Inheritance of black sigatoka disease resistance in plantain-banana (Musa spp.) hybrids

Black sigatoka (Mycosphaerella fijiensis Morelet), an airborne fungal leaf-spot disease, is a major constraint to plantain and banana (Musa spp.) production world-wide. Gaining further knowledge of the genetics of host-plant resistance will enhance the development of resistant cultivars, which is considered to be the most appropriate means to achieve stable production. Genetic analysis was conducted on 101 euploid (2x, 3x and 4x) progenies, obtained from crossing two susceptible triploid plantain cultivars with the resistant wild diploid banana Calcutta 4. Segregating progenies, and a susceptible reference plantain cultivar, were evaluated over 2 consecutive years. Three distinct levels of host response to black sigatoka were defined as follows: susceptible (< 8 leaves without spots), less susceptible (8–10) and partially resistant (> 10). Segregation ratios for resistance at the 2x level fitted a genetic model having one major recessive resistance allele (bs ₁) and two independent alleles with additive effects (bsr ₂ and bsr ₃). A similar model explains the results at the 4x level assuming that the favourable resistance alleles have a dosage effect when four copies of them are present in their respective loci (bs _i ⁴ ). The proposed model was further validated by segregation data of S ₁ progenies. Mechanisms of black sigatoka resistance are discussed in relation to the genetic model.     

The influence of exposure to ultraviolet radiation in simulated sunlight on ascospores causing Black Sigatoka disease of banana and plantain

 The influence of ultraviolet (UV) radiation in simulated natural sunlight on the viability of ascospores of Mycosphaerella fijiensis, the cause of Black Sigatoka disease in banana and plantain, has been investigated as part of a study to assess the windborne spread of this pathogen from mainland Central and South America into the Caribbean. Spores were killed following continuous exposure to UV radiation for periods of 6 h or over. This relatively short exposure time suggests that the distances over which viable spores can be transported will be determined not only by the speed of the wind but also the amount of cloud cover and the time off day that spore release occurs. On this basis, wind dispersal of viable spores over distances greater than a few hundred kilometres is unlikely. These conclusions are reinforced by an examination of historical reports of the arrival of the disease in previously uninfected areas of the Americas and Africa. Received: 10 February 1997 / Accepted: 30 March 1998     

Identifying different types of de-differentiated microspores from indica-japonica F1 hybrids with subspecies-differentiating RFLP probes in rice

The indica, japonica and intermediary types of de-differentiated microspores from indica-japonica hybrids were identified with 11 subspecies-differentiating RELP probes in rice (Oryza sativa L.). The results showed that the distribution of indica, japonica and intermediary types of de-differentiated microspores could be easily detected in a simple and quick way using the RFLP method. Moreover, the microspores from the same hybrid but inoculated onto different media, or microspores from different hybrids when inoculated onto the same medium, often displayed distinctive distribution curves of de-differentiated microspores types, indicating that the media employed in this experiment had high selectivity for the de-differentiation of certain types of microspores. The application of the RELP method to de-differentiated microspore identification is of great theoretical and practical significance in rice doubled-haploid breeding. Received: 27 February 1996 / Accepted: 14 June 1996     

Quantitative trait loci for resistance against Yellow rust in two wheat-derived recombinant inbred line populations

Yellow rust, which is a major disease in areas where cool temperatures prevail, can strongly influence grain yield. To control this disease, breeders have extensively used major specific resistance genes. Unfortunately this kind of resistance is rapidly lost due to pathogen adaptation. More-durable resistance against yellow rust can be achieved using quantitative resistance derived from cultivars with well-established durable resistance. The winter wheat Camp Remy has maintained a high level of resistance for over 20 years. In order to map quantitative trait loci (QTLs) for durable yellow rust resistance, we analysed a set of 98 F₈ recombinant inbred (RI) lines derived from the cross Camp Remy×Michigan Amber. We also mapped QTLs for adult resistance to yellow rust using the International Triticae Mapping Initiative RI population (114 lines derived from the cross Opata85×synthetic hexaploid). Two and five QTLs, respectively, were identified from these two populations. This work has highlighted the importance of the centromeric region of chromosome 2B and the telomeric regions of chromosomes 2AL and 7DS in durable yellow rust resistance. The same chromosomal regions are also implicated in resistance to other pathogens. Received: 8 December 2000 / Accepted: 17 April 2001     

