Components of partial resistance in phaseolus beans against an Ethiopian isolate of bean rust

Summary Banana weevil (Cosmopolites sordidus Germar) is a major limiting factor in the cultivation of plantains and bananas (Musa spp. L.) in sub-Saharan Africa. The larvae damage the crop by tunneling in the corm. Chemical control is feasible but not sustainable, whereas host plant resistance is safe and has long term benefits. Banana weevil damage and infestation levels and corm hardness were assessed in Musa germplasm in order to determine the genetic control and potential mechanisms of resistance to this insect pest. Susceptibility/resistance to weevil was measured by the percentage coefficient of infestation (PCI) and damage in cross sections (CS) of the corm. All plantains were equally susceptible to the banana weevil, while a wild banana accession and some cooking and dessert banana cultivars showed high levels of resistance. Differential genotypic responses were observed in euploid plantain-banana hybrids. Segregation results suggest that host plant response to weevil in Musa is controlled by gene(s) exhibiting partial dominance towards the resistant parent and modifier genes with additive and dosage effects for susceptibility in the plantain parent. In natural banana germplasm, resistant clones showed increased corm hardness, as measured by a penetrometer in longitudinal and cross sections of outer and central corm tissues. This might suggest a non-preference mechanism for weevil resistance. However, the lack of correlation between corm hardness with PCI and CS scores in the segregating progenies suggested that other mechanisms may be more important in conferring resistance to banana weevil.     

Levels of host plant resistance to banana weevil, Cosmopolites sordidus (Germar) (Coleoptera: Curculionidae), in African Musa germplasm

Forty-five Musa clones, including endemic and introduced cultivars plus hybrids, were evaluated for resistance against the banana weevil, Cosmopolites sordidus, in a field trial in Uganda. The predominant groups of staple crops, East African highland bananas (Musa spp. AAA) and plantains (Musaspp. AAB), as well as plantain-derived hybrids (AAB × AA), showed the highest levels of susceptibility to this pest. These were followed by dessert bananas (Musa spp. AAA), exotic bananas (Musa spp. ABB) and finally diploids of M. acuminata (AA). Hybrids of banana origin were highly resistant. Some East African highland cultivars, especially brewing types (e.g., Kabula, Bagandeseza, Ediirira), showed intermediate levels of resistance. Among the non-highland bananas, high levels of resistance were observed in Yangambi-Km5 (AAA), Cavendish (AAA), Gros Michel (AAA), Kayinja (ABB, Pisang Awak subgroup), Ndiizi (AB, Ney Poovan subgroup)and Kisubi (Ney Poovan subgroup). The highest resistance was observed in banana hybrids TMB2×7197-2, TMB2×8075-7 and the wild banana Calcutta-4 (AA). These were considered the best sources of resistance for a weevil resistance-breeding programme with the two hybrids commonly used as improved male parents. This revised version was published online in July 2006 with corrections to the Cover Date.     

Diversity of banana diploid hybrids: An assessment based on a hydroponic system

Bananas are typical to tropical and subtropical climate regions, where they have better conditions due to high air humidity and soil water availability. The production of drought-tolerant varieties is an alternative to expand the cultivation sites to drier areas. In this work, we present a hydroponic system for the genotype selection of Musa sp. (AA) subjected to water stress. A total of 14 banana genotypes were tested; 12 diploids and two commercial varieties with a known response to drought tolerance. The results show that the improved diploids have different responses with respect to water deficit, with the PMGB043 and PMGB044 genotypes considered more susceptible and the PMGB075 and PMGB099 genotypes more tolerant. Under water deficit conditions, most banana diploids maintain a high relative water content, but there is a reduction in aerial part growth and an increase in root dry mass. The accumulation of alcohol dehydrogenase, especially in control treatment plants, indicate that the glycolytic pathway was used to supply energy demand, showing potential flood-tolerant genotypes. The results are essential for the preimprovement of banana cultivars.     

Nematode resistance in bananas: screening results on some wild and cultivated accessions of Musa spp.

