Carlos Iglesias

Breeding for micronutrient density in edible portions of staple food crops: conventional approaches

Abstract Current and past efforts in breeding for industrial quality (processing, malting, baking, extruding, etc.), as opposed to yield, are reviewed as a prelude to discussion of the criteria that need to be met in breeding programs to improve the nutritional quality of crops for human consumption. In field crops, almost no attempts to improve nutritional value have been made. Recent studies in fact indicate that most criteria can be easily satisfied: existence of sufficient genetic variation, suitable selection methods and markers, workable heritabilities, and compelling reasons for doing so. However, establishing the efficacy in deficient human populations of elite material chosen by simple selection criteria is a major challenge that requires collaboration with human nutritionists. In some cases, developing marketing strategies for nutritionally superior types that may not – by color or other characteristics – appeal to target communities is also an issue breeders must bear in mind. Nevertheless, the fact recently established that desirable traits can be combined with high yield overcomes many obstacles. The widely demonstrated acceptance of new cultivars by farmers, in developing as well as industrialized countries, will ensure high impact of worthwhile improvements in nutritional value. To combine these new traits with high yield will increase the cost of breeding programs considerably, but the benefit–cost ratio is likely to be larger also.

Cassava breeding: opportunities and challenges

Although cassava is a major food crop, its scientific breeding began only recently compared with other crops. Significant progress has been achieved, particularly in Asia where cassava is used mainly for industrial processes and no major biotic constraints affect its productivity. Cassava breeding faces several limitations that need to be addressed. The heterozygous nature of the crop and parental lines used to generate new segregating progenies makes it difficult to identify parents with good breeding values. Breeding so far has been mainly based on a mass phenotypic recurrent selection. There is very little knowledge on the inheritance of traits of agronomic relevance. Several approaches have been taken to overcome the constraints in the current methodologies for the genetic improvement of cassava. Evaluations at early stages of selection allow for estimates of general combining ability effect or breeding values of parental lines. Inbreeding by sequential self-pollination facilitates the identification of useful recessive traits, either already present in the Manihot gene pool or induced by mutagenesis.

Variation of quality traits in cassava roots evaluated in landraces and improved clones

About 70 million people obtain more than 500 cal per day from cassava roots. The crop is fundamental as food security of poor rural communities, but little is known about variability of root nutritional and quality traits. Roots from 2457 genotypes comprising landraces and improved clones, were screened for their nutritional (cyanogenic potential, carotene, minerals, and sugars contents) and agronomic (dry matter content, color intensity, and postharvest physiological deterioration) traits. The objective was to assess the range of variation for the traits evaluated to define future research strategies. Results are mostly based on unreplicated measurements. Carotene contents in the roots ranged from 0.102 to 1.040 mg/100 g fresh tissue and correlated positively with color intensity (ρ = 0.860) and cyanogenic potential (ρ = 0.305). Average levels of Fe and Zn were 17.1 and 7.5 mg/kg, respectively. Many clones derived from Meso-America showed high protein levels in the roots, probably as a result of the introgression from wild relatives only found in that region. The observed values for carotene, proteins and minerals contents suggest the potential for improving the nutritive value of cassava.

Cross-species amplification of cassava (Manihot esculenta) (Euphorbiaceae) microsatellites: allelic polymorphism and degree of relationship

Microsatellite amplification was performed on cassava (Manihot esculenta) and six other different species (all wild) of the Manihot genus. We used ten pairs of microsatellite primers previously developed from cassava, detecting 124 alleles in a sample of 121 accessions of the seven species. The number of alleles per locus ranged from four to 21 alleles, and allelic diversity was greater in the wild species than in cassava. Seventy-nine alleles, including unique ones, were detected in the wild species but were not found in the crop. The lower level of heterozygosity in some wild species probably resulted from a combination of fine-scale differentiation within the species and the presence of null alleles. Overall, microsatellite primers worked across the genus, but, with increasing genetic distance, success in amplifying loci tended to decrease. No accession of M. aesculifolia, M. carthaginensis, and M. brachyloba presented a banding pattern at locus Ga-140; neither did one appear for M. aesculifolia at locus Ga-13. Previous work with amplified fragment length polymorphism (AFLP) markers and this microsatellite analysis show that these three wild taxa are the most distant relatives of the crop, whereas the wild forms M. esculenta subsp. flabellifolia and M. esculenta subsp. peruviana appear to be the closest.

Genetic potential and stability of carotene content in cassava roots

People in vast areas of the tropics suffer from vitamin A deficiency, resulting in progressive eye damage and eventually leading to blindness. Improving the content of vitamin A precursors in staple crops could alleviate or solve such a problem. The objective of this work was to study the range of variability for carotene content in a sub-set of the global cassava germplasm collection, and to determine the inheritance of carotenes, as well as their stability in response to different processing methodologies. Genotypes with more than 2 mg carotenes/100 g of fresh roots have been selected as parental material for population development. Although root colour is highly correlated with carotene content, a quantitative evaluation of genotypes selected by colour is required in order to increase the efficiency of selection. Relatively few major genes are involved in the determination of carotene accumulation in cassava roots. Stability of carotenes in response to different processing methods is genotypically dependant, representing a trait to be evaluated after selecting for high carotene concentration in fresh roots. The results from this work have provided the basis for defining future strategies for the improvement of the nutritional quality of cassava.

