On the number and size of cross combinations in a breeding programme of self-fertilizing crops

Summary The number and size of crosses in a breeding programme were discussed to the conclusion that the number of crosses rather than the size of a cross should be increased with a given total population size. The advantage due to this manipulation, however, is relatively small when such genotypes as improved for many (twenty-five or more) loci are to be obtained, or when a high proportion (around 0.1 or more) of available combinations have the potentiality of releasing the objective genotypes. In such a case, the ease with which crossing and cultivation can be carried out becomes the decisive factor.     

Number and size of cross progenies from a constant total number of plants manageable in a breeding program

Summary This short paper discusses the optimum number of crosses and size of progenies for a given total population size to minimize the risk of not finding any favourable genotype in the breeding material. It is shown that the number of crosses must generally be as large as possible. In special situations it may be advantageous to include also less promising crosses instead of enlarging the progenies of more promising crosses.     

Breeding crops for reduced-tillage management in the intensive, rice–wheat systems of South Asia

The importance of reduced tillage in sustainable agriculture is well recognized. Reduced-tillage practices (which may or may not involve retention of crop residues) and their effects differ from those of conventional tillage in several ways: soil physical properties; shifts in host–weed competition; soil moisture availability (especially when sowing deeply or under stubble); and the emergence of pathogen populations that survive on crop residues. There may be a need for genotypes suited to special forms of mechanization (e.g. direct seeding into residues) and to agronomic conditions such as allelopathy, as well as specific issues relating to problem soils. This article examines issues and breeding targets for researchers who seek to improve crops for reduced-tillage systems. Most of the examples used pertain to wheat, but we also refer to other crops. Our primary claim is that new breeding initiatives are needed to introgress favourable traits into wheat and other crops in areas where reduced or zero-tillage is being adopted. Key traits include faster emergence, faster decomposition, and the ability to germinate when deep seeded (so that crops compete with weeds and use available moisture more efficiently). Enhancement of resistance to new pathogens and insect pests surviving on crop residues must also be given attention. In addition to focusing on new traits, breeders need to assess germplasm and breeding populations under reduced tillage. Farmer participatory approaches can also enhance the effectiveness of cultivar development and selection in environments where farmers’ links with technology providers are weak. Finally, modern breeding tools may also play a substantial role in future efforts to develop adapted crop genotypes for reduced tillage.     

Frequencies of reciprocally compatible single,three-way and double crosses as determined by the number of S-homozygotes,the number of lines per S-homozygote and the types of inheritance of incompatibility in cruciferous crops

Summary The low number of useful S-alleles in several species and varieties of cruciferous crops (Brassica, Raphanus) may hamper the set-up of a practical programme of hybrid breeding based on sporophytic one-locus incompatibility. Few S-alleles may considerably reduce the number of reciprocally compatible crosses and therefore restrict the possibilities of selection for combining ability and for suitable S-alleles. This problem is discussed in this article. General formulae are derived for calculating the maximum number of reciprocally compatible cross combinations for three types of crosses (singles, three-ways and doubles), four types of S-allele interactions (I, II, III, IV: table 1), different numbers of S-homozygotes (1,2.....n) and different numbers of genotypes per S-homozygote (equal: x; unequal: x₁...._k....x_n).The following conclusions could be drawn. Selecting several lines per S-homozygote can to a certain degree compensate for a low number of S-alleles. An equal number of lines per S-homozygote in general is most favourable. If unequal numbers of lines per homozygote are obtained, then—with a constant total number of lines—the number of reciprocally compatible crosses is dependent on the variation of the number of lines and for certain inheritance types also on the distribution of the lines in relation to the nature of the S-alleles (recessivity-dominance). It is argued that a genetically very broadly based starting material is indispensable for a successful hybrid breeding programme.     

The application of the diallel cross to outbreeding crop species

Summary The use of the diallel cross as an aid in the breeding of outbreeding crop species is considered from two aspects. Firstly, for the strategic survey of populations as the initial material of a breeding programme and secondly for the tactical assessment of the genetic relationships of elite selected genotypes. Comparisons with alternative procedures indicate that the diallel crossing scheme is generally only applicable where dominance and gene interaction are likely to be of importance.Text of a paper presented to the Study Circle for Plant Breeding, Wageningen 1977.     