Bananas cultivated for export all belong to Cavendish cultivars and are all recognized as very susceptible to nematodes, particularly to the burrowing nematode Radopholus similis and the lesion nematode Pratylenchus coffeae. Even if there have been many changes in the management of banana nematodes in large commercial banana plantations, chemical control still remains most often the last resort method to manage the nematodes, although the number of registered products is definitely declining. Therefore, nematode control though genetic improvement is gaining new interest worldwide. In this study, 55 banana accessions mostly diploids from the Musa acuminata genome group (AA) but including some triploid accessions (AAA), some diploids of the Musa balbisiana genome group (BB) and some interspecific hybrids (AAB, AB) were evaluated for resistance to four nematode species R. similis, P. coffeae, Meloidogyne incognita and M. arenaria. These experiments were conducted in a growth chamber under controlled conditions. All banana accessions were susceptible to nematode species, although many different levels of susceptibility were detected. This study confirmed the good resistance status to R. similis of some cultivars from the Pisang jari buaya and Pisang batuau subgroups and the partial resistance of 17 diploid accessions significantly different from the susceptible reference cv. Grande Naine. This study also showed that 12 diploid accessions exhibited a partial resistance to P. coffeae, including some usual or potential genitors belonging to the wild diploids subspecies burmannica (cvs. Long Tavoy 1 and 2) and burmannicoides (cv. Calcutta 4). No source of resistance to Meloidogyne spp. was found. These screening results, combining for the first time four nematode species, are discussed within the scope of banana breeding in order to produce parental diploid lines with single or combined nematode resistances and further develop triploids that can substitute existing susceptible commercial cultivars.     

Field performance of embryogenic cell suspension-derived banana plants (Musa AAA, cv. Grande naine)

Growth and yield characteristics of two different clones of banana plants (Musa AAA cv. Grande naine) originating from four months old embryogenic cell suspensions were studied. These characteristics were compared with those plants produced by the conventional in vitro budding multiplication method. Two types of variants were observed during the acclimatization phase among 500 embryogenic cell suspension derived plants. The first type related to banana plants with `variegated or deformed leaves' were also observed in in vitro budding derived plants. The second type concerned `fasciated-leafed' plants. During the field growth, these two variant types produced plants morphologically similar to the other plants. Thus, none of the cell suspension derived plants exhibited off-type traits in the field. A Fisher block model was used to compare the field performances of the two clones produced through the two in vitro propagation techniques. The analysis of variance showed that there were no significant differences between the plants produced by either micropropagation techniques for the plant height and circumference, the length of the reference leaf, the number of nodal clusters of the inflorescence and of fruits, the bunch weight, the period of time between planting and flowering, and between planting and harvesting. This study showed that banana plants with an agronomical behaviour similar to those produced by the conventional in vitro budding method could be regenerated from embryogenic cell suspension. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genetic variation and phylogenetic relationships based on RAPD analysis in section Caulorrhizae, genus Arachis (Leguminosae)

Wild Arachis germplasm includes potential forage species, such as the rhizomatous Arachis glabrata and the stoloniferous A. pinto and A. repens. Commercial cultivars of A. pintoi have already been released in Australia and in several Latin American countries, and most of these cultivars were derived from a single accession of A. pintoi (GK 12787). Arachis repens is less productive as a forage plant than is A. pintoi. However, it can be crossed with A. pintoi, and thus has good potential as germplasm for the improvement of A. pintoi. Arachis repens is also used as an ornamental plant and ground cover. Many new accessions of these two stoloniferous species are now available, and they harbor significant genetic variability beyond that available in the few older accessions, previously available. Therefore, these new accessions need to be conserved, documented and considered in terms of their potential for crop improvement and direct commercial use. Sixty-four accessions of this new germplasm were analyzed using RAPD analysis. Most of the accessions of A. repens grouped together into a clearly distinct group. In general, the accessions from the distinct valleys of the Jequitinhonha, São Francisco and Paranã rivers did not group together, suggesting there is not a tight relation between dispersion by rivers and the geographic distribution of genetic variation in these species.     

