Participatory Plant Breeding

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1		Salvatore Ceccarelli	+ 2275 word(s)	2275	2022-02-16 03:59:51	\|
2	format correction	Peter Tang	+ 1 word(s)	2276	2022-02-17 02:21:23	\|

This entry is adapted from the peer-reviewed paper 10.3390/d14020126

Biodiversity in general, and agrobiodiversity in particular are crucial for adaptation to climate change, for resilience and for human health as related to dietary diversity. Plant breeding is a cyclic process during which breeders generate diversity, most commonly by making crosses; select, within the diversity generated during a varying number of years, which depends on the crop, the methodology and the type of variety to be produced; and eventually obtain as a final product a new variety, which in several countries must be distinct, uniform and stable for its seed to be legally commercialized. Participatory plant breeding (PPB) has been promoted for its advantages to increase selection efficiency, variety adoption and farmers’ empowerment, and for being more socially equitable and gender responsive than conventional plant breeding.

biodiversity participation decentralization human health climate change genotype x environment interaction breeding efficiency

1. The Importance of Biodiversity

Biodiversity in general, and agrobiodiversity in particular, are important to humans for several reasons: they increase resilience to climate changes by increasing the stability of ecosystem processes ^[1], benefit health through dietary diversity ^[2], are important for food security ^[3] and reduce the risk of yield losses ^[4]. A literature review ^[5] reported that diversified farming systems support substantially greater biodiversity, soil quality, carbon sequestration, and water-holding capacity in surface soils, energy-use efficiency, and resistance and resilience to climate change than uniform systems. In other words, biodiversity is beneficial in terms of several ecosystem services. Ecologists have long suspected that the loss of biodiversity could increase the risk of pandemics such as COVID-19 and a recent article ^[6] provides evidence of the underlying causes of this link.

Plant breeding has followed the changes in global food systems shifting towards a reduced number of crops ^[7], with markets’ preference for uniformity and standardization ^[8], thus becoming one of the major drivers of climate change, land-use change and biodiversity loss ^[9]. In other words, it is the requirements of industrial cultivation, husbandry and processing (and to some extent consumer demand) that determine the breeding objectives rather than nutritional value, taste, improved stress resistance or adaptation to natural conditions ^[10]. Related to this scenario is the evidence that global agriculture has grown more vulnerable to climate change ^[11].

2. Decentralized-Participatory Plant Breeding

Plant breeding is a cyclic process during which breeders generate diversity, most commonly by making crosses; select, within the diversity generated during a varying number of years, which depends on the crop, the methodology and the type of variety to be produced; and eventually obtain as a final product a new variety, which in several countries must be distinct, uniform and stable for its seed to be legally commercialized.

The cyclic nature of a plant breeding program implies that each year new crosses are made and therefore, at any moment in time, the breeder handles several generations of breeding material, representing different stages in the breeding cycle. These are usually grown and evaluated in one or more research stations. Their management, including data recording and processing, as well as seed sample storage, are the responsibility of the breeder’s team.

The entire process also includes setting objectives and, most importantly, defining a product profile and a client profile or, in other words, defining which type of variety(ies) to breed for, and for which typology of clients. A product profile is the complex of traits that farmers would prefer relative to the varieties they are already growing ^[12].

Depending on the size of the breeding program, regional, national or international, and on the crop, there could be a number of product profiles and client profiles, and also a number of contrasting physical environments to address, for example, irrigated vs. rainfed, organic vs. industrial, etc.

Faced with this complexity, breeders have historically debated between breeding for wide adaptation and breeding for specific adaptation ^[13]. Adaptation by natural selection is the fundamental principle of the theory of evolution. An early step in the process of adaptation and diversification is the differential adaptation of populations of the same species ^[14], hence an increase in within-species diversity. Therefore, in evolutionary terms, adaptation seems to refer to a physical environment represented by a location. However, in breeding terminology, adaptation has been referred to both time and space, since the concept is related to genotype by environment interactions (GEI).

3. Organization of a Participatory Plant Breeding Program

A plant breeding program becomes participatory when clients, generally farmers, but with no limitations to other stakeholders, participate or, as many prefer, collaborate with scientists in all the key steps of the breeding program ^[15].

Key steps are, for example, setting the objectives of the program, choosing the parents and the type of germplasm (for example, local vs. exotic) and developing the product profile, as well as methodological aspects such as the number of entries, the size of the plots, the agronomic management of the trials and the organization of the farmers’ selection process. Farmers can also be involved in making crosses and it has been argued that this is crucial for a program to be truly participatory. However, the researchers believe that, after the choice of the parents and the design of the crosses based on the definition of the objectives, making the crosses is merely a technical operation ^[15] and therefore, although a useful skill for the farmers to acquire, is not necessarily a measure of the level or quality of the participation.

A participatory plant breeding (PPB) program must maintain the cyclic aspect of plant breeding, because this is a condition for the program to become progressively more accurate in targeting the physical environment, in addressing the clients’ needs and in enhancing farmers’ skills. The cyclic aspect also allows the necessary flexibility to adapt to changes in objectives, product profiles and client profiles.

As decentralized selection is one of the two pillars on which PPB is based (the other is participation), it is important to test the breeding material in the actual selection environments at the earliest possible stage. A breeding method that was found suitable, particularly in self-pollinated crops, is the bulk method ^[16]^[17]^[18], which, in its original formulation, consists of advancing separately as bulks, each of the n crosses performed at the beginning of each breeding cycle—where n can vary from a few hundred in the case of regional programs, to several thousand in the case of international programs—until a satisfactory level of homozygosity is reached within each bulk.