Utilization of genetic resources for the introduction and adaptation of exotic vegetable crops: The case of pepino (Solanum muricatum)

Summary Pepino (Solanum muricatum), a vegetatively propagated plant from the Andean region used for its edible fruits, has been identified as a potential crop for greenhouse cultivation in Mediterranean regions. However, attempts for introducing it have been unsuccessful, either because of the low yield, poor fruit quality, or both. Screening of germplasm under Mediterranean conditions showed that sources of variation for high yield existed in the cultivated genepool and that wild species S. caripense and S. tabanoense could contribute to a considerable improvement of soluble solids content (SSC) of pepino. Progenies obtained after crossing genetically distant (AFLP-based) parental clones were heterotic for yield, and allowed the selection of clones with an improved combination of yield and SSC. As a result of this intraspecific programme, two new improved cultivars (‘Turia’ and ‘Valencia’), which outperform the rest of cultivars available, have been selected for cultivation in Mediterranean conditions. The interspecific programme involved the selection of clones with high yield and SSC in backcross generations. The results show that introgression of genes from the wild species can contribute to improve the SSC of pepino. The backcross programme is in an advanced stage and new cultivars derived from the interspecific crosses are expected to be released in a near future. New prospects for the future in pepino breeding include the genetic transformation, the establishment of a genetic map, and the use of the genomic information from related Solanaceae important crops. All the information obtained is illustrative for the breeding for adaptation of vegetatively propagated crops.     

Participatory plant breeding in maize: A case study from Gujarat, India

This paper describes how plant breeders and farmers worked together to produce improved varieties of maize for the low-resource farmers of the Panchmahals district of Gujarat, India. Initially, farmers tested a range of maize varieties in a participatory varietal selection (PVS) programme. However, none of these proved to be very popular with farmers, although farmers who had more fertile fields adopted the variety Shweta from Uttar Pradesh. Hence, in 1994 a participatory plant breeding (PPB)programme was begun to generate new, more appropriate varieties. Yellow- and white-endospermed maize varieties were crossed that had been either adopted to some extent following PVS or had attributes, such as very early maturity,that farmers had said were desirable. In subsequent generations, the population was improved by mass selection for traits identified by farmers. In some generations,farmers did this in populations which were grown by breeders on land rented from a farmer. Soil fertility management was lower than that normally used on the research-station. The breeding programme produced several varieties that have performed well in research-station and on-farm trials. One of them, GDRM-187, has been officially released as GM-6 for cultivation in hill areas of Gujarat state,India. It yielded 18% more than the local control in research-station trials, while being seven days earlier to silk. In farmers' fields, where average yields were lower, the yield advantage was 28% and farmers perceived GDRM-187 to have better grain quality than local landraces. PPB produced a variety that was earlier to mature than any of those produced by conventional maize breeding, and took fewer years to do so. The returns from PPB,compared to conventional breeding, are higher because it is cheaper and benefits to farmers are realised earlier. This revised version was published online in August 2006 with corrections to the Cover Date.     

What is the SMARTest way to breed plants and increase agrobiodiversity?

The evaluation and use of the vast diversity contained in plant genetic resources (PGR) is a main challenge for today’s plant breeding. The use of molecular markers has hugely increased the knowledge about genetic diversity and great hopes are raised about the potential of marker assisted selection [MAS; sometimes also termed SMART breeding (Selection with Markers and Advanced Reproductive Technologies)] to help increasing the use of PGR and maintaining crop genetic diversity. Another approach growing attention has been paid to over the past two decades and which also aims to increase variation in crops is evolutionary and participatory breeding (EPB). In this paper we discuss both the potential of marker-assisted breeding strategies and the potential of EPB breeding to contribute to the maintenance, increase and development of agrobiodiversity. The potentials of molecular markers in the evaluation and use of PGR and their documented contribution to agrobiodiversity are reviewed and results from guided interviews with scientists and breeders are given. Despite tremendous research efforts involving molecular markers, it is still difficult to obtain a clear picture how molecular markers contribute to the use of PGR in plant breeding. Minor and major crops do not benefit to the same degree from recent developments in marker technology. It therefore depends at least in part on economic considerations whether SMART breeding or EPB strategies or both are implemented in the breeding process of a crop. A general decision in favor or against MAS or EPB when breeding for diversity would not yield optimum results.     