Cytoplasmic genetic diversity in bananas and plantains

Summary Concerns over yield declines in bananas and plantains due to the spread of Black Sigatoka disease in Musa have drawn attention to the collection of Musa germplasm and its use in conventional and biotechnological improvement programs. This report demonstrates the use of chloroplast DNA (cpDNA) restriction fragment length polymorphisms (RFLPs) for differentiating cytoplasms of various Musa clones. DNA was extracted from lyophilized leaf blade tissue and digested with either Eco R1, Hind III, Bam H1 or Pst I. Southern blots onto nylon membranes were probed with radioactively labeled heterologous orchid and lettuce cpDNA fragments. Among the 14 Musa clones examined, a single balbisiana and four acuminata-type cytoplasms were differentiated. The ability to distinguish between cytoplasms and to place plants within a cytoplasmic grouping demonstrates the usefulness of RFLP technology in evaluating diversity and determining the ancestry of Musa clones.     

Analysis of meiotic behavior in selecting potential genitors among diploid and artificially induced tetraploid accessions of Brachiaria ruziziensis (Poaceae)

Sexuality is correlated with diploidy and apomixis with polyploidy in the Brachiaria genus. Brachiaria ruziziensis is a key species in Brachiaria breeding due to its obligate sexuality and intrinsic agronomic qualities. Interspecific crosses in the genus became feasible only when a few diploid accessions of B. ruziziensis were artificially tetraploidized and remained sexual. Hybridization has been done since, using natural tetraploid apomictic accessions of B. brizantha or B. decumbens as pollen donors. Twenty two accessions of B. ruziziensis from the Embrapa Beef Cattle germplasm collection (Campo Grande, MS, Brazil) were cytologically analyzed: 16 are natural diploids (2n = 2x = 18) and six are artificially induced tetraploids (2n = 4x = 36). The meiotic behavior in the 16 diploid accessions varied. The mean of meiotic abnormalities per accession ranged from zero to 24.46%. Meiotic behavior in the induced tetraploid accessions also varied with the mean of meiotic abnormalities ranging from 5.20% to 54.71%. The most common abnormalities observed in both the diploid and the tetraploid accessions, were those related to irregular chromosome segregation. In one tetraploid accession, with a high frequency of those, other irregularities involving chromosome orientation at metaphase plate and chromosome convergence to the poles, a meiotic mutation known as divergent spindle, were recorded. Meiotic behavior should be considered in selecting potential genitors for breeding.     

The potential of high-resolution BAC-FISH in banana breeding

The genetic complexity in the genus Musa has been subject of study in many breeding programs worldwide. Parthenocarpy, female sterility, polyploidy in different cultivars and limited amount of genetic and genomic information make the production of new banana cultivars difficult and time consuming. In addition, it is known that part of the cultivars and related wild species in the genus contain numerous chromosomal rearrangements. In order to produce new cultivars more effectively breeders must better understand the genetic differences of the potential crossing parents for introgression hybridization, but extensive genetic information is lacking. As an alternative to achieve information on genetic collinearity we make use of modern chromosome map technology known as high-resolution fluorescent in situ hybridization (FISH). This article presents the technical aspects and applications of such a technology in Musa species. The technique deals with BAC clone positioning on pachytene chromosomes of Calcutta 4 (Musa acuminata ssp. burmanicoides, A genome group, section Eumusa) and M. velutina (section Rodochlamys). Pollen mother cells digestion with pectolytic enzymes and maceration with acetic acid were optimized for making cell spread preparations appropriate for FISH. As an example of this approach we chose BAC clones that contain markers to known resistance genes and hybridize them for establishing their relative positions on the two species. Technical challenges for adapting existing protocols to the banana cells are presented. We also discuss how this technique can be instrumental for validating collinearity between potential crossing parents and how the method can be helpful in future mapping initiatives, and how this method allows identification of chromosomal rearrangements between related Musa species and cultivars.     

