Genetic and physical mapping of telomeres and macrosatellites of rice

Telomeres and telomere-associated satellites of rice were genetically and physically analyzed by pulsed-field gel electrophoresis (PFGE) using Arabidopsis telomeric DNA and rice satellite sequences as probes. We demonstrate that Arabidopsis telomeric sequences hybridize to rice telomeres under the conditions of high stringency. Using the Arabidopsis probe, multiple, discrete telomeric fragments could be identified on pulsed-field gel blots of rice DNAs digested with rare-cutting restriction enzymes. Most of the telomeric bands larger than 300 kb are physically linked with satellite bands as revealed by PFGE. Some of the telomeric and satellite bands segregate in a Mendelian fashion and are highly reproducible. Three such telomeric bands have been mapped to the distal ends of RFLP linkage groups: Telsm-1 on chromosome 8, Telsa-1 on chromosome 9 and Telsm-3 on chromosome 11. One segregating satellite band was mapped to an internal region of chromosome 10. Telomeric fragments were shown not only to be genetically linked to but also physically linked (based on PFGE) to the terminal RFLP markers. The physical distance from telomeric sequences to a distal RFLP marker, r45s gene, on chromosome 9, is 200 kb while the distance from telomeric sequences to RG98, a terminal RFLP marker on chromosome 11, is 260 kb. Physical maps of the telomere regions of chromosome 9 and chromosome 11 are presented.     

Molecular mapping of the py-1 gene for resistance to corky root rot (Pyrenochaeta lycopersici) in tomato

 We report the molecular mapping of the py-1 gene for resistance to corky root rot [Pyrenochaeta lycopersici (Schneider and Gerlach)] in tomato using RAPD and RFLP marker analysis. DNA from near-isogenic lines (NILs) of tomato differing in corky root rot resistance was screened with 575 random oligonucleotide primers to detect polymorphic DNAs linked to py-1. Three primers (OPW-04, OPC-02, OPG-19) revealed polymorphisms between the NILs. Twelve resistant and eight susceptible DNA pools derived from segregating F₃ families were used to confirm that the RAPD markers were linked to the py-1 gene. Two of the linked amplified fragments, corresponding to OPW-04 and OPC-02, were subsequently cloned and mapped on the tomato molecular linkage map as RFLPs. These clones were located between TG40 and CT31 on the short arm of chromosome 3. Further analysis with selected RFLP markers showed that 7% (8.8 cM) of chromosome 3 of the resistant line ‘Moboglan’ was introgressed from the L. peruvianum donor parent. Three RFLP markers (TG40, TG324, and TG479) from the introgressed part of chromosome 3 were converted to cleaved amplified polymorphism (CAP) markers for use in a polymerase chain reaction (PCR) assay. These PCR markers will allow rapid large-scale screening of tomato populations for corky root rot resistance. Received: 2 January 1998 / Accepted: 12 January 1998     

Patterns of genetic variation of the genusCapsicum (Solanaceae) in Mexico

The evolutionary relationships of 186 accessions ofCapsicum from Mexico were studied through enzyme electrophoresis. A total of 76 alleles representing 20 genetic loci coding for nine enzyme systems were observed and the allelic variations of enzymes were studied for geographical distribution. Allele frequencies were used to estimate the apportionment of gene diversity within and between populations and to construct a dendrogram based on a similarity matrix containingNei genetic distances. — The gene diversity estimates suggest that the structure ofCapsicum populations in Mexico consists of predominantly homozygous genotypes presumably due to a self-pollinated breeding system and population bottlenecks. Significant genetic differentiation was found mainly between populations of differing geographical regions.—Based on the results of this study, three species of domesticatedCapsicum can be identified in Mexico,C. annuum var.annuum, C. chinense, andC. pubescens. Semidomesticated and wild forms include two species,C. frutescens andC. annuum var.glabriusculum. A sharp geographical division results between the latter species;C. frutescens was collected exclusively in the southeastern states of Oaxaca, Chiapas, and Tabasco; whereas wild and semidomesticated forms from the rest of the country areC. annuum. Based upon the similarity of enzyme genotypes of semidomesticated and wild forms, the primary center of domestication of cultivatedC. annuum was estimated to be the region comprising the states of Tamaulipas, Nuevo Leon, San Luis Potosi, Veracruz, and Hidalgo in eastern Mexico. A possible second center of domestication is suggested to be localized in the state of Nayarit, western Mexico.     

Molecular mapping of genes for resistance to rice blast (Pyricularia grisea Sacc.)

