroculture asepsis in Uapaca kirkiana.
In vitropropagation of Uapaca kirkianais feasible with sprouts excised from the preconditioned trees (Mng’omba, 2007). Shoot multiplication was effective using 1/2 strength Murashige and Skoog (MS) medium plus 2.5 mg/l of IBA (indole butyric acid) medium. Results will be improved by micropropagating Uapaca kirkianain such a way that stock plants are not stressed and ensuring
mycorrhiza is present. Repeated exposure of the difficult-to-root microcuttings ofPappea capensisto 0.5 mg/l IBA improved rooting from 42 to 62%, and the number of roots per plantlet averaged 3 (Mng’omba, 2007). Somatic embryos of P. capensis were successfully germinated into plants (65%), and 65% of plantlets survived after hardening off in a mist chamber.
Graft compatibility increased more with homografts than with heterografts between Uapaca kirkiana clones, species and provenances. Uapaca kirkiana and U. nitida had weak compatibility and they may exhibit delayed incompatibility. Although JatrophaandUapaca kirkianabelong to same family, there is outright incompatibility or early rejection. The technique seems promising for the detection of early incompatibility between close and distant related propagule sources. On the basis of a series of results, reproducible micropropagation protocols have been developed for the rapid multiplication of mature Uapaca kirkianaandP. capensis(Mng’ombaet al., 2007a, b).