Phylogenetic relationships as in Ocimum revealed by RAPD markers

Genetic relationships were examined among thirty germplasm accessions belonging to five Ocimum species using RAPD markers. A very high degree of polymorphism (98.20%) was observed. UPGMA cluster analysis of genetic similarity indices grouped all the accessions into two major clusters corresponding to previously reported botanical sections. Intra-clustering within the two clusters precisely grouped the accessions belonging to one species in one sub-cluster as expected from their genetic background. Our results show that RAPD technique is a sensitive, precise and efficient tool for genomic analysis in Ocimum species, that may be useful in future studies, by assigning new unclassified germplasm accessions to specific taxonomic groups and reclassifying previously classified accessions of other Ocimum species by traditional criteria on a more objective basis. This revised version was published online in August 2006 with corrections to the Cover Date.     

Analysis of bulked and redundant accessions of Brassica germplasm using assignment tests of microsatellite markers

This study was conducted to determine if Brassica germplasm bulks created and maintained by the USDA-ARS North Central Plant Introduction Station (NCRPIS) were made with genetically indistinguishable component accessions and to examine newly identified putative duplicate accessions to determine if they can be bulked. Using ten microsatellite primer pairs, we genotyped two bulks of B. rapa L. ssp. dichotoma (Roxb.) Hanelt comprising four accessions and three bulks of B. rapa L. ssp. trilocularis (Roxb.) Hanelt comprising fourteen accessions, as well as four pairs of putatively duplicate accessions of B.␣napus L. Assignment tests on ten individual plants per accession were conducted using a model-based clustering method to arrive at probabilities of likelihood of accession assignment. The assignment tests indicated that one of the two bulks of B. rapa ssp. dichotoma involves genetically heterogeneous accessions. It was observed in the B. rapa ssp. trilocularis bulks that the component accessions could be differentiated into groups, with misassignments observed most frequent within groups. In B. napus, only one of the four pairs of putative duplicates showed significant genetic differentiation. The other three pairs of putative duplicates lack differences and support the creation of bulks. The results of the assignment tests were in agreement with cluster analyses and tests of population differentiation. Implications of these results in terms of germplasm management include the maintenance and/or re-creation of some Brassica germplasm bulks by excluding those accessions identified as being unique in this study.     

Osmoregulative capacity in birdseed millet under conditions of water stress. I. Variation in Setaria italica and Panicum miliaceum

Experiments involving 14 accessions of Panicum miliaceum L. (Proso millet) and 11 accessions of Setaria italicaL. (Foxtail millet) have demonstrated variability in the degree of osmoregulative capacity among these accessions. Birdseed millet is generally claimed to be sensitive to drought stress, apparently because of a shallow root system. Accessions with high osmoregulative capacity demonstrate at least some drought tolerance. Osmoregulative capacity was measured on flag leaves of headed millet plants in pots undergoing water stress in a controlled environment chamber. Osmoregulative capacity was determined from the relationship between osmotic potential and leaf water potential; and the logarithmic relationship between osmotic potential and relative water content. The group of accessions of S. italica showed an overall level of osmoregulative capacity which was greater than that observed for the group of P. miliaceum accessions. Four accessions of S. italica(108042, 108463, 108541 and 108564) and one accession of P. miliaceum (108104)demonstrated high osmoregulative capacity. Differences of 1.05 MPa or more between observed and estimated osmotic potential were found at relative water contents of80 % among these accessions. The extent of osmoregulative capacity was associated with osmotic potential at full turgor and the rate of decline in osmotic potential as leaf water potential declined. This revised version was published online in August 2006 with corrections to the Cover Date.     

Genomic variation and genetic relationships in Ipomoea spp.