Two dominant genes conferring complete resistance to specific isolates of the rice blast fungus, Pyricularia grisea Sacc., were located on the molecular map of rice in this study. Pi-l(t) is a blast resistance gene derived from the cultivar LAC23. Its map location was determined using a pair of nearly isogenic lines (NILs) and a B₆F₃ segregating population from which the isoline was derived. RFLP analysis showed that Pi-l(t) is located near the end of chromosome 11, linked to RZ536 at a distance of 14.0±4.5 centiMorgans (cM). A second gene, derived from the cultivar Apura, was mapped using a rice doubled-haploid (DH) population. This gene was located on chromosome 12, flanked by RG457 and RG869, at a distance of 13.5+-4.3 cM and 17.7+-4.5 cM, respectively. The newly mapped gene on chromosome 12 may be allelic or closely linked toPi-ta. (=Pi-4(t)), a gene derived from Tetep that was previously reported to be linked to RG869 at a distance of 15.4±4.7 cM. The usefulness of markers linked to blast resistance genes will be discussed in the context of breeding for durable blast resistance.     

Use of isogenic lines and simultaneous probing to identify DNA markers tightly linked to the tm-2a gene in tomato

The Tm-2a gene of tomato confers resistance to the viral pathogen, tobacco mosaic virus. Like many economically important plant genes, Tm-2a has been characterized phenotypically and by classical linkage analysis, yet nothing is known about its gene product. We report here the isolation of two DNA clones which are very tightly linked to the Tm-2a gene. These clones were identified by testing 122 genomic clones as hybridization probes against Southern blots consisting of DNA from pairs of nearly isogenic lines with or without the Tm-2a gene. Screening such a large number of clones in a short period of time was facilitated by co-labeling and simultaneous probing of sets of up to 10 random genomic clones. Tightly linked clones were distinguished by the fact that they exhibited one or more restriction fragment length polymorphisms between the nearly isogenic lines. Tight linkage of the clones with Tm-2a was verified in a segregating F(2) population. Both mapped to the same locus 0.4 +/- 0.4 centimorgans away from Tm-2a and may provide starting points for a genomic ;;walk' to this gene. Due to the availability of isogenic lines in many plant species, the strategy outlined in this paper should be widely applicable for selecting DNA clones tightly linked to genes of interest.     

Low temperature effect on selective fertilization by pollen mixtures of wild and cultivated tomato species

Summary In vitro pollen germination of cultivated tomato, Lycopersicon esculentum Mill., is inhibited by an ambient temperature of 5°C, more so than pollen from a Peruvian ecotype of Lycopersicon hirsutum Humb. & Bonpl. originating from an altitude of 3200 m. The frequency of L. hirsutum gametes contributing to hybrid zygote formation is more than doubled when controlled fertilizations with pollen mixtures of the two species occurs at 12/6°C as compared to crosses with the same mixtures at 24/19°C. The results suggest that differential selection at the gametophytic level occurs in response to low temperature regimes. To our knowledge this is the first time in higher plants that alteration of an environmental factor has been demonstrated to change selection values of male gametophytes in a fashion predicted by the ecology of the parental sporophytes.     

Characterization of the effect of introgressed segments of chromosome 7 and 10 from Lycopersion chmielewskii on tomato soluble solids,pH, and yield

Three chromosomal segments from the wild tomato L. chmielewskii have been introgressed into the L. esculentum genome. Using molecular markers they have been mapped to the middle and terminal regions of chromosome 7 (7M, 7T respectively), and to the terminal region of chromosome 10 (10T). This study was conducted to further clarify the physiological influence of the introgressed segments of chromosome 7 and 10 on tomato soluble solids (SS), and other fruit and yield parameters. The effect of the 10T segment was evaluated using five lines that differ for the presence of this segment. As previously reported this segment increased fruit pH with no significant effect on SS. Sixty-four BC2F5 backcross inbred lines (BILs) were developed from a cross using LA1501 (an L. esculentum line that contains the 7M and 7T fragments from L. chmielewskii) as the donor parent, and VF145B-7879 (a processing cultivar) as the recurrent parent. BILs were classified in four groups (+ +, inbreds without either of the L. chmielewskii segments; 7M +, lines with only the 7M segment; + 7T, inbreds with only the 7T segment, and 7M7T, inbreds with both segments) based on RFLP information, and then compared to each other for all the parameters under study. Inbreds homoyzgous for the 7M fragment displayed greater SS (26%) and higher pH (0.10) than the control group (+ +). The 7L fragment did not influence either SS or pH, but was observed to significantly increase fruit yield by 12% when compared to the recurrent parent. A gene or genes that increase yield without affecting SS or pH may have potential in the development of commerical cultivars.