Effect of genome-plastome interaction on meiosis and pollen development in Oenothera species and hybrids

 Oenothera villaricae Dietrich and Oe. picensis Dietrich, complete translocation heterozygotes, are fully interfertile, giving rise to six discrete classes of true-breeding hybrids from a reciprocal cross. Associated with each parent and hybrid is a characteristic abortive non-staining pollen fraction easily distinguished from fully developed pollen under the light microscope. Pollen abortion has been associated with translocation rings in other angiosperm species, and may characterize such systems. The abortive pollen fraction is significantly different between reciprocal Oenothera hybrids, however (P<0.001), indicative of partial cytoplasmic control. Pollen abortion is most severe in the F₁ hybrid generation, and ameliorates with successive generations of hybrid self-fertilization. Three-way analysis of variance shows significant effects on pollen stainability (a measure of the non-abortive fraction) for nucleus, cytoplasm and selfed hybrid generation, individually or in combinations. This result suggests a combined nucleocytoplasmic basis for the pollen abortion. Correlated with the observation of increased pollen abortion in Oenothera hybrids are meiotic findings of broken chromosome rings (chains, univalents), asymmetric anaphase chromosome distributions and trinucleate tetrads. To test the hypothesis that such anomalous meiotic events play a role in the mechanism of pollen abortion, meiotic disjunction frequency was determined for each parent, F₁ and F₉ selfed hybrid accessions. Three-way analysis of variance shows levels of significance comparable to those noted for pollen stainability (P<0.001) for effects of nucleus, cytoplasm and selfed hybrid generation on disjunction frequency. A high degree of correlation (r ²=0.984) is noted between disjunction frequency and pollen stainability. We conclude that the abortive pollen grains are indeed the products of nondisjunctional meiotic events, which themselves are consequences of hybrid nucleocytoplasmic incompatibility. Received: 28 May 1997 / Revision accepted: 23 July 1997     

Changes in DNA-marker frequencies associated with response to contrasting selection methods in Arabidopsis

 Recurrent selection for specific combining ability (RS-SCA) and S1 family performance (RS-S1) were compared in replicated selection programs initiated from a common C0 base population of Arabidopsis. Three cycles of selection for aerial biomass were completed in each of two replicate programs of each selection method. Response to selection was measured on the basis of per se, S1 progeny, and testcross performance with a common tester. All selection programs improved testcross performance. Testcross gain per cycle in RS-S1 (7.15% cycle^-1) and RS-SCA (5.31% cycle^-1) were not significantly different. Performance of S1 progeny and populations per se significantly improved over cycles of selection using RS-S1 but were unchanged by RS-SCA. Codominant molecular marker-allele frequencies were recorded for each population at 22 polymorphic loci. Trends in marker-allele frequencies were tested by linear regression. Significant trends in marker-allele frequencies pooled over replicate programs were detected at 8 and 7 loci in the RS-S1 and RS-SCA programs, respectively. Marker alleles at 2 loci significantly changed frequency in response to both RS-S1 and RS-SCA programs. Marker alleles at 6 loci significantly changed frequency only in response to RS-SCA. Marker alleles at 6 other loci showed significant linear trends pooled over replicates only in RS-S1. No markers revealed increases in the frequency of different marker alleles within loci using the two selection methods. Possible genetic causes of marker frequency changes are discussed, as well as breeding implications.     

Organization of an Arabidopsis thaliana gene cluster on chromosome 4 including the RPS2 gene, in the Brassica nigra genome

 Genetic and physical maps, consisting of a large number of DNA markers for Arabidopsis thaliana chromosomes, represent excellent tools to determine the organization of related genomes such as those of Brassica. In this paper we report the chromosomal localization and physical analysis by pulsed-field gel electrophoresis (PFGE) of a well-defined gene complex of A. thaliana in the Brassica nigra genome (B genome n=8). This complex is approximately 30 kb in length in A. thaliana and contains a cluster of six genes including ABI1 (ABA-responsive), RPS2 (resistance against Pseudomonas syringae, a bacterial disease), CK1 (casein kinase I), NAP (nucleosome-assembly protein), X9 and X14 (both of unknown function). The Arabidopsis chromosomal complex was found to be duplicated and conserved in gene number at different levels in the Brassica genome. Linkage group B1 had the most-conserved arrangement carrying all six genes tightly linked. Group B4 had an almost complete complex except for the absence of RPS2. Other partial complexes of fewer members were found on three other chromosomes. Our studies demonstrate that by this approach it is possible to identify ancestrally related chromosome segments in a complex and duplicated genome, such as the genome of B. nigra, permitting one to draw conclusions as to its origin and evolution. Received: 11 July 1997 / Accepted: 9 October 1997     