Random amplified polymorphic DNA (RAPD) markers were used to develop genetic fingerprints and analyse genetic relationships among 29 Ipomoea accessions from different geographical locations around the world, including unique wild species, and reproducible profiles were obtained for all accessions using random decamer primers. The primers generated 46 polymorphic markers, one primer alone having 10 products, enabling the discrimination of all 29 accessions. A high level of genetic variability in sweet potato collections was suggested by the degree of polymorphism. Half of the Japanese land races were closely related while accessions from Papua New Guinea and The Philippines were distinct and exhibited the greatest genetic diversity. The wild species Ipomoea gracilis and Ipomoea tiliacea formed a group distinct from the cultivated sweet potato. The wild tetraploid accession K233 and the species Ipomoea trifida were progressively more related genetically to the cultivated sweet potato and are the probable progenitors of Ipomoea batatas, and may be suitable as germplasm for genetic enhancement. RAPDs proved to be useful for sweet potato systematics and should be valuable for germplasm management, gene tagging and efficient choice of parents in breeding programmes.     

Stability of cassava plants at the DNA level after retrieval from 10 years of in vitro storage

Summary Cassava (Manihot esculenta Crantz) germplasm collections are conventionally maintained by continuous vegetative propagation in the field. Tissue culture techniques provide a more convenient way to conserve germplasm. The cassava in vitro gene bank held in trust at CIAT comprises nearly 6000 accessions. A study was carried out to determine whether any DNA rearrangements resulting from in vitro storage under slow growth could be detected by molecular analysis in retrieved plants. RFLPs with homologous probes, RAPDs with twenty primers and DNA fingerprinting with M13 probe were tested to detect variation at DNA level in cassava plants after ten-years in vitro storage. The molecular marker data obtained in this study supports the stability of the cassava germplasm under the in vitro storage conditions described in this work.     

Inheritance of dwarfism in plantain (Musa spp., AAB group)

Plantain (Musa spp., AAB group) is a major food crop in the humid lowland tropics of Africa, the Caribbean and Latin America. Lodging, caused by strong winds occurs periodically in these areas. Therefore, development of dwarf plantain cultivars and elucidation of the inheritance of dwarfism is desirable. A heterozygous normal plantain cultivar (2n= 2x) with long false-internodes (19.4 ± 0.9 cm) was crossed with a homozygous wild banana (2n= 2x) with short false-internodes (6.9 ± 0.4 cm) to develop a test-cross segregating population. A total of 74 euploids (2x, 3x and 4x) were produced. Forty-one normal and 24 dwarf diploids were obtained, which fits a 2: 1 trisomic test-cross segregation ratio for one locus. Dwarfism is controlled by a single recessive gene, dw. At the tetraploid level, two normal and six dwarf hybrids were produced, suggesting that the dw locus is close to the centromere and that there is a dosage effect of the dw allele at the tetraploid level.     

Beet mild yellowing virus resistance derived from wild and cultivated Beta germplasm

Sugar beet (Beta vulgaris ssp. vulgaris L.) has a relatively narrow genetic base and several programmes have sought to evaluate the potential for introducing novel traits from wild and cultivated Beta germplasm into the crop. In particular, resistance to important sugar beet diseases has been identified within individual Beta accessions. We report here the successful transfer of resistance to Beet mild yellowing virus (BMYV) from garden beet, fodder beet and leaf beet accessions to progeny populations in initial crosses with sugar beet. Twelve plant populations derived from different Beta accessions were inoculated with viruliferous aphids carrying BMYV and the virus content of individual plants subsequently quantified by an ELISA test. Seven populations were significantly more resistant than a control sugar beet cultivar (P ≤ 0.05). BMYV resistance was successfully inherited in BC₁ and BC₂ generations, suggesting that resistance could potentially be introgressed from these sources into elite sugar beet lines.     

