Tomato chromosome 6: a high resolution map of the long arm and construction of a composite integrated marker-order map

Integration of molecular and classical genetic maps is an essential requirement for marker-assisted breeding, quantitative trait locus mapping and map-based cloning. With respects to tomato, such maps are only available for the top part of chromosome 1, for chromosome 3 and for the short arm and the centromere proximal part of the long arm of chromosome 6. Employing an L. esculentum line carrying an L. hirsutum introgression we constructed an integrated linkage map for the telomere proximal part of the long arm of tomato chromosome 6, thereby completing the integrated map published previously. With an average map distance of only 0.6 cM the map provides detailed information on the relative position of molecular markers and several traits of economical importance, such as the fruit color marker B. Furthermore, two additional crosses using lines containing L. pennellii introgressions were performed to address the question as to how the recombination frequency in a marked interval on the long arm of chromosome 6 is affected by introgressed segments from different origins. It is concluded that recombination is not merely affected by the local level of homology but also by surrounding sequences. Combination of all the linkage data generated in various crosses described in this and other studies enabled the construction of the first integrated map of an entire tomato chromosome. This map carries 42 loci and shows the position of 15 classical genes relative to 59 molecular markers.     

Tomato chromosome 1: high resolution genetic and physical mapping of the short arm in an interspecific Lycopersicon esculentum × L. peruvianum cross

 A detailed map of part of the short arm of chromosome 1 proximal to the Cf-4/Cf-9 gene cluster was generated by using an F₂ population of 314 plants obtained from the cross between the remotely related species Lycopersicon esculentum and L. peruvianum. Six markers that cosegregate in an L. esculentum×L. pennellii F₂ population showed high recombination frequencies in the present interspecific population, spanning an interval of approximately 13 cM. Physical distances between RFLP markers were estimated by pulsed field gel electrophoresis of high-molecular-weight DNA and by identifying YACs that recognized more than one RFLP marker. In this region 1 cM corresponded to 55–110 kb. In comparsion with the value of 730 kb per cM averaged over the entire genome, this reflects the remarkably high recombination frequencies in this region in the hybrid L. esculentum×L. peruvianum progeny population. The present data underline the fact that recombination is not a process that occurs randomly over the entire genome, but can vary dramatically in intensity between chromosomal regions and among populations. Received: 20 May 1996 / Accepted: 10 September 1996     

Alignment of molecular and classical linkage maps of rice,Oryza sativa

Application of genetic linkage maps in plant genetics and breeding can be greatly facilitated by integrating the available classical and molecular genetic linkage maps. In rice, Oryza sativa L., the classical linkage map includes about 300 genes which correspond to various important morphological, physiological, biochemical and agronomic characteristics. The molecular maps consist of more than 500 DNA markers which cover most of the genome within relatively short intervals. Little effort has been made to integrate these two genetic maps. In this paper we report preliminary results of an ongoing research project aimed at the complete integration and alignment of the two linkage maps of rice. Six different F₂ populations segregating for various phenotypic and RFLP markers were used and a total of 12 morphological and physiological markers (Table 1) were mapped onto our recently constructed molecular map. Six linkage groups (i.e., chr. 1, 3, 7, 9, 11 and 12) on our RFLP map were aligned with the corresponding linkage groups on the classical map, and the previous alignment for chromosome 6 was further confirmed by RFLP mapping of an additional physiological marker on this chromosome. Results from this study, combined with our previous results, indicate that, for most chromosomes in rice, the RFLP map encompasses the classical map. The usefulness of an integrated genetic linkage map for rice genetics and breeding is discussed.Abbreviations RFLP restriction fragment length polymorphism - chr chromosome - cM centiMorgan     

Microcolinearity between a 2-cM region encompassing the grain protein content locus<Emphasis Type="Italic"> Gpc-6B1</Emphasis> on wheat chromosome 6B and a 350-kb region on rice chromosome 2

