Inheritance of useful traits in cassava grown in subhumid conditions

A diallel study among nine parental clones of cassava was conducted in the subhumid environment on the northern coast of Colombia. Analysis of variance suggested significant effects for the six variables analysed: fresh‐root yield, harvest index, root dry matter content, height of first branching, reaction to thrips and plant‐type scores. General and specific combining ability effects and their interaction with the environment were significant for most of the variables as well. Results suggested that dominance plays a particularly important role in the cases of fresh‐root yield and harvest index but had relatively little importance in the reaction to thrips, dry matter content or height of first branching. Specific breeding approaches are suggested for these traits, depending on the relative importance of additive or non‐additive effects in their inheritance. The correlations among different traits were also analysed and in several cases their magnitude reached statistical and biological significance.     

The closest wild relatives of cassava ( Manihot esculenta Crantz)

The origin of cassava has long eluded researchers. The botanical origin, the geographical origin, the area of domestication, all are disputed matters. The argument advanced here is that, once the wild ancestor is identified, speculation about the taxonomy and the geographical cradle of the root crop will narrow considerably, as a number of species and geographical areas will automatically become excluded from consideration in matters of classification, domestication and evolution. Current knowledge about the three subjects shows that discoveries on the botanical origin of manioc have progressed a great deal, discoveries on the geographical origin are in progress, and discoveries on the area where the beginning of cultivation happened are incipient. In this paper, five Brazilian Manihot species are suggested as the closest wild relatives of cassava. One of them (M. esculenta ssp. flabellifolia) is regarded as the wild progenitor of modern cultivars and thus part of the primary genepool of the root crop. Another species (M. pruinosa) is regarded as the nearest species to the GP1 of cassava and can hardly be separated from the wild strain M. esculenta ssp. flabellifolia on morphological grounds. This revised version was published online in July 2006 with corrections to the Cover Date.     

Heredity of seventeen isozyme loci in cassava (Manihot esculenta Crantz)

Summary Starch gel electrophoresis was used to assess isozyme polymorphism in two Manihot species. Crude extracts were obtained from leaves and pollen. Ten enzymes were examined for their polymorphism in a germplasm collection of 365 cultivated plus 109 wild accessions, mainly from Africa. The inheritance of these enzymes was examined using 13 intra and interspecific progenies. Seventeen polymorphic loci were found for the ten enzyme systems, with 59 alleles. All the markers showed disomic heredity and three linkage groups were identified.     

Combining ability and patterns of inheritance for plant and fruit traits in Japanese quince (Chaenomeles japonica)

Phenotypic variation and pattern ofinheritance was investigated for plant andfruit traits in Japanese quince (Chaenomeles japonica, Maloideae). Japanesequince is a minor fruit crop in Latvia andLithuania, and is presently being developedfor production of juice, aroma and fruitfibre in Sweden and Finland. General andspecific combining ability was estimatedfor five plant vegetative traits, eightfruit yield and morphology traits, and fivefruit biochemistry traits in a studypopulation consisting of 14 hybridfamilies. Thorniness seems to be controlledprimarily by a single locus. All othertraits studied appear to be governedprimarily by additive genes, as judged fromgeneral combining ability estimates.Significant specific combining ability(SCA) was detected for yield, plant heightand some fruit traits. However, therelative contribution of SCA to the overallgenetic variance was in general low. Anefficient breeding strategy for Japanesequince could therefore be based onrecurrent selection.     

Somatic embryogenesis from floral tissue of cassava (Manihot esculenta Crantz)

The aim of this study was to examine the embryogenic potential of floral material of the cassava cultivar MCOL 1505. Macerated immature inflorescences were found to be highly embryogenic, with almost 78% of the explants producing somatic embryos. Somatic embryos were also produced from whole male florets and half florets although at much lower rates. No regeneration was obtained from anther, microspore or floret wall tissue. Somatic embryos derived from immature inflorescences were regenerated via organogenesis and the plants derived from this process were assessed in terms of phenotype and ploidy level. If haploid plants could be produced by this method, this would have significant implications in assisting traditional cassava breeding, as this would allow homozygosity to be reached more rapidly. In a crop such as cassava, which is highly heterozygous in nature, the use of haploids in a breeding programme could considerably shorten the time taken to produce new desirable cultivars. This is the first report on plant regeneration through somatic embryogenesis from floral tissue of cassava. This revised version was published online in July 2006 with corrections to the Cover Date.     