Trials conducted in farmers’ fields are expected to be less precise than in a research station; therefore, the choice of suitable experimental designs is crucial. One design, which has proved to be useful in increasing the precision of field trials, particularly in the early stages of a breeding program when there is a large number of genetic materials to be tested and relatively little seed available, is the partially replicated design in rows and columns allowing the control of field variability in two directions ^[19]. The relative genetic gains for the p-rep designs are significantly higher than those obtained with an unreplicated design with replicated checks, such as the augmented design. The precision can be further increased by generating experimental designs incorporating variable replications and correlated errors with an R-program package called DiGGer ^[20].

Usually, at least three stages of selection between bulks are conducted. In the second and third stages, trials are replicated among different farmers within the same villages to capture the effect of different agronomic managements. At the end of the three stages, the bulks are at the F₆ generation with a high frequency of homozygotes.

Other methods are equally suitable and a number of them have been recently reviewed ^[21], and methods suitable for cross-pollinated and vegetatively propagated crops have been also described in detail elsewhere ^[17]. Whatever methodology is used, it should allow bringing into farmers’ fields as much genetic diversity as possible, from which farmers can choose. The only exception is represented by traits with high heritability, namely with low GEI, which could be conveniently selected within a research station, provided the breeder knows their most desirable expression by farmers. Typical examples of such traits are seed colour, phenology, quality traits and disease resistance.

Another essential feature of a PPB program is that it should maintain the cyclic aspect of a plant breeding program, without which it is no longer a breeding program but simply an experiment on plant breeding.

PPB generates diversity through two mechanisms, which act simultaneously. The first is decentralized selection, which, because of differences in soil characteristics, climatic conditions, agronomic practices, farmers’ preferences, gender differences and social conditions among locations, inevitably determines the selection of different genetic material by different farmers in different locations, thus increasing agrobiodiversity in space ^[22]. The second is the continuous flow of new genetic materials, which favours a rapid turnover of the varieties, thus increasing agrobiodiversity in time. This has been well documented in Syria ^[23].

On one hand, the agrobiodiversity in time and space generated by PPB is an ecological barrier to the spreading of pests, but on the other, it makes measuring the impact of a PPB program a challenging task. This is because the impact of a plant breeding program is usually measured as the area planted with the varieties generated by the program, or by the number of farmers growing them, or by the percent share of the seed market in the case of private breeding companies. In the case of a PPB program, and as a consequence of the rapid identification and turnover of new varieties, which are selected for specific adaptation, it is unlikely that a single variety expands to a large area before being replaced by another variety.

The cyclic nature of a PPB program caused a remarkable increase over seasons in farmers’ skills in a number of countries where PPB was practised; farmers moved from an almost passive participation to various degrees of active participation ranging from suggesting new selection criteria to indicating desirable crosses ^[24].

One form of participation in plant breeding, which has been described as participatory variety selection (PVS), is when farmers collaborate in the final stages of an otherwise conventional breeding program (namely centralized) to choose among a restricted number, normally around 10–20, of nearly finished breeding lines. Although the method has the disadvantage of considerably limiting the choices of the farmers, it is easier to organize because of the small number of lines and has the merit of being a possible entry point, which could eventually lead to implementing a PPB program. However, the fact that PVS is easier to organize has implied that PVS has been often organized outside breeding programs by organizations not engaged in plant breeding. This usually leads to lack of continuity and, consequently, to a limited benefit to farmers.

4. The Scientific Basis of PPB

A PPB program has the same scientific basis of a conventional breeding program. It differs because the evaluation and the selection of the breeding material are carried out in farmers’ fields in collaboration with farmers and other stakeholders to be as inclusive as possible. The locations are chosen to represent the target population of environments and the actual sites where the breeding material is evaluated and selected are chosen, within each location, together with the farmers.

As indicated earlier, the PPB program can achieve a high level of precision by adopting the most advanced experimental designs and statistical analysis. Furthermore, each of the three stages of selection, even if conducted with bulks, is in fact a Multi Environment Trial (MET), namely, field trials conducted in a number of locations and years. Therefore, at an early stage of the PPB program it is possible to obtain information on the bulk responses to the locations and to study the nature of GEI.

The analysis of MET is particularly useful to continuously fine tune the organization of a PPB program. For example, the use of GGE biplot ^[25] allows the grouping of locations on the basis of the repeatability of GL interactions. In other words, if n locations, among those used routinely in the breeding program, consistently rank the breeding materials in the same way year after year, n − 1 of those are redundant and can be discontinued as testing sites, leaving only one. The resources saved can then be used by expanding the program to new location(s). This applies also to MET conducted in conventional breeding programs, but in the case of a PPB program, the ranking of breeding material at different locations, for example for grain yield, must be validated with the ranking of farmers preferences before deciding on redundancy.

Therefore, a PPB properly organized is not only capable of controlling the obscuring effects of uncontrollable within-field, site-to-site, and year-to-year heterogeneity, which has been considered as a limit to PPB’s efficiency ^[26], but also of capturing, to some extent, genotype x management (GM) interactions, as different genetic materials under selections may respond differently to soil fertility, fertilizers, or to other agronomic management techniques. This is achieved by replicating trials at the same selection stage in fields differing in agronomic management within the same village. For example, if, in a village, farmers differ in their fertilizer use depending on their wealth, trials can be planted on the property of different farmers to measure the differences among the breeding material for their response to fertilizer without the confounding effect of climatic conditions.

In fact, the demonstration that it was possible to organize a plant breeding program in partnership with farmers with the same scientific structure used in a research station was the strategy deliberately used to facilitate the institutionalization of PPB.

Eventually, PPB can take full advantage of some of the molecular techniques available to plant breeding such as marker-assisted selection (MAS) and genomic selection, which allow for a greater number of traits to be included in the selection process ^[27].

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