«Biotechnology-Assisted Participatory Plant Breeding: Complement or Contradiction? PPB Monograph No. 3 Ann Mane Thro and Charlie Spillane 1 7 ...»
The biological bases oC toxidty-precursor compounds of cyanidc called cyanogens-are found in al) cassava. Toxic cyanide is releascd when these cyanogens come into contact with an enzyme released by damaged cell walls when cassava is chewed or chopped. In cassava~dependent cultures, processing to remove cyanogens is typically women's principal activity. Processing is lengiliy and labor-intensive, but if toxic cassava is eaten after rushed or inadequate processing, paralysis or death can result, especially if the consumer already has poor general nutrition.
In sorne oCthe world's most disadvantaged areas, particularly in sub-Saharan Africa, farmers deliberately grow toxic cassava as their basic staple. They ~xplain their choice by describing this crop as more drought-tolerant, higher-yielding.
superior in processing quality C traditional Coods, and disease- and insector resistant. Moreover, in these areas, the higher the toxicity oC the varieties, the lower th ~ risk of theCt oC plants Crom the fields oC vulnerable female -h eaded households. Processing bulky cassava roots is a difficult operation to hide in a small cornmunity, and the perishability and bulkiness oC th~ roots mak~s it djfficult to carry away stolen roots to process them elsewhere. In a s urvival economy, where trede is n ot an option due to remoteness and civil unrest, these protective advantages may outweigh the accompanying disadvantages.
Appropriate biotechnology interventions may exist thal could benefit wo m ~n coping with s uch sit u a tions. But what they would be is not imrnediately obvious to outsiders. Possible objectives are to alleviate the toxicity risk, reduce !he labor burden on women caused by proc~ssing, and promot~ marketing, while at the same time supporting loca1 food security strategj~s.
With support from the Swedish International Development Agency (SIDA), researchers from !he Swedish Agricultural University (SLU) oi Uppsala, and the Ministries of Agriculture of Malawi and Tanzania are using sociological and molecular data to elucidate womcn farroers' objectives and processes in !he use of toxic cassava. The practiee of growing toxie cassava has apparently spread into Malawi and Tanzania from \Vest and Central Africa, although 'sweet' (lowcyanogen) cassava is also grown by all farmcrs. The two countries are now among those most affected by the paradox between cyanogen toxicity and the essentiaJ role oC cyanogens in rood security. Cassava varieties are commonly renamed as they pass from fanner to farmer, so researchers working without molecular markers have been unable to assess and validate oral histories oC the spread and value oC toxic cassava. The better understanding oCfanners' objectives achieved by the study will, it is hoped, form the basis for appropriate support to Carroers' diversity management strategies. probably through farmer~ led PPB.
SOURCES: H. Rosling (pers. comm. ); Chiwona-Karltun et al (1997, 2000);
Thm et al (1994).
Biotechnology-Assisled PPB: Complement o,. Contradiction?
Box6 Molecular markers tbrow light Carmers' selectioDs of OD pear' miUet Jandraces in West Africa A molecular marker study of fanners' landraces oC pearl millet in West Africa revealed that the crop management practices oC neighboring fanne rs led to the selection oC different genotypes o( the same named landrace, and similar genotypes of different-named landraces.
Eight samples were colJected of each o( four landraces of pearl millet. The four landraces were identified by name by the local farmers and were visually distinct.
Samples were (rom the fields oC (our different fann ers in two villages in Chana; no field was less than 200 meters away from any other.
Molecular analysis showed thal, while lhe phenotypic characteristics which identified a landrace were maintained across farmers, the genetic profiles oC two different landraces grown by the same farmer were more similar than those oC the same landrace gTown by two different farmers. Farme rs' conscious or subconscious selection practices were shaping genetic diversity at the fann leveL While holdmg a few major genes constant, they were selecting for specific phenotypic traits that indicated adaptation to their own field s or micro-sites.
This sludy has important implications ror the maintenance of oo -farm genetic diversity and also for oo-farro crop improvement. It suggests that, in addition to the names of landraces, the names oC farmers, farmers' evaluation oí the variety, dates of sampling, and eco-geographic details are equaJly important fo r the purposes of gennplasm identification and genebank records. It also suggests that diversity, at ¡east in these areas of Chana, is better represented by samples from each fanner lhan by samples oC each 'variety'. In the case oC a disaster, if materials had to be re-supplied to an area, researchers would know that the name of the variety a farmer grew before míght not be enough infonnation to get 1 0caJly ada pted seed back into that farmer 's field, since a variety with a different name could conceivably be claser lo the original genotype.