The primary gene pool of cassava (Manihot esculenta Crantz subspecies esculenta, Euphorbiaceae)

The primary gene pool (GP-1) of a crop is composed of gene reservoirs that cross easily with the domesticated, while the crosses regularly produce fertile offspring. The GP-1 is further subdivided into cultivated and wild gene pools. The cultivated gene pool encompasses commercial stocks of the crop, as well as landraces. The wild GP-1 of the crop comprises putative ancestors and closely related species that show a fair degree of fertile relationships with the domesticate. Two South American wild subspecies of cassava (M. flabellifolia and M. peruviana) were proposed as natural members of the wild GP-1 of the crop. Another Brazilian species(M. pruinosa) is morphologically so close to both wild subspecies that it may turn out as another member of the wild GP-1. This revised version was published online in July 2006 with corrections to the Cover Date.     

RAPD-based genetic diversity study of fifty cassava (Manihot esculenta Crantz) genotypes

Fifty cassava clones were studied using RAPD technique. They included landraces from the Wenchi, Nkoranza, Dormaa Ahenkoro and Asonafo districts of the Brong Ahafo region of Ghana and three improved varieties. Genetic diversity of these genotypes was studied using four primers, OPK-01, OPR-02, OPR-09 and OPJ-14. A total of 41 different bands were detected. Levels of polymorphic fragments detected by the four primers ranged from 90% to 100%. By pooling bands from individual accessions together, mean number of fragments per accession per primer ranged from 5.50±1.04 for the Improved cultivars to 7.00±0.71 for populations of landraces from Dormaa. Mean frequencies of fragments not detected by the primers for the accessions were 0.524±0.12, 0.460±0.12, 0.561±0.12 and 0.523±0.12 for landraces from Wenchi, Nkoranza, Dormaa Ahenkro, Asonafo and the Improved varieties, respectively. The grand mean frequency of individuals showing fragments not present in populations was 0.522±0.10. Genetic diversity estimates ranged from 0.290 to 0.425 (mean 0.352±0.05) for primer OPK-01, 0.001 to 0.381 (mean 0.309±0.06) for primer OPR-02, 0.335 to 0.344 (mean 0.283±0.04) for primer OPR-09 and 0.152 to 0.352 (mean 0.261±0.07) for primer OPJ-14. Within the accessions mean gene diversity estimates were 0.316±0.03, 0.293±0.09, 0.331±0.02, 0.322±0.07 and 0.247±0.03 for accessions from Wenchi, Nkoranza, Dormaa Ahenkro, Asonafo districts and the Improved varieties, respectively. Interpopulational genetic divergence ranged from 0.069 to 0.203 (mean 0.119±0.04). Rate of nucleotide substitution among the landraces was 9.8 per cent per site per year, while that for the Improved varieties was 15 per cent. This revised version was published online in August 2006 with corrections to the Cover Date.     

Evaluation of cassava (Manihot esculenta Crantz) germplasm collections using RAPD markers

Summary Deterioration of cotton, Gossypium hirsutum L., seed can adversely affect stands, seedling vigor, and subsequent performance of plants, but deterioration can be lessened by improved genetic resistance. This study was conducted to evaluate the relationship of physical and germination characteristics of cotton seed and to determine the inheritance of resistance to seed deterioration. Physical (weight, volume, density, and imbibition rate) and germination (germination of non-deteriorated seed and deteriorated seed) characteristics were determined for eight cotton genotypes. In addition, imbibition rate and the germination characteristics were determined for all possible F₁, including reciprocals, combinations of the eight genotypes. Variation among the parents for germination of deteriorated seed and imbibition rate were significantly, highly correlated (r=–0.97) and intrinsically related. Resistance to seed deterioration tended to increase as seed weight and volume decreased. Significant genetic effects were found for imbibition rate and germination of both non-deteriorated and deteriorated seed. However, general combining ability (GCA) accounted for only 21% of the variation in crosses sums of squares for germination of non-deteriorated seed. In contrast, GCA accounted for 90% of the variation for germination of deteriorated seed and imbibition rate. A breeding approach involving selection of early generation seed for ability to survive hot water treatment, then evaluating later generations for imbibition rate is suggested as a means to improve resistance to seed deterioration in cotton.     