Combining ability and heterosis under pest epidemics in a broad-based global wheat-breeding population

Wheat breeders rarely apply population improvement schemes or select parental sources according to combining ability and heterotic patterns. They rely on pedigree selection methods for breeding new cultivars. This experiment was undertaken to assess the advantages of using diallel crosses to define combining ability and understand heterosis in a broad‐based wheat‐breeding population across different environments affected by yellow rust. Sixty‐four genotypes derived from a full diallel mating scheme were assessed for grain yield in two contrasting growing seasons at two locations for two consecutive years. Parental genotypes showed significant combining ability for grain yield that was affected by yellow rust and genotype‐by‐environment (GE) interactions, both of which affected heterosis for grain yield. Significant GE interactions suggested that decentralized selection for specific environments could maximize the use of this wheat germplasm. Cultivar effects and specific heterosis were the most important factors influencing grain yield. Some crosses capitalized on additive genetic variation for grain yield. This research shows the power of available quantitative breeding tools to help breeders choose parental sources in a population improvement programme.     

How to maintain improved cultivars

Improved cultivars loose their identity and healthiness unless maintained properly. Contaminating and degrading forces, such as outcrossing, volunteer plants, mixing, natural selection, mutation and seed-borne diseases, are at the root of this. Maintenance selection can prevent this deterioration. How it is carried out depends on the reproduction system of the crop. Crops are therefore classified into four categories; typical cross-pollinating crops, self-pollinating crops with a substantial amount of outcrossing, typical self-pollinating crops with little outcrossing, and the vegetatively reproduced crops. Generally some of the “breeder seed” is used to plant a small plot with spaced plants. A fair number of healthy plants of the cultivar type is selected and the seed is harvested per plant. The progenies of the selected plants are grown in small plots. Non-uniform or deviating plots and plots with a seed-borne disease are removed. The seed of the progenies that are healthy, uniform and similar (and so of the cultivar type) are harvested per progeny to be tested next season on larger plots. The same selection is applied and only the seed of the progenies that are healthy, uniform and similar are harvested together to produce the “breeder seed”. The details of this maintenance selection vary with the reproduction system, the multiplication rate of the crop and the possibilities available to the breeder. Seven crops, potato, common bean, barley, wheat, faba bean, quinoa and maize are discussed here as they represent the different reproduction systems and multiplication rates, while being important Andean food crops.     

The relationship between the regeneration system and genetic variability in the cucumber (Cucumis sativus L.)

Durum wheat has been subjected to intense breeding in Italy due to its local economic importance. Four groups of Italian cultivars representative of different breeding eras were compared in northern Syria for yield potential and morphophysiological features at a moderately favourable site, and drought tolerance at a stressful site. Group 1 included indigenous landraces; Group 2 comprised genotypes selected from exotic landraces (released in 1920's–1930's); Group 3 included genotypes selected from crosses or mutagenesis involving Group 2 materials (1950's–1960's); Group 4 comprised genotypes selected from crosses between CIMMYT and Group 2 materials (from 1970's). Under moderately favourable conditions, a yield increase of 1.03 t ha–¹ was observed from Group 1 through Group 4, corresponding to a genetic gain of about 0.017 t ha–¹ per year. Such increase was only partly accounted for by a parallel increase in spike fertility and seed weight. Plant stature decreased dramatically from Group 1 to Group 4; a remarkable reduction of height was already attained in Group 3, before the introduction of dwarfing genes from bread wheat. Earliness of heading and grain filling duration increased consistently across breeding phases, the length of crop cycle remaining almost unaltered. No significant improvement of drought tolerance resulted from the breeding activity, suggesting the need to put more emphasis on selection for real stress tolerance rather than escape. Overall variation for morphophysiological traits, assessed by a principal components analysis, highlighted the great diversity among the Group 1 cultivars, while variation within Groups 3 and 4 was lower. Indigenous landraces, little used in the breeding history, as well as novel, unexploited exotic germplasm sources could contribute to broaden the crop genetic base in the region.     

