The release of new sweet-acid banana cultivars resistant to black leaf streak, Sigatoka leaf spot and Fusarium wilt is important for domestic markets in tropical and subtropical countries. Common ...current breeding strategies consist of selecting tetraploid AAAB hybrids directly from crosses between AAB sweet-acid cultivars pollinated with AA clones carrying disease-resistance genes. However, this crossing pathway is hampered by low gamete fertility and the rare occurrence of desired 2N gametes on the AAB female side (N=3X=33 chromosomes). We propose an alternative pathway which aims to create new triploid hybrids directly from AB landraces. Natural AB clones are sterile but their AABB tetraploid counterparts obtained by colchicine treatment are fertile. This gamete fertility was made profitable in crosses with AA and BB accessions to generate AAB and ABB hybrids. We present here agronomical results of various progenies involving an in vitro synthesized tetraploid ‘Kunnan’ (AABB) and several AA and BB clones. These first results suggest the very high potential of this new strategy for the release of well-performing new hybrids combining higher productivity, disease resistance and better fruit quality. Hybrids with a high added-value, produced in this way, could be made available for evaluation within the ProMusa network.
In banana, the maternal transmission of chloroplast DNA and paternal transmission of the mitochondrial DNA provides an exceptional opportunity for studying the maternal and paternal lineage of ...clones. In the present study, RFLP combined with hybridization of heterologous mitochondrial and chloroplastic probes have been used to characterize 71 wild accessions and 131 diploid and 103 triploid cultivated clones. In addition to Musa acuminata and Musa balbisiana, other species from the four Musa sections were studied to investigate their contribution to the origin of cultivated bananas. These molecular analyses enable the classification of the Musa complex to be discussed. Results ascertain relationships among and between the wild accessions and the mono- and interspecific diploid and triploid bananas, particularly for the acuminata genome. Parthenocarpic varieties are shown to be linked to M. acuminata banksii and M. acuminata errans, thus suggesting that the first center of domestication was in the Philippines-New Guinea area.
Banana hybrids with resistance to Yellow Sigatoka and Black Leaf Streak disease were evaluated for resistance to the burrowing nematode Radopholus similis and to the lesion nematode Pratylenchus ...coffeae in a growth chamber at 24-28°. Plants produced by tissue culture were acclimatised for 6 weeks prior to inoculation. Forty-five days after inoculation with nematodes, the root systems were processed and nematode numbers assessed. Two cultivars of Grande Naine (Musa AAA, Cavendish subgroup, ITC1256 and cv902) and one cultivar of Yangambi Km5 (Musa AAA, Ibota subgroup, ITC1123) were used respectively as susceptible and resistant controls. Results based on multiplication rates and root infestations showed that three of these hybrids (FB918, FB919 and FB924) were not significantly different from the resistant control Yangambi Km5 with a lower multiplication of R. similis. Similarly four of these hybrids (FB918, FB919, FB920, FB924) showed a lower multiplication of P. coffeae, not significantly different from the same resistant control. This is the first study that shows a partial resistance to both nematode species, R. similis and P. coffeae within synthetic hybrids of M. acuminata, adding an important extra value to these dessert banana hybrids formerly bred to resist to Mycosphaerella leaf spot diseases.
Nuclear genome size variation was studied in Musa acuminata (A genome), Musa balbisiana (B genome) and a range of triploid clones differing in genomic constitution (i.e. the relative number of A and ...B genomes). Nuclear DNA content was estimated by flow cytometry of nuclei stained by propidium iodide. The A and B genomes of Musa differ in size, the B genome being smaller by 12% on average. No variation in genome size was found among the accessions of M. balbisiana (average genome size 537 Mbp). Small, but statistically significant, variation was found among the subspecies and clones of M. acuminata (ranging from 591 to 615 Mbp). This difference may relate to the geographical origin of the individual accessions. Larger variation in genome size (8.8%) was found among the triploid Musa accessions (ranging from 559 to 613 Mbp). This variation may be due to different genomic constitutions as well as to differences in the size of their A genomes. It is proposed that a comparative analysis of genome size in diploids and triploids may be helpful in identifying putative diploid progenitors of cultivated triploid Musa clones. Statistical analysis of data on genome size resulted in a grouping which agreed fairly well with the generally accepted taxonomic classification of Musa.
A partial molecular linkage map of the Musa acuminata diploid genome is presented. This map is based on 58 RFLP, four isozyme and 28 RAPD markers segregating in an F2 population of 92 individuals. A ...total of 90 loci was detected, 77 of which were placed on 15 linkage groups while 13 segregated independently. Segregation distortions were shown by 36% of all loci, mostly favoring the male parent. Chromosome structural rearrangements were believed to be one of the main causes of these distortions. The use of genetic linkage data to further the genetic and evolutionary knowledge of the genus Musa, as well as to help improve the design of breeding strategies, is discussed.