A molecular and embryonic study of apomixis in cassava (Manihot esculenta Crantz)

In cassava, apomixis fixes heterosis and avoids transmission of systemic pathogens which complicate vegetative propagation of the crop. A combination of evidence from maternal inheritance of RAPD markers and the structure of the embryonic sac in large progeny sets of two distinct genotypes have further confirmed the occurrence of apomixis in cassava. We could advance further on earlier reports of the detection of apomixis in four ways: (1) we could arrive at an estimate of the rate of facultative apomixis in the range of 2%; (2) we detected the occurrence of apomixis in a second genotype, derived from a different interspecific cross; (3) apomictic behavior was demonstrated in an F₁ individual and (4) parallel embryonic evidence was generated that corroborate the potential occurrence of apomixis by apospory. The fact that apomixis was detected in an F₁ interspecific hybrid hints to the possibility of directly transferring genes for apomixis from a wild relative to cultivated cassava. This revised version was published online in July 2006 with corrections to the Cover Date.     

A cytogenetic study of bilateral sexual polyploidization in cassava (Manihot esculenta Crantz)

Microsporogenesis and megasporogenesis were cytogenetically and histologically analysed in three cassava clones:‘Rayong 1′,‘Rayong 60′,‘M. mga’ and two hybrid lines,‘OMR 3641‐1’ and ‘OMR 3641‐1’ to elucidate the evolution of sexual polyploids in cassava. At telophase II, formation of 17‐21 micronuclei per pollen cell plate was observed in 16 out of 351 cell plates in ‘M. mga’. Micronuclei were observed at low (0.3‐2.3%) frequencies, at the sporad stage in all clones. Monads, dyads, triads and tetrads, which are established sources of high ploidy levels were observed at low (2.6%) and high (22.2%) frequencies. Megasporogenesis in ‘Rayong 1’ and ‘Rayong 60’ showed a lack of second meiotic divisions after a successful first division that resulted in partly unreduced embryo sacs with 2n eggs, suggesting another unrecognized and, as yet, unreported source of sexual polyploid formation in cassava. Meiotic abnormalities during microsporogenesis and megasporogenesis are implicated as being responsible for the formation of mixoploids (triploids and tetraploids) in cassava breeding programmes. A cytogenetic mechanism resulting in bilateral sexual polyploids through different gametic fertilization pathways in cassava is suggested and its role in breeding is briefly discussed.     

Mating system in an experimental garden composed of cassava (Manihot esculenta Crantz) ethnovarieties

Crossing patterns were investigated in an experimental garden of ethnovarieties ofManihot esculenta (Euphorbiaceae) inPiracicaba, São Paulo, Brazil. A model of evolutionary dynamics for cassava presupposes genetic recombination by means of crossing within cassava gardens as a source that amplifies genetic diversity. Quantitative analysis of mating system parameters was performed using progeny arrays assayed for eight allozyme markers. The multilocus outcrossing rate (t _m)estimate (0.915±0.04)revealed that outcrossing was prevalent, but that a low level of self-pollination also occurred. The multilocus outcrossing rate ranged from 0.69 to 1.00 among eight varieties. The high value found for the outcrossing rate indicates that the ethnovarieties studied are preferentially allogamous. Genetic recombination occurred through crossing within the cassava garden, in agreement with an assumption of the model of evolutionary dynamics for this species.     

