«Biotechnology-Assisted Participatory Plant Breeding: Complement or Contradiction? PPB Monograph No. 3 Ann Mane Thro and Charlie Spillane 1 7 ...»
Efforls are now unde r way lo isolate the genes that prornote or impede recombination (Moore, 1998). Once this is done, it m ay be possible to develop 'gene cassettes', in which these genes are co n trolled by inducible promoters. These cassettes would be u sed to generate experimentallines for u se by farmers or formal breeders. Crossed into breeding populations, they would e ither enhance recombination or reduce it, to protect favora ble gen e combinations from rearran gement.
Such approaches m ay give farmer-led PPB grea ter control over recombination rates within their populations.
Apomixis is a natura lly occurring phenomenon whereby sorne plant species produce true seeds without fertilization and recombination. lt has been described in over 400 diffe rent plant species, only a few oC which are crops. The harnessing oC apomixis genetics for plant breeding may make it possible to develop true -breeding hybrids which retBi n their yield advantages Qver generations, making it unnecessary fo r farrners to buy new seed cach year. In contrast to gene-based enhancemen ts, the provision of an apomictic trait could permit new stra tegies based on the control of recombination in conventional breeding and selection. There have been s ignificant advances in recent Biotec1mology·Assisted PPB: Complemellt or Conlradiction?
years towards the goal of harnessing apomixis in a number of crop plants (Grossniklaus el al, 1998). Howevcr, a considerable amount of fu rther research wiU probably be necessary to develop the technology for wide spread u se in breeding (Jefferson, 1994; D. Wood, pers.
The development of apomictic varieties will require the use of inducible promotors that can be switched on and off (Jefferson, 1994).
Retaining th e ability to switch back to a sexual phase of recombination will be nccessary to permit the incorporation of new genes into the apomictic background. The genetic engineerin g of apomixis should make it possible to develop an inducible apomictic gene cassette, perhaps one that is inclependent of erop species.
Many commentators feel that the development of inducible apomixis could h a ve a profound effect on PPB (Jefferson, 1994; P. Richards, T. Hodgkin, D. Wood, pers. cornms.). Inducible apomixis·bascd plant breeding could be done on a modest scale at regional or locallevel, m ain ly by farmers' groups. Access to inducible apomixis through PPB would allow farmers to screen, select, and enhance germplasm much more efficiently and productively, with minírnal outside intervention.
One cornmentator s uggested that, until inducible apomixis is fully dcveloped, PPB projects involving clonally propa gated crops with a sexua l cycle could be used to provide insights into farmers' interest in the technology ancl the likclihood of wiclespread adoption (P. Richards, pers. co mm.J.
The authors of the 1998 Bellagio Apomixis Declaration expect easyto-use apomixis to permit:
New breeding procedures and strategies based on individual plants (existing methods are based on the synthesis of observations of en tire plant families). An exceptional individual plant could irnmediately become a variety Immediate genetic fixation of any desired plant individual, including those generated by wide crossing two difTerent species, which are often sterile at present. This could expand the a ccessibili ty and use of a wider diversity of genetic resources Fast and flexible plant breeding. Commentators have emphasiz,ed the advantages of apomixis for responding to changing micro· environments, cropping conditions, pathogen populations, and market opportunities. It is also felt that apomixis could promote more sustainable agro·ecosystem rnanagement (Jefferson, 1994) Development oC hybrid cultivars in almost every crop species.
Farmers sowing seed harvested from F) hybrids would experie nce minimal decrease in yield. The a uthors of the Bellagio Dec1aration and other cornmentators (e.g., A. Ebert, pers. cornm.) feel that this will greatly in crease resource-poor farmers' access to the yield benefits of heterosis, without changing traditional seed saving Biotechrwlogy as a Set of Tools for Fonnal and Informa l Plant Breeding
Sorne commentators \'I¡a rn of possible u nwanted side-effects. If farmers usin g landraces turn to apom ic tic h y brids that maintain thcir yield advantage down the ge neratio n s, they could become dependent on external sourccs to provide im provcd genotypes, jus t as they are when they adopt conventional improved verieties ¡Sma le, 1997; S. S mith, R.