The conservation of the linear order (colinearity) of genetic markers along large chromosome segments in wheat and rice is well established, but less is known about the microcolinearity between both genomes at subcentimorgan distances. In this study we focused on the microcolinearity between a 2.6-cM interval flanked by markers Xcdo365 and Xucw65 on wheat chromosome 6B and rice chromosome 2. A previous study has shown that this wheat segment includes the Gpc-6B1 locus, which is responsible for large differences in grain protein content (GPC) and is the target of a positional cloning effort in our laboratories. Twenty-one recombination events between Xcdo365 and Xucw65 were found in a large segregating population (935 gametes) and used to map 17 genes selected from rice chromosome 2 in the wheat genetic map. We found a high level of colinearity between a 2.1-cM region flanked by loci Xucw75 and Xucw67 on wheat chromosome 6B and a 350-kb uninterrupted sequenced region in rice chromosome arm 2S. Colinearity between these two genomes was extended to the region proximal to Xucw67 (eight colinear RFLP markers), but was interrupted distal to Xucw75 (six non-colinear RFLP markers). Analysis of different comparative studies between rice and wheat suggests that microcolinearity is more frequently disrupted in the distal region of the wheat chromosomes. Fortunately, the region encompassing the Gpc-6B1 locus showed an excellent conservation between the two genomes, facilitating the saturation of the target region of the wheat genetic map with molecular markers. These markers were used to map the Gpc-6B1 locus into a 0.3-cM interval flanked by PCR markers Xucw79 and Xucw71, and to identify five candidate genes within the colinear 64-kb region in rice.     

Mapping a new nematode resistance locus in Lycopersicon peruvianum

Accessions of the wild tomato species L. peruvianum were screened with a root-knot nematode population (557R) which infects tomato plants carrying the nematode resistance gene Mi. Several accessions were found to carry resistance to 557R. A L. peruvianum backcross population segregating for resistance to 557R was produced. The segregation ratio of resistant to susceptible plants suggested that a single, dominant gene was a major factor in the new resistance. This gene, which we have designated Mi-3, confers resistance against nematode strains that can infect plants carrying Mi. Mi-3, or a closely linked gene, also confers resistance to nematodes at 32°C, a temperature at which Mi is not effective. Bulked-segregant analysis with resistant and susceptible DNA pools was employed to identify RAPD markers linked to this gene. Five-hundred-and-twenty oligonucleotide primers were screened and two markers linked to the new resistance gene were identified. One of the linked markers (NR14) was mapped to chromosome 12 of tomato in an L. esculentum/L. pennellii mapping population. Linkage of NR14 and Mi-3 with RFLP markers known to map on the short arm of chromosome 12 was confirmed by Southern analysis in the population segregating for Mi-3. We have positioned Mi-3 near RFLP marker TG180 which maps to the telomeric region of the short arm of chromosome 12 in tomato.     

Mapping of ripening-related or -specific cDNA clones of tomato (Lycopersicon esculentum)

Summary Nineteen ripening-related or -specific clones from Lycopersicon esculentum were mapped via RFLP analysis using an F₂ population from the cross L. esculentum x L. pennellii and cDNA or genomic clones of known map location. The map produced using cDNA and genomic clones of known map location corresponded well with previously published maps of tomato. The number of loci detected for each ripening-related or-specific clone varied from one to seven. These loci were located on all 12 chromosomes of the tomato genome. There was no significant clustering of ripening-related or-specific genes. Regions of very low recombination were observed. The clone for polygalacturonase (TOM6) mapped to a single region on chromosome 10, the same chromosome as the nor and alc ripening mutants. To fine map this chromosome, two backcross populations were produced from the cross of L. esculentum x L. pimpenillifolium, in which the esculentum parents used were homozygous for either the alc or the nor. The coding region for polygalacturonase is functionally unlinked to either of these two ripening mutants.     