For several years, CIRAD (Centre de coopération internationale en recherche agronomique pour le développement) has invested in the genetic improvement of banana and plantain (Musa spp.) with the goal ...to make production, both for export trade and local consumption, more environmentally sustainable. A crossing strategy was developed to obtain triploid hybrid varieties directly from diploid plant material. Molecular tools have now been developed to support this conventional cross-breeding scheme. Molecular markers have given new insight in Musa genetic diversity, leading to a better understanding of the relationships between modern cultivars and their ancestors, thereby providing critical information for the choice of parent material. The Musa genome structure and genetic maps have been investigated, clarifying the mating behaviour of gametes during meiosis and giving insight in how to manage ploidy and interspecificity. Genes involved in character expression have been identified, and genome sequencing is ongoing as a result of an international effort. The development of these molecular tools provides breeders with important information to increase the efficiency of genetic improvement efforts. This paper presents how the molecular approach is actually implemented in CIRAD’s breeding strategy.
The origin of triploid export banana cultivars was investigated. They all belong to Cavendish and Gros Michel subgroups of triploid clones and have a monospecific Musa acuminata origin. The ...appearance of these cultivars is thought to be result of hybridization between partially sterile diploid cultivars producing non reduced gametes and fertile diploids producing normal haploid gametes. To trace these diploid ancestors we compared the RFLP patterns, revealed by 36 probe/enzyme combinations, of 176 diploid clones representing the worldwide available variability with that of clones from the Cavendish and Gros Michel subgroups. This lead us to the identification of the common putative diploid ancestor of cultivars from Cavendish and Gros Michel subgroups which contributed to triploid cultivar formation through the production of 2n restitution gametes. For cultivars of Gros Michel subgroup we also propose a normal gamete donor that may have complemented the triploid allele set.
Restriction fragment length polymorphisms (RFLPs) were used as markers to determine the transmission of cytoplasmic DNA in diploid banana crosses. Progenies from two controlled crosses were studied ...with heterologous cytoplasmic probes. This analysis provided evidence for a strong bias towards maternal transmission of chloroplast DNA and paternal transmission of mitochondrial DNA in Musa acuminata. These results suggest the existence of two separate mechanisms of organelle transmission and selection, but no model to explain this can be proposed at the present time. Knowledge of the organelle mode of inheritance constitutes an important point for phylogeny analyses in bananas and may offer a powerful tool to confirm hybrid origins.
We have developed a statistical model for the flashover of a 45° vacuum-insulator interface (such as would be found in an accelerator) subject to a pulsed electric field. The model assumes that the ...initiation of a flashover plasma is a stochastic process, that the characteristic statistical component of the flashover delay time is much greater than the plasma formative time, and that the average rate at which flashovers occur is a power-law function of the instantaneous value of the electric field. Under these conditions, we find that the flashover probability is given by 1−exp(−EpβteffC/kβ) , where Ep is the peak value in time of the spatially averaged electric field E(t) , teff≡∫E(t)/Epβdt is the effective pulse width, C is the insulator circumference, k∝exp(λ/d) , and β and λ are constants. We define E(t) as V(t)/d , where V(t) is the voltage across the insulator and d is the insulator thickness. Since the model assumes that flashovers occur at random azimuthal locations along the insulator, it does not apply to systems that have a significant defect, i.e., a location contaminated with debris or compromised by an imperfection at which flashovers repeatedly take place, and which prevents a random spatial distribution. The model is consistent with flashover measurements to within 7% for pulse widths between 0.5 ns and 10μs , and to within a factor of 2 between 0.5 ns and 90 s (a span of over 11 orders of magnitude). For these measurements, Ep ranges from 64 to 651kV/cm , d from 0.50 to 4.32 cm, and C from 4.96 to 95.74 cm. The model is significantly more accurate, and is valid over a wider range of parameters, than the J. C. Martin flashover relation that has been in use since 1971 J. C. Martin on Pulsed Power, edited by T. H. Martin, A. H. Guenther, and M. Kristiansen (Plenum, New York, 1996). We have generalized the statistical model to estimate the total-flashover probability of an insulator stack (i.e., an assembly of insulator-electrode systems connected in series). The expression obtained is consistent with the measured flashover performance of a stack of five 5.72-cm-thick, 1003-cm-circumference insulators operated at 100 and 158kV/cm . The expression predicts that the total-flashover probability is a strong function of the ratio Ep/k , and that under certain conditions, the performance improves as the capacitance between the stack grading rings is increased. In addition, the expression suggests that given a fixed stack height, there exists an optimum number of insulator rings that maximizes the voltage at which the stack can be operated. The results presented can be applied to any system (or any set of systems connected in series) subject to random failures, when the characteristic statistical delay time of a failure is much greater than its formative time.
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