Riley, D. Duviek, pers. cornrns.). There is a risk of 10ss of diversity and gene tic stagnation (D. Du vi ck, pers. comm.). However, traditional landraces \Vould not always be di splaced ; in many traditional fanning system s, modern varieties and landraces are maintained together (e.g., Bellón, 1991 ; Brush, 1995; Smale an d Heisey, 1995; Wood and Lcnne, 1997). A number of secd indus try cornmentators have expressed eon cern th at the widespread use of apornictic varieties might lead to redueed investment in public: or private-sector formal breeding, including activities to source n ew germplasm and create new diversity.
~ Ifthis were to occur, then genetic progress would pla teau, leading to stagnant yields, declining genetie diversity, and, over time) high er risk of crop failure caused by diseases and insects (S. Smith, R. Riley, pers.
The value of a pomixis tcchno logy in the long term would depend greatly on what farmers (and their formal -sector partners) did with it, which in turn would depend on whether they find it easier to create improved apomictic hybrids than to use existing methods to improve pure En es or ope n-pollinated varieties, and on the extent to which they continu e to access verieti es from outside thei r ferming system s (D.
Duvick, pers. comm. ).
Controllable male-sterllity systems
Male sterility is a u sefu l trait for promoting cross-pollination and recombination. It is also widely u sed in the production of F 1 hybrid seeds. Ho\Vever, ma le-sterile lines are not yet available for all crops.
And there may be problems associated with ¡ts u se in sorne crops, such as the lack of suitable restorer lines or the vulnerability lo disease of genetically uniform cytoplasm in the progeny.
Biotechnology·Assisted PPB: Complernent or Contradiction?
While nuclear male-sterile (NMS) mutants have been observed in many plant species (Kaul, 1988), the lack of homozygous breeding lines has precluded their use in hybrid seed production (Williams, 1995).
Regardless of whether th e NMS gene is dominant or recessive, at most 50% of the progeny of any cross \Vil! be male·sterile (Rao et al, 1998).
The problem then arises of how to eliminate th e 50% non-male s terile progeny. Simple and elegant genetic engineering technologics h ave been developed to Qvercome this problem, allowing 100% male-sterile progeny to be produced (Marian i et al, 1991). These technologies also incorporate the ferti hty restoration necessary for the production of F ¡ hybrids. A number of potentially useful transgenic technologies in which male sterili ty can be induced in any crop species have now been devcloped (e.g" Yistra et al, 1994 ; Mariani et al, 1990). Early tran sge nic technologies had the disadvantage of requiring t\Vo 1ines for fertility restoration. Transgenic one-tine male sterility technologies have now been developed, in which conditional rnale sterility can be induced by applying a non-Ioxic chemical (e.g., Kriele el al, 1996).
No male -steriHty technologics appropriate for the production of F¡ hybrid seeds solely by farmers have yet been adopted by them, even if they have been developed (M. Gale, pers. comm.). Howevcr, sin gle transgene-conditional male- or female-sterility technologies could be of use in sorne PPB applications, ir directional cross-pollination is desirable but is not easy to achieve with existing germplasm. Bidinger et al (1994) have demonstrated that hetcrosis can be used to improve pearl millet landraces with ou t any major loss in adaptation, by topcrossing locally adapted landraces with high-yielding male-sterile lines.
Coupled with emerging developments in fletd-level inducible promoters, advances in transgenic maJe· and female·sterility technologies suggest that simpler systems for the generation of hybrid seed could be developed. Current approa ches to F 1 hybrid secd production are bascd on the strip-plan ti ng oC female and maJe (pollen donor) inbred lines, which are then crossed. The fe male lines are emasculated by hand or chemically by spraying. The use of field-Ievel inducible promoters Jinked to transgenes which promote mate s terility (in the female inbred line) or female sterility (in the male inbred line) could allow breeders to plant a mLxture of fema1e- and male-sterile plants, induce ste rility, and harvest the entire plot for hybrid seed.
Such approaches could conceivably be used to facilitate heterosis breeding by farmers.