Tomato chromosome 1: high resolution genetic and physical mapping of the short arm in an interspecific Lycopersicon esculentum×L. peruvianum cross

A detailed map of part of the short arm of chromosome 1 proximal to the Cf-4/Cf-9 gene cluster was generated by using an F₂ population of 314 plants obtained from the cross between the remotely related species Lycopersicon esculentum and L. peruvianum. Six markers that cosegregate in an L. esculentum×L. pennellii F₂ population showed high recombination frequencies in the present interspecific population, spanning an interval of approximately 13?cM. Physical distances between RFLP markers were estimated by pulsed field gel electrophoresis of high-molecular-weight DNA and by identifying YACs that recognized more than one RFLP marker. In this region 1?cM corresponded to 55–110?kb. In comparsion with the value of 730?kb per cM averaged over the entire genome, this reflects the remarkably high recombination frequencies in this region in the hybrid L. esculentum×L. peruvianum progeny population. The present data underline the fact that recombination is not a process that occurs randomly over the entire genome, but can vary dramatically in intensity between chromosomal regions and among populations.     

Physical mapping of RFLP markers on four chromosome arms in maize using terminal deficiencies

Terminal deficiencies (TDs) generated by the r-XI deletion system in maize were used to physically map RFLP markers on the short arm of chromosome 2 (2S) and the long arm of chromosome 6 (6L), chromosome 8 (8L), and chromosome 10 (10L). Five TDs on 2S, 8 on 6L, 10 on 8L, and 20 on 10L were isolated using the recessive morphological markers lg1, py1, j1(gl18), and sr2, respectively, for selection. Two exceptional TDs on 2S and 8L also have a second breakpoint on the long arm of chromosome 2 (2L) and 8L, respectively. The physical mapping of RFLP probes in relation to TD breakpoints was done by Southern hybridization. The five TDs on 2S divide chromosome 2 into four regions, all of which are distinguishable by RFLP markers. Likewise, three remaining chromosome arms are divided by TDs into RFLP-marked regions: 8 TDs divide 6L into five regions, 10 TDs divided 8L into seven regions, and 20 TDs divide 10L into three regions. The linear order of the physical map of 6L and 8L is consistent with that of the genetic maps, but that of 2L and 10L is not. Four groups of markers on 2S as well as 2L, and two on 10L are in reverse order in the physical map compared with the genetic maps. Other intriguing results are that breakpoints of TDs on 6L and 8L are distributed throughout the selected region, but most of those on 2L and 10L cluster in a region near the centromere; a single TD arose after fertilization. Received: 17 March 1997 / Accepted: 26 June 1997     

High-density physical maps reveal that the dominant male-sterile gene Ms3 is located in a genomic region of low recombination in wheat and is not amenable to map-based cloning

In wheat it is essential to know whether a gene is located in a high or low recombination region of the genome before initiating a map-based cloning approach. The objective of this study was to explore the potential feasibility of map-based cloning of the dominant male-sterile gene Ms3 of wheat. High-density physical maps of the short arms of the group-5 chromosomes (5AS, 5BS, and 5DS) of Triticum aestivum L. were constructed by mapping 40 DNA markers on a set of 17 homozygous deletion lines. One hundred RFLP loci were mapped: 35 on 5AS, 37 on 5BS, and 28 on 5DS. A consensus physical map was colinearly aligned with a consensus genetic map of the group-5 short arms. Sixteen of the 17 markers in the consensus genetic map encompass a genetic distance of 25 cM and correspond to the distal region (FL 0.56–0.97) of the consensus physical map. Two rice probes, RG463 and RG901, previously identified to be linked to markers CDO344 and CDO749 (group-5 short arm of wheat), respectively, in the genetic map of rice chromosome 12, map between FL 0.56 and 0.63 in the consensus map. Thus at least a part of the group-5 short arm is homoeologous to a region of chromosome 12 of rice. The genetic map of chromosome arm 5AS was constructed using a population of 139 BC₁ plants derived from a cross between the euploid wheat ”Chris” carrying a dominant male-sterile gene Ms3 and a disomic substitution line in which chromosome 5A of T. aestivum cv Chinese Spring was substituted by chromosome 5A from Triticum turgidum ssp. dicoccoides. The map has a genetic length of 53.4 cM with 11 DNA markers. The initial map showed that the gene Ms3 cosegregated with three markers, WG341, BCD1130 and CDO677. High-resolution mapping using an additional 509 BC₁ plants indicated that the marker WG341 was closely linked to Ms3 at a genetic distance of 0.8 cM. The Ms3 was mapped physically in the region spanning 40% of the arm length from the centromere of 5AS. Therefore, map-based cloning of the Ms3 is not feasible, although WG341 can be used as a useful tag for the Ms3 gene for breeding purposes. Received: 12 December 2000 / Accepted: 26 January 2001     