Field assessments of gene flow from transgenic to cultivated rice (Oryza sativa L.) using a herbicide resistance gene as tracer marker

Development of plant genetic engineering has led to the deployment of transgenic crops and, simultaneously, to the need for a thorough assessment of the risks associated with their environmental release. This study investigated the occurrence of gene flow from transgenic rice to non-transgenic rice plants under agronomic conditions using a herbicide resistance gene as a tracer marker. Two field experiments were established in the paddy fields of two main Mediterranean rice-growing areas of Spain and Italy. In both locations analyses of phenotypic, molecular and segregation data showed that pollination of recipient plants with pollen of the transgenic source occurred at a significant frequency. A gene flow slightly lower than 0.1% was detected in a normal side-by-side plot design. Similar results were found in a circular plot when the plants were placed at 1-m distance from the transgenic central nucleus. A strong asymmetric distribution of the gene flow was detected among this circle and highest values (0.53%) were recorded following the direction of the dominant wind. A significant lowest value (0.01%) was found in the other circle (5 m from the transgenic plants) as was expected according to the characteristics of rice pollen. Such circular-field trial designs could also prove to be very useful in studying the gene flow to other commercial cultivars of rice with the aim of establishing strategies to prevent pollen dispersal from commercial transgenic fields to the neighbouring conventional fields. Received: 23 February 2001 / Accepted: 31 March 2001     

Contributions of subpopulations to total gene diversity

 The concept of the partitioning of genetic diversity into a component within and a component among populations (F _ST - or G _ST -statistics) can be easily expanded to compute the contribution of single subpopulations to total gene diversity. A subpopulation contributes to total gene diversity with its single-population gene diversity plus the (weighted) mean of Nei’s minimum genetic distances to all subpopulations. The suggested method allows one to unambiguously rank subpopulations according to the amount they contribute to the total gene diversity. Genetic polymorphisms at four isozyme gene loci of Alnus acuminata in Costa Rica are used to illustrate the procedure and its biological interpretation. Received: 15 August 1998 / Accepted: 8 September 1998     

RFLP mapping of resistance to chlorosis induction by Pyrenophora tritici-repentis in wheat

Tan spot, caused by Pyrenophora tritici-repentis, is an economically important disease in major wheat production areas. The fungus can produce two genetically distinct symptoms on leaves of susceptible wheat genotypes: tan necrosis (nec) and extensive chlorosis (chl). Our objectives were to determine the number of genes conditioning resistance to tan spot in a population of wheat recombinant inbred lines, and map the chromosomal location of the resistance genes using RFLPs. Conidia produced by the P. tritici-repentis isolate Pti2 (nec+chl+) were used to inoculate seedlings of 135 recombinant inbred lines derived from the cross of the synthetic hexaploid wheat W-7984 with Opata 85. A subset of the population was inoculated with conidia produced by the isolates D308 (nec−chl+) and 86-124 (nec+chl−). Inoculated seedlings were rated on a scale of 1 to 5 based on lesion type. Necrosis-inducing culture filtrate produced by the isolate 86-124 was also used to screen the entire population. A map consisting of 532 markers was employed to identify significant associations between marker loci and tan spot resistance. The entire population was insensitive to culture filtrate produced by the isolate 86-124, and the entire subset was resistant to conidial inoculation of the same isolate. The population segregated for reaction to isolates D308 and Pti2, indicating that this population segregates for resistance to extensive chlorosis only, and not to tan necrosis. RFLP analysis indicated the presence of a gene with a major effect in 1AS, a gene with a minor effect in 4AL, and an interaction between the 1AS gene and a gene in 2DL. Together, these loci explained 49.0% of the variation in this population for resistance to tan spot produced by the isolate Pti2. Two regions one in 1BL and one in 3BL, were significantly associated with resistance to extensive chlorosis, but were not significant in the multiple regression model. It should be feasible to introgress these resistance loci into adapted genetic backgrounds by using a marker-assisted selection scheme. Received: 30 March 1996 / Accepted: 31 May 1996     