Leaf shape × sowing density interaction affects chickpea grain yield

Modifying plant architecture is considered a promising breeding option to enhance crop productivity. Modern chickpea (Cicer arietinum L.) cultivars with either compound (wild‐type) or simple leaf shapes are commercially grown but the relationships between leaf shape and yield are not well understood. In this study, a random sample of ‘Kabuli’ type progeny lines of both leaf types, derived from two crosses between modern American simple leaf cultivars and early‐flowering wild‐type breeding lines, were planted at different sowing densities. Leaf area development and final grain yield in genotypes of the two leaf types responded differently to changes in sowing densities. Compound leaf lines attained higher leaf area indices and higher grain yields at both low and high sowing densities. Yield responses of the simple leaf lines to increasing sowing density were significantly higher compared to compound leaf genotypes in two of three field experiments. The prospects for utilizing the simple leaf trait as a breeding target for short‐season growing areas are discussed.     

Genetic interpretations of the effects of bulk breeding on four populations of beans (Phaseolus vulgaris L.)

Summary The effects of bulk breeding on yield and its components in two high-yeilding and two low-yielding bean populations when grown at crop densities are interpreted in terms of gene action and selection. There was considerable additive variation in the crosses, dominance effects were variable but tended to be negative. The results were compared with results obtained at low density by another worker with the same populations, and this showed that interpretations of gene action in hybrid vigour depend on the test environment used. The interpretations were also compared with plant breeding interpretations made on the same material (Hamblin, 1977), and it was found that a single pattern of response to natural selection, in plant breeding terms, may be explained by more than one genetic interpretation and, conversely, a single genetic interpretation may result in different patterns of response in plant breeding terms. The value of composite crosses in plant breeding are discussed and it is concluded that there is a greater chance of producing improved lines from a large number of simple crosses than from a smaller number of composite crosses.     

Anther-Culture ability in Secale cereale L.

Success in anther culture of rye (Secale cereale L.) has mainly been restricted to genotypes whose genome is partly determined by S. vavilovii Grossh. The aim of this study was to identify true S. cereale genotypes with anther-culture ability. The plant material used included the semi-wild standard genotype SC35, six single crosses of the ‘Petkus’ and 10 of the ‘Carsten’ gene pool, plus a double cross between SC35 and one of the ‘Carsten’ single crosses. Three anther-culture experiments were performed. Several single crosses within the ‘Carsten’ gene pool produced green plants. The ‘Petkus’ single crosses tested did not show anther-culture ability. As expected, the induction and regeneration rates of SC35 were higher than those of all other genotypes tested. The performance of the double cross was intermediate between its parents. The results demonstrate for the first time that anther culture in rye is a valuable tool in plant breeding. The data also show the possibility of transferring anther-culture ability from high- to low-responding genotypes.     

Molecular evidence of outcrossing rate variability in <Emphasis Type="Italic">Brassica napus</Emphasis>

A leafy crop of Brassica napus L. called nabicol has been grown by farmers in northwestern Spain for many years, being an important horticultural product during the winter season. A collection of landraces of a Brassica napus leafy crop called nabicol is kept at ‘Misión Biológica de Galicia’ (CSIC-Spain) which can be used to search for desirable characteristics or to produce new commercial varieties to release in the market. The assessment of the mating system of nabicol landraces is particularly important to carry on adequate breeding and genetic conservation programs. The objective of this work is to estimate the outcrossing rate in nabicol under controlled pollinator conditions using SSRs. Pairs of flowering plants taken from nabicol landrace MBG-BRS0039 were placed in separated isolation cages and bumble bees (Bombus sp.) were released for facilitating the crosses between plants. A seed sample from each plant in the cross was analyzed by SSRs that were polymorphic in the parental population. We found that the crop is partially allogamous and that there is genetic variation for the outcrossing rate among individuals. Several consequences for the maintenance and genetic improvement of the crop are discussed. The existence of genetic variability for this trait is a valuable tool that will allow us to study the genetic mechanisms underlying the mating system of this crop.     