Efficient production of transgenic plants by Agrobacterium-mediated transformation of cassava (Manihot esculenta Crantz)

An efficient and reproducible method was developed for Agrobacterium-mediated transformation of embryogenic suspension cultures of cassava. LBA4404(pTOK233), containing the nptII, hph and gus marker genes, was used in the experiments. Chemical selection by means of kanamycin was used to establish 1037antibiotic resistant callus lines, of which 526 showed GUS expression. Of the 241 callus lines that were transferred to maturation medium 219formed somatic embryos. Thirty-seven of the 38 lines that were transferred to germination medium produced plants. GUS-positive plants could be obtained from 31 lines; in 14 of those lines 100% of the produced plants were GUS-positive, the remaining 17 lines yielded GUS-positive plants at an average of 72%. The transgenic nature of these plants was confirmed by Southern blot analysis. This revised version was published online in July 2006 with corrections to the Cover Date.     

Isozyme electrophoregrams of sixteen enzymes in five tissues of cassava (Manihot esculenta Crantz) varieties

Summary Methodology, based on starch and polyacrylamide gel electrophoresis, was developed for determining isozyme electrophoregrams (patterns) of 16 enzymes of cassava (Manihot esculenta Grantz) varieties as potential genotype markers. Extracts of five different tissues (root, stem, leaf, petiole and bud) were examined. In general, the nodal portions of the shoots gave isozyme patterns with the largest number of bands. Petiole extracts gave similar results but bud extracts gave poor patterns. The limited number of varieties that were examined could be distinguished by sequential classification on the basis of the isozyme patterns of acid phosphatase, esterase, glutamate oxaloacetase transaminase and phosphoglucoisomerase.Joint publication of the Centro Internacional de Agricultura Tropical and the Department of Plant Science (No. 716), University of Manitoba. Presented in part at the 2nd International Symposium on Biochemical Approaches to Identification of Cultivars and Evaluation of Their Properties, 5–9 May, 1985, Braunschweig, West Germany.     

Production of embryogenic tissues and regeneration of transgenic plants in cassava (Manihot esculenta Crantz)

Disorganised embryogenic tissues have been utilised as target tissues for transgene insertion and transgenic plant regeneration in cassava (Manihot esculenta). The production of friable embryogenic callus in fourteen geographically diverse cassava cultivars, from which eleven were established as embryogenic suspension cultures, is reported. Embryogenic tissues were similar in nature in all cultivars tested although there was variation in the time required to generate friable callus and the growth rates of suspension cultures. Regeneration of plants has been achieved from eight cultivars but varied significantly in efficiency, with cv. TMS 60444and Line 2 from Zimbabwe being the most responsive. Tissues from the remaining eight cultivars became arrested at globular and torpedo stages of regeneration indicating that they most likely process an inherent ability to produce plants but require further research to allow this to be realised. Significant numbers of transgenic plants containing transgenes for putative resistance to important viral diseases of cassava in addition to visual marker genes have been regenerated. Transgenic plants from three the cultivars TMS 60444, Bonoua Rouge and M.Col 1505 were recovered after particle bombardment of embryogenic suspension cultures. Correlation's have been made between abnormal leaf morphology and plant vigour with the use of embryogenic suspension cultures for transgene insertion. As an result friable embryogenic callus is now being successfully utilsed as the target tissue for genetic transformation and plant regeneration at ILTAB. This revised version was published online in July 2006 with corrections to the Cover Date.     

Combining ability of common bean (Phaseolus vulgaris) genotypes for root traits across diverse environments

Root system architecture is important for common bean (Phaseolus vulgaris) adaptability to diverse environments. Beans employ complex adaptive root mechanisms for coping with multiple stresses in production environments. Understanding genetic control of root traits is central to improvement of common bean for adaptation to marginal environments. The objectives of this study were to (i) determine combining ability of root and agronomic traits and (ii) estimate the heritability and genetic correlation of root and agronomic traits in common bean. Four bean lines with superior root traits were crossed with four locally adapted varieties in a North Carolina II mating scheme to generate 16 crosses. The 16 F₁s were selfed and advanced to F₂ generation. Eight parents and their F₂ progenies were evaluated in an alpha-Lattice design with two replications. General and specific combing ability mean squares were significant (p ≤ .05) for all traits measured. General predictability ratios ranged from .47 to .68 across locations suggesting that both additive and non-additive gene action modulate root traits and seed yield. Positive and significant (p ≤ .05) phenotypic and genetic correlations revealed significant association between root traits and yield. Moderate to high heritability estimates of between .43 and .67 were realized. Such estimates point to possible deployment of a successful selection programme. Genotype AFR398 displayed significant positive GCA effects among its crosses for both root and agronomic traits hence a potential candidate genotype for inclusion in a bean genetic improvement programme for marginal environments.     