SOURCE: Busso et al (1998).
subsequen t managerncnt of the population in many different environments under natura l or weak selectlon pressures (Goldringer et al, 2000). Molecular marker analyses allowed adaptive changes in pathogen resistance and multilocus diversity to be tracked across populations and over time. In addition, outcrossing rates were determined in order to assess the optimallevels of geneflow that might be promoted between difTerent sub-popula tions. Although no individual farmer seleetion pressures were applied lo the populations, the program's approach and findings are similar to those of the study on local-Ievel gen eflow in ma ize condueted by Louette e l al (1997).
Goldringer el al (2000) suggest that their evolutionary breeding model may be su itablc for PPB where uniformity of the material s produced is not requ ired.
Biorechnology as a Set 01 Tools for Formal and Infonnal Plant Breeding The choice of a cost-effective molecular marker technique depend s on program objeetives (Karp et al, 1997). Sorne teehniques (e.g., isozyrnes, RAPDs) are simpler to use, while others are more difficult bU l a lso more accu rate or s en s itive (e.g., AFLPs, microsatelli tes, SCARs, etc.). Where the re is sufficient polymorphism, isozyme an a lysis may yield enough information to be the technology of choice. For instan ce, 12 isozyme sys tems allowed the differentiation of 95% of culti valed clones of Hevea (Leeonte et a l, 1994). A 'ponable la boratory' based on these enzymes h as been developed, a Uowing nursery finge rprinting of high -yielding clones u sed in industrial plantations. For other species or objectives, other DNA markers may be required to achieve s ufficient reso lving power. Most PPB programs would need the assistance of an advanced biotechnology la boratory to conduct DNA analysis of germplasm. Ma n y su eh laboratories may be interes ted in the analysis of seleetion by farmers (e.g., Busso et al, 1998).
Th e advent of DNA chip, micro-array, and nanomachine technology is likely to mcrease the throughput of molecular m a rker and DNA analyses in the coming years, by increasing the speed and lowcring the cost of processing large numbers of samples (e.g., Walter et al, 2002;
Gibson, 2000; Chee et al, 1996). This eould open the way to simpler evaluation of gene frequencies in a single mixture of DNA representing a popula tion, greatly facilitating the spatial and temporal monitoring of the m olecular eve nts underlying either dynamic conservation or PPB eITorts (Seeond et al, 1997). It should be possible to bu1k rnany plants in samples [or analysis and so to obtain information on many loei in one or a few high-throughput experiments. However, such teehnologies are s tiU well beyond the reach of most biotechnology researchers, many of whom are competing to conduct the initial experiments on the firstgeneratíon DNA chips currentIy under development.
Understanding the dynamics of farmer-directed genetic change, especially among r esource-poor farmcrs, may not rank high compared to other research objectives. To the authors ' knowledge, no farmers' groups h ave spontaneously chosen the understanding of genetie variatíon and gene-flow processes in their material as a priority research objective. Paradoxically, therefore, such research- although condueted at the field level- m ay be as 'upstream' as many laboratory projects, in the s en se that it is not pereeived as providing short-run benefits by its end users. However, farmers have a keen sense of urgency cegarding varietal improvcment and have in many cases requested outside intervention in support of this. D. Duvick (pers.
comm.) notes that studies of population dyna mics of farmers' varieties can become over-academic becau se of the fascinating data they generate for specialists. It is at this point that they run the grcatest danger of losing practical relevance foc farme rs. He suggests that a11 such studies should be guided by the ques tion, Are molecular markerassisted methods the most efficient way of helping farmcrs get the germplasm tbey want?
Biotechnology-Assisted PPB: Complemen.t or Contradiction?
Tools for Selecting Germplasm Relating fanners' criteria to researchers' tools Farmers may use vcry different selection critena from forma l breeders and biotechnologists to evaluate germplasm. The fact that sorne modern crop varieties are not adopted is a clear indication of the gap. Indeed, the very concept of 'adoption' implies that forma l breeders and biotechnologists need to improve th eir understa nding of what farmers mean by a 'preferred variety' (M. Fregene, pe rs. cornm.). If different social groups of farmers (Le., disaggregated by sex, incorne, ethnicity.
age, etc.) h a ve different preferences, th en breeders need to understan c1 the se as weU (K. Schmidt, P. Eyzagu irre. pe rs. comms.).