Consequences of a decentralized participatory barley breeding programme on changes in SSR allele frequency and diversity in one cycle of selection

Changes in allele type, allele frequency and genetic diversity because of selection by individual farmers and breeders were assessed using simple sequence repeats (SSRs) during one cycle of selection in a decentralized participatory barley breeding programme. Selection by both breeders and farmers resulted in the loss of a number of alleles in the majority of the locations, with more alleles lost in the heterogeneous breeding materials than in the fixed genotypes, indicating selection against undesirable traits uncovered in the heterogeneous breeding materials that are presumably linked to SSR alleles. After selection, significant allelic frequency changes were observed at several loci in both the germplasm groups. As the selection was conducted independently in each location, an allele had a chance of being selected in more than one location, and therefore considering the whole study area the allelic composition and diversity of the original genetic materials was maintained after the selection. The study showed the importance of decentralized participatory plant breeding in maintaining genetic diversity that helps stabilize and sustain production in unpredictable production conditions.     

The choice of locality for plant breeding when selecting for high yield and general adaptation

Summary The choice of localities for initial yield testing of selections in a plant breeding programme is often arbitary. We describe a method that allows a more objective approach based on the correlation between the performance of varieties in a locality and their performance over a large area. The method was checked using data from the Western Australian Department of Agriculture regional wheat variety testing programme, and it was found possible to identify combinations of localities which produced significant predictions over both years and genotypes.     

Plant breeding and diversity: A troubled relationship?

Plant breeding collects, induces and rearranges genetic diversity followed by selection. Breeding may contribute to diversity in farmers’ fields or significantly reduce it. History has numerous examples of both. The diversity of many crops have gone through domestication, dispersal and modernization bottlenecks. Between these major decreasing processes, diversity has picked up through different evolutionary processes, and plant breeding affected by policies. Major negative effects of plant breeding on diversity have been recorded following the modernization bottleneck, but alternative breeding strategies have come up as well, both in the formal system and in the interphase between formal and farmers’ seed systems. Multiline breeding and participatory plant breeding are introduced as examples to also analyse effects of current developments in technology and policy. This paper intends to shed some light on the questions: how will current developments in technology and policy affect crop genetic diversity? Are we heading for a new bottleneck—either a molecular or a policy bottleneck, or a combination of both? Or could the future become more diverse? We look at the relationship between breeding, policies, and crop genetic diversity in farming systems with a birds-eye view. Notably because of current policy trends we warn for a new diversity bottleneck.     

Collaborative and consultative participatory plant breeding of rice for the rainfed uplands of eastern India

We describe a participatory plant breeding (PPB) programme in rice for the rainfed uplands of eastern India. Collaborative participation (farmers grew and selected in segregating materials in their fields) and consultative participation (farmers selected among progenies in researchers' plots) were used. The PPB was started with only two crosses and, of these, Kalinga III/IR64 was the most successful and produced two released varieties: Ashoka 200F from collaborative breeding and Ashoka 228 from consultative breeding. Both yielded significantly more than control varieties, including Kalinga III, in research trials and in participatory trials in farmers' fields. Qualitative data from participatory trials was highly informative, statistically analysable, and cheaper to obtain than quantitative data. In low-yielding research-station and on-farm trials the two new upland varieties showed no significant genotype × environment interaction with the check varieties. However, in higher-yielding All India Coordinated Rice Improvement Project trials, both varieties were more adapted to low yielding environments than the national check variety. Farmers liked the varieties for their early maturity, improved lodging resistance, higher fodder and grain yield, long-slender grains and excellent cooking quality. Before certified seed production, farmer groups have produced large quantities of seed that have spread widely through informal channels. The returns from PPB, compared to conventional breeding, were higher because it cost less, the genetic gains per year were higher, and the benefits to farmers were realised earlier. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effectiveness of selection at microtuber crop level in potato

Correlation coefficients were computed between microtuber crops and normal seed crop in order to study the selection efficiency for agronomic characters in potato at microtuber crop level. Two types of microtuber, green and white, were produced by inducing microtuberization under light and dark conditions, respectively. Genotypic differences were significant for various characters in both microtuber and normal seed crops. Green and white microtubers were equally useful for selection, judging by the crops raised from them. Selection at microtuber crop level was found to be highly effective for plant height, stem habit, tuber colour and general impression, and moderately effective for internode length, leaf length, leaflet width, stem pigment, plant vigour and tuber shape. Selection efficiency was low for tuber yield, tuber number, average tuber weight, number of nodes, number of stems, leaf colour and number of eyes. The implications of these results for potato breeding and germplasm evaluation programmes are discussed and a procedure for selecting superior genotypes at microtuber crop level is outlined.