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     

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.     

Genomic and biological diversity of the African cassava geminiviruses

The virological situation of cassava in Africa is increasing in complexity due to the number and types of viruses isolated from different locations within the continent. Here, we report the complete nucleotide sequences of both A and B components of two geminivirus species infecting cassava in the Ivory Coast and review the current knowledge of the molecular and biological diversity of the African cassava geminiviruses. As a whole, newly obtained sequences are compared with those of the African cassava mosaic geminiviruses identified to date. Results indicate that all isolates of African cassava mosaic virus (ACMV), irrespective of their geographical origin are clustered together with little or no variation in their genomic sequence. On the contrary, the genomes of the East African cassava mosaic virus (EACMV) are more genetically diverse due to the frequent occurrence of recombinations within their two components. Indeed, the EACMV-like viruses vary so much that their classification is becoming problematic. In addition, there is also a large range of phenotypic symptom variation for each of these virus species, irrespective of the location of isolation. Furthermore, it has been shown that ACMV and EACMV can be synergistic in cassava, resulting in a greater DNA accumulation and consequently inducing severe symptoms. For all these reasons, this paper initiates a discussion concerning the species demarcation for cassava geminivirus. This revised version was published online in July 2006 with corrections to the Cover Date.     

Standardisation of true cassava seed (TCS) programme with special emphasis on more homogeneous, CMD resistant progenies

The propagation of cassava through true seeds (sexual seeds) rather than by clones is a promising option due to its manifold advantages such as enhancing the multiplication rate, keeping the dreaded cassava mosaic disease (CMD) under check, longer seed viability, ease of storage and transport. The high genetic heterogeneity and consequent variation among seedlings is the major stumbling block in sexual propagation. In the present study, a CMD resistant exotic accession MNga-1 and a promising cultivar Ambakadan with profuse fruit setting, seed output and male sterility were identified to be promising parents for the TCS programme. The rate of sexual propagation could be more than 20-fold over the traditional clonal propagation. Seed treatment with 1% KNO₃ or 300 ppm GA promoted uniform seed germination and seedling vigour and reduced the transplanting period from 45 days after planting (DAS) to 30 DAS. Removal of taproots of seedlings while transplanting enhanced tuber development. Tuber yield of first clones (C₁) was significantly superior to that of the seedlings. The dry matter content and starch output of seedlings and first clones were comparable to that of the commercial varieties. Similarly, the HCN and cooking quality of seedlings and first clones were at acceptable levels. In the open pollinated (OP) progenies of the Ambakadan the CMD infection increased drastically due to secondary spread of the pathogen. The hybrid progenies of Ambakadan and the CMD resistant line MNga-1 revealed higher percentage of CMD free seedlings and first clonal progenies in the evaluation trials conducted at CTCRI Thiruvananthapuram and ARS, Peddapuram during 2001–2002 and 2002–2003. Nearly homogeneous hybrid population resistant to CMD could be obtained by systematic roguing at seedling and first clonal stage.     

基于SNP和InDel标记的巴西木薯遗传多样性与群体遗传结构分析

As a typical tropical crop, cassava (Manihot esculenta Crantz) has the characteristics of drought resistance, barren resistance, high biomass and so on. In addition to being used for food and forage, it can also be used for production, processing and starch extraction. Due to highly heterozygous cassava genome, breeding is more difficult. Enriching the genetic diversity of cassava germplasm, comprehensively evaluating its genetic background and traits, and discovering superior alleles that control excellent traits are of great significance for cassava breeding in the future. In order to analyze the genetic diversity, genetic relationship and population structure of cassava germplasm in Brazil, 7946 SNPs and 1997 InDels molecular markers were used. Population structure analysis was performed by ADMIXTURE software, and principal component analysis was performed by GCTA software. Brazilian cassava was divided into nine subgroups, and was roughly consistent with the results of cluster analysis using PHYLIP. Among them, subgroup 1, subgroup 2, subgroup 4, subgroup 6, and subgroup 8 could be clustered together respectively, while the samples of other subgroups could be roughly clustered, and there was a certain cross between the samples. The genetic diversity of cassava germplasm in Brazil (0.274) was higher than the genetic diversity level of cassava germplasm in China and Nigeria. Subgroup 5 of Brazil cassava had a relatively high genetic diversity (0.29). The genetic differentiation of subgroups was low (the genetic differentiation vary from 0.03 to 0.15), but higher than domestic cassava germplasm. The genetic distance between cassava accessions varied from 0.084 to 0.297, with the average of 0.228. The results of this study can provide a basis for subsequent association analysis to identify great alleles and introduction.     

Identification of levels of resistance to cassava root rot disease (<Emphasis Type="Italic">Botryodiplodia theobromae</Emphasis>) in African landraces and improved germplasm using <Emphasis Type="Italic">in vitro</Emphasis> inoculation method

Cassava root rot disease is an increasing problem in Africa where yield losses of about 80% have been recorded. We evaluated 290 African landraces and 306 improved genotypes from the germplasm collections of the International Institute of Tropical Agriculture (IITA), for sources of resistance using root slice laboratory assay. Disease severity was assessed quantitatively by direct percentage estimation (PS) and by use of a rating scale (RS). Both methods of assessment were compared for identification of variability in the germplasm, and genotypes were classified into response groups using an enlarged rank-sum method that combined the PS and RS assessments. The two scoring methods revealed continuous variation (P < 0.001) for resistance in the sets of germplasm. Disease assessments based on PS and RS were highly correlated in both the improved germplasm (r = 0.75) and the landraces (r = 0.72). Based on PS assessment, 50 improved genotypes (16.3%) and 53 landraces (18.3%) showed significantly lower disease scores than the resistant control. The rank-sum method separated each set of collections into highly resistant, resistant, moderately resistant, moderately susceptible, susceptible and highly susceptible groups. Fifty-nine improved genotypes (16.4%) and 61 African landraces (16.9%) were identified as either highly resistant or resistant. Generally, these genotypes exhibited resistance by limiting the growth of the pathogen (reduced amount of invaded surface area). This type of rate-reducing resistance is highly heritable and a quantitative trait which can be harnessed in breeding. Genotypes subsets were identified for further studies into the genetic basis of resistance to root rot disease.     

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.     

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.     

预切种式木薯排种机构设计与试验

针对当前预切种式木薯播种器存在充种效果差、合格指数低的问题，该研究设计了一种由落种滑板、主动辊筒、型孔摩擦带、支撑辊组、从动辊筒及储种箱等组成的预切种式木薯型孔摩擦带式精密排种机构。阐述了排种机构的基本结构和工作原理。对关键部件参数进行设计，通过理论分析确定影响排种机构充种性能的主要因素为型孔摩擦带型孔形状、型孔数量、型孔摩擦带安装倾角、型孔摩擦带速度以及种茎层厚度。基于离散元法（discrete element method,DEM）建立种茎群-型孔摩擦带仿真模型，通过单因素仿真分析各因素对充种性能及种茎群规律的影响。以充种合格指数和漏充指数为评价指标，通过二次回归正交旋转组合仿真分析确定最优参数，利用Design-Expert数据软件，建立各试验因素与评价指标的数学回归模型，并进行参数优化。结果表明，影响充种合格指数和漏充指数的因素主次顺序为型孔摩擦带速度、型孔摩擦带安装倾角和种茎层厚度，圆整后的最优参数为型孔摩擦带速度0.6 m/s、型孔摩擦带安装倾角45°，种茎层厚度250 mm，此时的模型预测充种合格指数为94.63%，漏充指数为3.42%，重充指数为1.95%。在最优参数下...