The pattern of genetic diversity in the Andean grain crop quinoa (Chenopodium quinoa Willd). I. Associations between characteristics

Summary A germplasm collection comprising 294 accessions of quinoa was evaluated at Cambridge, England, to assess its potential as a break crop for arable agriculture at temperate latitudes. The origins of the accessions ranged from 40°S to 2°N and from sea level to an altitude of 3800 m, and 19 discrete and continuous characteristics describing the plants' pigmentation and morphology and the duration of developmental phases were noted or measured. There was significant variation among accessions for all the continuous characteristics, and the plant and inflorescence dimensions were all fairly strongly correlated, but the associations between the durations of developmental phases were surprisingly weak, suggesting that there is great scope for manipulation of the pattern of development through breeding. Contingency tables revealed associations between pairs of discrete characteristics. The means and variances of many continuous characteristics differed according to the discrete characteristics: for example accessions with branched plants took longer to germinate and were more variable in this respect than those with unbranched plants. Sometimes one form of a discrete characteristic was associated with a homogeneous group of accessions but the alternative form was not: such a relationship would be easily lost in a multivariate analysis. On the basis of there relationships, and using knowledge of the accessions' origins, they were classified into seven groups. It is concluded that the plant characteristics required for temperate agriculture are present to a large extent in the accessions from near sea level in southern-central Chile, but that the seed characteristics are scattered throughout the germplasm.     

RFLP characterization of Indian Musa germplasm for clonal identification and classification

Summary Nineteen single-copy clones isolated from a PstI genomic library (cv. Maiden Plantain), and eight Vigna chloroplast DNA clones were used to probe total genomic DNA digests of 57 genotypes of Musa from India. The 19 genomic clones detected a total of 107 polymorphisms among the 57 genotypes. Principal coordinates and phenetic analyses of these data placed cultivars and species into distinct groups that were in general agreement with a previously published RAPD-based classification of these same plant materials. The 107 polymorphisms were sufficient to differentiate each clone from every other clone. Heterologous Vigna chloroplast DNA probes were used to characterize the cytoplasm of Musa cultivars and species. PCO analysis of these RFLPs were detected both within and between the generally recognized genome groups, indicating multiple hybridization pathways in the origin of hybrid clones. Data presented demonstrate that RFLPs are sufficiently abundant to classify Musa germplasm and that genetic relationships among Musa cultivars, based upon RFLP data, are in general agreement with relationships determined by analysis of morphology and RAPDs.     

Survey of the Bru1 gene for brown rust resistance in Brazilian local and basic sugarcane germplasm

Bru1 is currently the major gene conferring brown rust resistance in sugarcane, and diagnostic markers are available. A survey for the presence of this gene was conducted on 391 genotypes including Brazilian cultivars, clones and basic germplasm. The efficiency of these markers for identifying resistant cultivars and artificially inoculated basic germplasm was also evaluated. The Bru1 frequency among cultivars (73.5%) suggests this gene is the prevalent source of brown rust resistance in Brazilian sugarcane breeding programmes. Most of the cultivars known to be resistant were positive for Bru1, although other genes for resistance could be present in lines not having Bru1. Only 17.8% of the basic germplasm accessions were positive for the Bru1 gene, and a low correlation between Bru1 diagnostic markers and brown rust severity was observed for basic germplasm accessions. Overall, Bru1 diagnostic markers proved to be efficient identifying resistant cultivars and clones and have potential to be in screening brown rust resistance in Brazilian breeding programmes.     

Genetic relationships among 23 Ficus accessions using inter-simple sequence repeat markers

Ficus genus is widely distributed in all the climatic stages and great diversity. The exploration of genetic diversity is a pre-requisite for genome organization in the wild species and the related domesticated ones. Inter-Simple Sequence Repeat (ISSR) markers were used to assess the identification of 23 important Ficus accessions and determination of the genetic relationships among these accessions. Out of 21 ISSR primers tested, five primers produced 116 detectable fragments, of which 106 (91.3%) were polymorphic across the accessions. Each of the five primers produced a fingerprint profile unique to each of the accessions studied, and thus could be solely used for their identification. Thirteen unique bands specific to nine species were detected. These may be converted into species-specific probes for identification purposes. Genetic relationships among these accessions were evaluated by generating a similarity matrix based on the Dice coefficient and the Unweighted Pair Group Method with Arithmetic Average (UPGMA) dendogram. The results showed a clear cut separation of the 23 Ficus accessions and were in broad agreement with the morphology. Both molecular and morphological markers will be useful for preservation of the Ficus germplasm.