Comparison of wheat physical maps with barley linkage maps for group 7 chromosomes

Comparative genetic maps among the Triticeae or Gramineae provide the possibility for combining the genetics, mapping information and molecular-marker resources between different species. Dense genetic linkage maps of wheat and barley, which have a common array of molecular markers, along with deletion-based chromosome maps of Triticum aestivum L. will facilitate the construction of an integrated molecular marker-based map for the Triticeae. A set of 21 cDNA and genomic DNA clones, which had previously been used to map barley chromosome 1 (7H), were used to physically map wheat chromosomes 7A, 7B and 7D. A comparative map was constructed to estimate the degree of linkage conservation and synteny of chromosome segments between the group 7 chromosomes of the two species. The results reveal extensive homoeologies between these chromosomes, and the first evidence for an interstitial inversion on the short arm of a barley chromosome compared to the wheat homoeologue has been obtained. In a cytogenetically-based physical map of group 7 chromosomes that contain restriction-fragment-length polymorphic DNA (RFLP) and random amplified polymorphic DNA (RAPD) markers, the marker density in the most distal third of the chromosome arms was two-times higher than in the proximal region. The recombination rate in the distal third of each arm appears to be 8–15 times greater than in the proximal third of each arm where recombination of wheat chromosomes is suppressed.     

Molecular mapping of the centromeres of tomato chromosomes 7 and 9

The centromeres of two tomato chromosomes have been precisely localized on the molecular linkage map through dosage analysis of trisomic stocks. To map the centromeres of chromosomes 7 and 9, complementary telo-, secondary, and tertiary trisomic stocks were used to assign DNA markers to their respective chromosome arms and thus to localize the centromere at the junction of the short and long arms. It was found that both centromeres are situated within a cluster of cosegregating markers. In an attempt to order the markers within the centric clusters, genetic maps of the centromeric regions of chromosomes 7 and 9 were constructed from F₂ populations of 1620Lycopersicon esculentum × L. pennellii (E × P) plants and 1640L. esculentum × L. pimpinellifolium (E × PM) plants. Despite the large number of plants analyzed, very few recombination events were detected in the centric regions, indicating a significant suppression of recombination at this region of the chromosome. The fact that recombination suppression is equally strong in crosses between closely related (E × PM) and remotely related (E × P) parents suggests that centromeric suppression is not due to DNA sequence mismatches but to some other mechanism. The greatest number of centromeric markers was resolved in theL. esculentum × L. pennellii F₂ population. The centromere of chromosome 7 is surrounded by eight cosegregating markers: three on the short arm, five on the long arm. Similarly, the centric region of chromosome 9 contains ten cosegregating markers including one short arm marker and nine long arm markers. The localization of centromeres to precise intervals on the molecular linkage map represents the first step towards the characterization and ultimate isolation of tomato centromeres.     

Genetic linkage map of ISSR and RAPD markers in Einkorn wheat in relation to that of RFLP markers