Mapping wound-response genes involved in post-harvest physiological deterioration (PPD) of cassava (Manihot esculenta Crantz)

The genome locations of the wound-response genes that were expressedduring the post-harvest physiological deterioration (PPD) of cassava, suchas phenylalanine ammonia lyase, β-1.3 glucanase, hydroxyprolinerich glycoprotein, catalase, 1-aminocyclopropane 1-carboxylate, cysteineprotease inhibitor, aspartic protease, a partial cDNA for serine/threonineprotein kinase and peroxidase, have been identified on the frameworkmolecular genetic map of cassava. Also, molecular markers linked toputative quantitative trait loci (QTLs) influencing PPD of cassava weremapped using an F1mapping population derived from elite parentallines (TMS 30572 × cm 2177-2). A molecular linkage map previouslyconstructed based on the segregation of 240 RFLP, 100 RAPD, 85microsatellite and five isoenzyme markers on 144 F1 individuals wasused for the QTL mapping.A set of 10 molecular markers with a significant association with putativeQTLs for PPD were identified based on probability values < 0.005in order to minimize the detection of false positives. Based on single-markerregression, eight putative QTLs located on the linkage groups G, P, L, U,and X of the female-derived framework map were found to explain between 5–12% of the phenotypic variance of the PPD. In the male-derived frameworkmap, two putative QTLs on linkage groups C and L explained 13% and11% of this variance, respectively. This study thus identified the majorgenome regions of cassava related to physiological post-harvestdeterioration, thereby providing tools for the identification of gene(s)controlling this trait.

Propagating cassava (Manihot esculenta) by sexual seed

Commercial cassava production from true cassava seeds (TCS) appears to be a promising option for reducing or eliminating several of the production constraints associated with vegetative propagation. The most important contribution of TCS would be to reduce virus build-up in vegetative material and to resolve the problems of stake storage, low multiplication rate and the long growth cycle. Preliminary results suggest that the root yield potential of TCS is comparable with that of traditional vegetative propagation. Improvements in the capacity of true seed progenies to germinate and establish in the field can be achieved either genetically or through pre-planting seed treatments such as coating the seed with rock phosphate. Development of a successful TCS technology will require a multidisciplinary approach, involving basic studies in cassava breeding, physiology, agronomy, socio-economics and other fields.

Sensitivity of cassava (Manihot esculenta Crantz) clones to environmental changes

Abstract The performance of 15 cassava (Manihot esculenta Crantz) clones in 14 environments in Colombia was analyzed to determine the possibility of improving stability of root-yield in cassava in association with minimum acceptable yields. The specific objectives were to study the relationship among agronomic traits, to evaluate genotypic sensitivity to changes in the environment, and to characterize and determine the representativeness of the evaluation environments. Correlations found between root yield and related physiological or quality traits were in a favorable direction for breeding purposes. This indicates that when selecting for a complex set of traits, indices might be established with major emphasis on traits with high heritability and/or stability. For some traits (number of commercial roots and length of stem with attached leaves) the range of genetic variability was broader in favorable environments. Variation among evaluation sites was greater than variation across years. The results indicate that intermediate to low genotypic sensitivity in terms of cassava root yield andry matter content can be combined with improved potential for dry matter production per unit area. In order to improve the performance and stability of cassava gene-pools, representative sites should be selected within the priority agro-ecosystem, to evaluate the genetic base for at least two years before selection is made. Association between the mean and the sensitivity coefficient for different traits was either nonsignificant or positive for breeding purposes. Improvement in the mean of traits can be made independently from, or in relation to, genotypic ability to react to environmental changes.

Toward the construction of a molecular map of cassava (Manihot esculenta Crantz): comparison of restriction enzymes and probe sources in detecting RFLPs

The construction of a detailed genetic map of cassava (Manihot esculenta Crantz), classified as a tetraploid species, depends on the ability of cloned sequences to detect polymorphisms. As a first step in developing this map, 200 cloned nuclear sequences generated with different restriction enzymes were hybridized to total digested DNA from eleven cultivated lines and one wild Manihot species, M. aesculifolia. Polymorphism was detected less frequently with both BamHI and EcoRI genomic probes than with PstI, HindIII and XbaI genomic probes. DNA digested with HpaII, DraI and TaqI displayed less polymorphism, whereas DNA digested with EcoRI and EcoRV displayed more polymorphism like that found in lettuce, rice and tomato (Landry et al., 1987; McCouch et al., 1988; Miller and Tanksley, 1990). Four-cutter restriction enzymes displayed less frequency of polymorphism when compared with six-cutter restriction enzymes. Polymorphism displayed by DraI was extremely low, indicating that regions rich in adenine and thymine may not be hot spots for mutation in cassava. Polymorphism detected between cultivated genotypes and M. aesculifolia was dramatically higher than that found among cultivated genotypes.