Tools for Enhancing Germplasm
Many [armers need germplasm contain ing variation that is unavailable to them in locally available germplasm, whether landraces or modern varieties (Wood and Lenne, 1997). Locally adapted varieties that are othenvise excellent may lack useful traits following genetic erosion
BiOlechnology as a Se, of Tools for Formal and Informal Plan! Breeding
caused by events such as war or natural disasters (so-ca1led 'bottlenecking events', see Boxes 10 and 11), as a result of genehc drift or simply because the traits are nol found in that crop. In environmenls subject to extreme fluctuation, such as drylands that are marginal for croppiqg, sorne landraces may ha vc a narrow genetic base due to past bottlenecking events (Spillane and Gepts, 2000). Suitable germplasm may even be lacking in the centres of diversity for a crop. For instance, local landraces of wheat in Ethiopia were shown to lack resistance to stem rust (Puccinia graminis) and leaf rust (P. recondita) and were consequently confined to highland areas where disease pressure was low (BeIay et al, 1995).
Introducing exotic germplasm can bring substantial benefits to farmers. However, most plant breeders, formal and informal, are reluctant to use exotic or unadapted material due lo its initially de tri mental efTects on their elite or adapted breeding material (Kannenberg and Falk, 1995; Duvick, 1996). Crosses with exotic material can result in the parallel introduction of inferior alleles and the disruption ofuseful co-adapted gene complexes (Duvick, 1984).
Adaptation can be negatively affected by such changes. Such disincentives to use exotic germplasm may be felt more slrongly by informal than by formal breeders, who do not have to eat or sell their early-generation progeny.
What starting materlals to choose?
Choosing the starting genetic material s is the crucial first step for any PPB prograrn (Witcombe et al, 1996; Witcombe and Virk, 2001). The choice will depend on the program's objectives. When the program wishes only to consider existing locally adapted landraces, the choice will be limited to these. But when important agronomic characteristics are lacking in Iocally available germplasm, the inclusion of exogenous material wilI be necessary. The extent to which farmers participate in making such decisions in existing programs, even the participatory ones, is often not clear.
Many PPB programs take as their point of departure an implicit assumption that the participatory approach will increase on-farm genetic diversity. However, this assumption may not be valid, because phenotypic diversity does not necessarily equate with gcnetic diversity (Wood and Lenné, 1997; Spillane and Gepts, 2000). Additionally, it has been suggested that widesprcad adoption by farmers of varieties [rom participatory projccts could as easily lead to the contiguous planting of genetically similar varieties over Iruge arcas as conventional plant breeding has done, with the concomitant risk of genetic erosion and in creased vulnerability to pests and diseases (Witcombe, 1999b).
la unched and al periodic intervaJs subsequently. Molecular genetic characterizatio n offarmers ' materia l at different stages ofthe program would h elp monitor the situation over time, enabling researchers and farmers to identify th e breeding activi ties most nceded. For example, in mass selection of self-pollinated crops it may be important to main tain a number of individuallines to ensure adequate genetic diversity in the population. Molecular ma rker analysis of rogued versus selected plants woutd in dicate the effects of selection on the genetic base over lime and the relative importance of different genes to farmers.
For both formal and informal breeders, the surest way of aehieving genetie gain is lo eross genotypes lhat are already known lo perform well under their target conditions. Consequently, a plant breeding program that needs to show early results may use only a modest amount of gcnetic variation in th e initial erossing design lo produce material tha t can be predicted lo perform well (D. Ouvick, pers. comm.). The need lo obtain good rcsults quickly is as common a constraint in PPB as in conventional breeding, particularly when rcsource -poor farmers with an urgent need to improve their livelihoods are involved. However, when a progra m has to meet a need that cannat be met using proven material, a greatcr range of genetie diversity is required, bringing in unadapted or even unrelated genotypcs or genes. In this case, most progeny of erosses will prove unusable in the short term o Better selection tools (and aften additional generations of recombination) are needed to extraet the rare favorable reeombinants of th ese crosses. Biateehnology can provide sueh tools (D. Duviek, pcrs.
comm.), making it more feasible for PPB to incorporate new or unrelated genetie variation.
Introducing exogenous variation
In many cases exotie germplasm must undergo 'pre-breeding' or 'trait enrkhment' before it can be usefut (Simmonds, 1993). Th.i s is a strong argu ment for sorne degree of outside support to farmers' breeding efforts (D. Ou vick, pers. comm.). including the use of biotcehnology tools where th ese are the key to either providing new variation or making effieient use of it.