 The potential of PCR-based markers for construction of a genetic linkage map in Einkorn wheat was investigated. From a comparison of polymorphisms between two Einkorn wheats, Triticum monococcum (Mn) and T. boeoticum (Bt), we obtained 49 polymorphic bands produced by 33 primers for inter-simple sequence repeat (ISSR) and 36 polymorphic bands shown by 25 combinations of random amplified polymorphic DNA (RAPD) primers for mapping in 66 individuals in the F₂ population. Although 44 ISSR fragments and 29 RAPD fragments statistically showed a 3 : 1 segregation ratio in the F₂ population, only 9 markers each of the ISSR and RAPD bands were able to be mapped on the RFLP linkage map of Einkorn wheat. ISSR markers were distributed throughout the chromosomes. The mapped positions of the ISSR markers seemed to be similar to those obtained by the RFLP markers. On the other hand, 4 of the 9 RAPD markers could map the RFLP marker-poor region on the short arm of 3A^m, suggesting a potential to map novel regions containing repetitive sequences. Comparisons of the genetic linkage map of Einkorn wheat to the linkage map and cytological map of common wheat revealed that the marker orders between the two maps of Einkorn wheat and common wheat coincided except for 4A, which harbors chromosome rearrangements specific for polyploid wheats, indicating a conservatism between the two genomes. Recombinations in Einkorn wheat chromosomes took place more frequently around the centromere and less at the distal part of chromosomes in comparison to those in common wheat. Nevertheless, recombinations even in Einkorn wheat chromosomes were strongly suppressed around the centromere. In fact, the markers located within 1 cM of the centromere were located almost in the central part of the chromosome arm. Received: 7 June 1997 / Accepted: 17 June 1997     

Precise mapping of a locus affecting grain protein content in durum wheat

Grain protein content (GPC) is an important factor in pasta and breadmaking quality, and in human nutrition. It is also an important trait for wheat growers because premium prices are frequently paid for wheat with high GPC. A promising source for alleles to increase GPC was detected on chromosome 6B of Triticum turgidum var. dicoccoides accession FA-15-3 (DIC). Two previous quantitative trait locus (QTL) studies found that the positive effect of DIC-6B was associated to a single locus located between the centromere and the Nor-B2 locus on the short arm of chromosome 6B. Microsatellite markers Xgwm508 and Xgwm193 flanking the QTL region were used in this study to develop 20 new homozygous recombinant substitution lines (RSLs) with crossovers between these markers. These 20 RSLs, plus nine RSLs developed in previous studies were characterized with four new RFLP markers located within this chromosome segment. Grain protein content was determined in three field experiments organized as randomized complete block designs with ten replications each. The QTL peaks for protein content were located in the central region of a 2.7-cM interval between RFLP markers Xcdo365 and Xucw67 in the three experiments. Statistical analyses showed that almost all lines could be classified unequivocally within low- and high- protein groups, facilitating the mapping of this trait as a single Mendelian locus designated Gpc-6B1. The Gpc-6B1 locus was mapped 1.5-cM proximal to Xcdo365 and 1.2-cM distal to Xucw67. These new markers can be used to reduce the size of the DIC chromosome segment selected in marker-assisted selection programs. Markers Nor-B2 and Xucw66 flanking the previous two markers can be used to select against the DIC segment and reduce the linkage drag during the transfer of Gpc-6B1 into commercial bread and pasta wheat varieties. The precise mapping of the high GPC gene, the high frequency of recombinants recovered in the targeted region, and the recent development of a tetraploid BAC library including the Gpc-6B1 DIC allele are the first steps towards the map-based cloning of this gene.Communicated by J. Dvorak     

RFLP analysis of the size of chromosomal segments retained around the Tm-2 locus of tomato during backcross breeding

Summary Genes introduced into cultivated plants by backcross breeding programs are flanked by introgressed segments of DNA derived from the donor parent. This phenomenon is known as linkage drag and is frequently thought to affect traits other than the one originally targeted. The Tm-2 gene of Lycopersicon peruvianum, which confers resistance to tobacco mosaic virus, was introduced into several different tomato cultivars (L. esculentum) by repeated backcrossing. We have measured the sizes of the introgressed segments flanking the Tm-2 locus in several of these cultivars using a high density map of restriction fragment length polymorphic (RFLP) markers. The smallest introgressed segment is estimated to be 4 cM in length, while the longest is over 51 cM in length and contains the entire short arm of chromosome 9. Additionally, RFLP analysis was performed on remnant seed from different intermediate generations corresponding to two different backcross breeding programs for TMV resistance. The results reveal that plants containing desirable recombination near the resistance gene were rarely selected during backcrossing and, as a result, the backcross breeding method was largely ineffective in reducing the size of linked DNA around the resistance gene. We propose that, by monitoring recombination around genes of interest with linked RFLP markers, one can quickly and efficiently reduce the amount of linkage drag associated with introgression. Using such a procedure, it is estimated that an introgressed segment can be obtained in two generations that is as small as that which would otherwise require 100 backcross generations without RFLP selection.     