Genetic analysis of durable leaf rust resistance in winter wheat

Quantitative resistance that delays the epidemic development of leaf rust in wheat is an important source for durable resistance breeding. The Swiss winter wheat variety ’Forno’ shows a high level of quantitative resistance against leaf rust. This resistance has been effective for more than 10 years and can therefore be considered to be durable. In order to map quantitative trait loci (QTL) for durable leaf rust resistance we analysed 204 F₅ recombinant inbred lines (RILs) of the cross between the winter wheat ’Forno’ and the winter spelt ’Oberkulmer’ for their level of leaf rust resistance (LR) and leaf tip necrosis (LTN) in four different environments. Both traits showed a continuous distribution and were significantly correlated (r=−0.5). Across environments we detected 8 QTL for leaf rust resistance (6 inherited from ’Forno’) and 10 QTL for the quantitative expression of LTN (6 inherited from ’Forno’). Of the 6 QTL responsible for the durable leaf rust resistance of ’Forno’, 1 major QTL coincided with a thaumatin locus on 7BL explaining 35% of the phenotypic variance. Four QTL for LR coincided with QTL for LTN. At these loci the alleles of ’Forno’ increased the level of resistance as well as the extent of LTN, indicating pleiotropy. Received: 1 July 1999 / Accepted: 30 July 1999     

Molecular characterization of a tomato polygalacturonase gene abundantly expressed in the upper third of pistils from opened and unopened flowers

 A polygalacturonase (PG) gene, TPG7 (Lyces;Pga1;8), has been cloned from tomato (Lycopersicon esculentum Mill., cv. Rutgers). RNA blot analysis reveals that TPG7 is highly expressed in pistils (ovary removed) from unopened and fully open flowers. Dissection of mature pistils demonstrated that TPG7 expression is limited to the top third (stigmatic region) of the pistils. This is contrasted with another tomato PG, TAPG4, which is also expressed in the same region of the pistil but only in mature pistils from fully open flowers. Hybridization of the TPG7 probe to anther RNA was nil to none and was barely detectable in RNA from leaf and flower abscission zones. The TPG7 polypeptide shares 39% sequence identity with the tomato fruit PG and between 63% and 73% sequence identities with six other tomato PGs. Received: 15 March 1999 / Revision received: 6 October 1999 / Accepted: 7 Oktober 1999     

Mapping of the Rf-3 nuclear fertility-restoring gene for WA cytoplasmic male sterility in rice using RAPD and RFLP markers

The cytoplasmic male sterility (CMS) of wild-abortive (WA) cytoplasm has been widely used for breeding hybrid rice. Two restorer genes for the CMS have been found by traditional genetic analysis. To tag the restorer genes we used a set of near-isogenic lines (NILs) of Zhenshan 97 carrying different genotypes for fertility restoration from IR24, to perform RAPD analysis. From the survey of 720 random primers, six RAPD markers were identified to be associated with Rf-3. Three of these OPK05-800, OPU10-1100 and OPW01-350, were mapped on chromosome 1. Two populations from the crosses between Zhenshan 97 A and a near-isogenic restorer line ZSR21 and between Zhenshan 97 A and IR24 were used for mapping Rf-3. The three RAPD markers and three RFLP markers, RG532, RG140 and RG458, were found to be closely linked to Rf-3 in the two populations. The same location of Rf-3 was also found in a population from the cross of IR58025 A//IR36/IR58025 B. At the RG532 locus, different alleles were found between two CMS lines, Zhenshan 97 A and IR58025 A, and between two restorer lines, IR24 and IR36. The use of these molecular markers closely linked to Rf-3 in facilitating the development of hybrid rice is discussed. Received: 3 January 1996 / Accepted: 17 May 1996     

Assessment of a species-specific element (Brep 1) in banana

 The nuclear genome of wild-type banana accessions was investigated for repetitive elements. We report here the occurrence, in the banana genome, of a sequence family of species-specific repetitive elements: Brep 1. This sequence family is distributed throughout the Musaceae with various copy numbers. The two species Musa acuminata and M. schizocarpa carry the highest copy numbers in contrast to M. balbisiana and tested representatives of different other sections. PCR primers were defined in the core consensus sequence for specific amplifications, which allow representatives of this sequence family to be easily detected in wild and cultivated banana clones. Sequence data were analysed and hypotheses on the evolution of banana cultivars from the wild-type banana clones are discussed. Received: 17 January 1997 / Accepted : 7 March 1997