Nuclear DNA content and in vitro induced somatic polyploidization cassava (Manihot esculenta Crantz) breeding

Summary The diploid (2C) amount of DNA in cassava (Manihot esculenta Crantz) is 1.67 picograms (pg) per cell nucleus. This value corresponds to 772 mega-base pairs in the haploid genome. The size of the nuclear genome in cassava is very small in comparison with other Angiosperms. Flow cytometry techniques were used to screen ploidy levels in a large population of in vitro plantlets treated with colchicine and oryzalin (3,5-dinitro-N⁴,N-dipropylsulphate). Culture of axillary node cuttings for 48 hours in liquid medium supplemented with 2.5 to 5.0 mM colchicine in combination with 2% dimethyl sulfoxide (DMSO) resulted in a high frequency (23 to 42%) of non-chimeric tetraploids in the V₃ generation. Although mixoploidy may persist in as many as four cycles of vegetative propagation of node cuttings, solid (non-chimeric) tetraploids can be identified by flow cytometry among in vitro plantlets and then rapidly propagated for field testing. A somatic polyploidization system is proposed for implementation in cassava breeding programmes.Dedicated to the memory of the late Dr. Novak. Correspondence to M. van Duren     

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, -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.     

Genetic linkage map of cassava (Manihot esculenta Crantz) based on AFLP and SSR markers

To generate a genetic linkage map of cassava ( Manihot esculenta Crantz), 58 F₁ progenies from a cross between Rayong 90 (female) and Rayong 5 (male) were examined in amplification fragment length polymorphism (AFLP) and simple sequence repeat (SSR) analyses. A total of 469 polymorphic markers consisting of 378 AFLPs generated from 76 primer combinations and 91 SSRs were identified. These markers were analyzed using the joinmap ^® 3.0 package program to construct a genetic linkage map. A total of 33 linkage groups of a common map were constructed from 119 AFLPs and 18 SSRs, spanning 1095 cM with an average of 7.99 cM between markers. The genetic linkage map generated in this study will be useful for genetic studies in cassava particularly for the identification of genetic markers linked to traits of interest, although the complex cassava genome suggests that maybe a long term objective.     

Histological analysis of embryo-sac formation and detection of meiotic diplospory in cassava (Manihot esculenta Crantz)

Summary Embryo-sac formation in six cultivated Asian cassava clones and one unknown wild type, was histologically investigated to determine apomictic potential in the genus Manihot. All the 6 clones were found to possess one single sexual embryo-sac containing 6–8 nuclei at maturity. However, one clone, Rayong 3 possessed two functional embryo-sacs at 12 hours after controlled pollination (HACP). The larger embryo-sac was 6-nucleate (all 3 nuclei of the egg apparatus, 1 polar and 2 antipodal cells) and located towards the false micropylar or the nucellar beak region. The smaller embryo-sac also contained a total of 6 nuclei and approximately 10 m³ or 1/2 the volume of the former and located towards the chalazal pole. Percentage sexuality estimates indicated a 100% sexual reproduction in embryo-sacs of all the clones analysed. However, apomeiotic relationships indicate a low (0.3%) degree of meiotic diplosporous embryo-sac formation, thus suggesting facultative apomixis in cv. Rayong 3. This is the first embryological identification and evidence of apomeiosis and apomictic potential in cassava.Contribution from Plant Breeding Laboratory, Applied Genetics & Biotechnology Division, Faculty of Agriculture, Miyazaki University, No. 91.