Sorne say that farmers are biased towards selecting tra its that a re easy to distinguish visually in a parental or progeny plant (Wood a nd Lenné, 1997). Such selection h as, for example, led to extreme phenotypic diversity in the color of bean seeds and maize kernels.
These 'peacock' trai ts may be either qualitative or quanti tative.
Conversely, it is difficult for farmers to select for traits th at are not easy to see, such a s resistance to sheath blight in rice. Farmers are probably' aware of desirable quan titative traits (e.g., high yield) which are diffic ult to control and rctain between generations. However, lhey are unlikely to be interested in subjecting their crops to majo r losses in order to select for phenotypic traits whose evalu a tion requires destruc uve testing, such as pest and disease resistan ce.
The extent to which farmers can visualize or 'perceive' different traits will have a bearing on their success in selecting for individual traits. While it may seem obvious that farmers interpret the look and performa n ce of a plant as de sira ble or undesirable for certain traits, it is not obvious how they do this and how they use thi s information in their selection efforts. Very Httle is known about how lhe phenotypic descri ptors that farmers u se for selection correlate with those u sed by plant breeders or genebank curators. For instance, there is little information on how farrners perceive the phenotypic trait markers used in conve ntional ge netic linkage rnaps (e.g., Kinoshita, 1995) or on how they characterize germplasm accessions. Detai1ed farmer participatory research work has, however, been done on the definition of Brazilia n farmers' selection criteria in cassava (e. Iglesias, L.A. Hernández, W.
FUkuda, pers. comms.; Iglesias and Hernández-Romero, 1997). The objectives of identifying farmers' descriptors and definitions were to enable farmers and formal breeders to 's peak the same language' and, when possible, to 'translate ' fa rmers' descriptors so that a given descriptor (or a highly correlated trait) can be meas ured and quantified in order to study inheritance a nd design effective breeding strategies.
Integra ted mul tid isciplinary approaches involving erop geneticists, a nthropologists, agronomi s ts, and socio-economists are likely to be valuable in gaining a be tter understand ing of farmers' seleetion criteria.
Biotechllology as Set ofTools lor Formal Qnd Informal Plant 8reeding Q Without new selection tools a nd techniques for farmers, interaction between farmers and researchers to improve the efficiency of lrait selection will, then, tend to be limited to lraits that farmers can easily \risualize' or 'perceive' through non-destructive evalu atíon, su eh as heading date, plant height, seed weight, and so on. But ir simple diagnostic tools that ¡ncrease throughput can be developed for use by farmers as well as formal-sector breeders, this \Voutd \Vide n the variety of traits that could be evaluated. For ínstance, where farmers have to meet exacting fODd safety standard s, diagnostic lools for detecting undesirable compounds, su eh as aflatoxin in groundnu t, could be userul. These and other tools can help re source-poor farmers create a surplus of uniform, hígh-quality produce, enabling them to enter new markets (Box 7).
Similarly, the use of MAS is likely to be most powerful when it is integrated with social and agronomic studies of the phenotypic critería used by farmers. The advent of molecular and linkage maps may allow collaborative participatory selection efforts th a t compleme nt or integrate farmers' 'visible' critería with the invisible ones that are also important for many traits.
Quality standards tend lO be highly specific, requiring measurement (e.g., minimum levels of a given,,¡tamin, Creedom Crom insect c1amage, a specific dry matter, starch. or protein content). Sorne biotechnologies can help Carmers meet lhese standards. For instanee, diagnostic kits can allow farmers to test C levels OI" oC desirable and undesirable compounds, sllch as starch or aflatoxins. Several modern biotechnologies can help Canners or farmers' groups involved in seed multiplication and dissemination improve the quality oC their seed (Cromwcll et al, 1993). The application oC simple diagnostic tests for seed-transmitted diseases can a110w Carmers' groups 10 seU disease-free seed al a premium. Using tissue culture, farroers can generate large amounts of disease-free planting materials, especial ly in vegetatively propagated craps.