Inheritance and genetic mapping of fruit sucrose accumulation in Lycopersicon chmielewskii

Fruit of the domestic tomato (Lycopersicon esculentum Mill.) accumulate soluble sugars primarily in the form of the hexoses, glucose and fructose. In contrast, the predominant sugar in fruit of the wild tomato relative, L. chmielewskii, is sucrose. In the present study, the inheritance and linkage relations of sucrose accumulation were examined in interspecific L. esculentum x L. chmielewskii populations. In backcrosses to either the wild or domestic tomato, segregation for sucrose accumulation permitted qualitative analysis of the trait and indicated monogenic recessive control, although deviations from Mendelian inheritance were observed in some populations. This major gene, designated sucr, was mapped in F₂, F₃, and BC₁F₂ populations using a set of 95 informative RFLP and isozyme markers covering the tomato genome. A map location near the centromere of chromosome 3 was established, with tight linkage to the genomic clone TG102. Association of sucrose accumulation with yellow fruit, encoded by an allele of the r gene, permitted alignment with the classical map, thereby confirming the map location of sucr. A linkage map of the region surrounding sucr was obtained by monitoring recombination between flanking markers in the back-crosses to tomato. A cDNA clone of tomato fruit acid invertase, TIV1, was mapped to TG102 and sucr, with no recombination between the two RFLP markers observed in over 1700 meiotic products. Despite the tight linkage, TG102 and TIV1 hybridize to distinct restriction fragments, hence do not represent the same gene. The genetic data strongly suggest that sucr is an allele of the invertase gene and thus support previous biochemical studies that demonstrated low invertase activity in sucrose-accumulating fruit. L. hisutum, another low-invertase, sucrose-accumulating species, was hybridized with L. chmielewskii and the resulting F₁ plants accumulated sucrose, indicating that genetic control of soluble sugar composition is conserved in these two species.     

Genetic characterization of apospory in tetraploid Paspalum notatum based on the identification of linked molecular markers

Tetraploid Paspalum notatum (bahiagrass) is a valuable forage grass with aposporous apomictic reproduction. In a previous study, we showed that apospory in bahiagrass is under the control of a single dominant gene with a distorted segregation ratio. The objective of this work was to identify molecular markers linked to apospory in tetraploid P. notatum and establish a preliminary syntenic relationship with the genomic region associated with apospory in P. simplex. A F₁ population of 290 individuals, segregating for apospory, was generated after crossing a completely sexual plant (Q4188) with a natural aposporous apomictic plant (Q4117). The whole progeny was classified as sexual or aposporous by embryo sacs analysis. A bulked segregant analysis was carried out to identify molecular markers co-segregating with apospory. Four hundred RAPD primers, 30 AFLP primers combinations and 85 RFLP clones were screened using DNA from both parental genotypes and aposporous and sexual bulks. Linkage analysis was performed with cytological and genetic information from the complete progeny. Cytoembryological analysis showed 219 sexual and 71 aposporous F₁ individuals. Seven different molecular markers (2 RAPD, 4 AFLP and 1 RFLP) were found to be completely linked to apospory. The RFLP probe C1069, mapping to the telomeric region of the long arm of rice chromosome 12, was one of the molecular markers completely linked to apospory in P. notatum. This marker had been previously associated with apospory in P. simplex. A preliminary map of the chromosome region carrying the apospory locus was constructed.     

Characterization and mapping of a gene controlling shoot regeneration in tomato

The superior regeneration capacity of Lycopersicon peruvianum was introduced into the cultivated tomato Lycopersicon esculentum by backcrossing hybrid material with the tomato genotype VF11. In segregating material derived from these backcrosses, the ability to regenerate shoots on root explants cultured on a zeatin-containing medium, was highly correlated with the ability to regenerate shoots on established callus cultures. The efficient shoot-regenerating root explant system permitted us to study the genetics of this trait and to locate the genes involved, using a set of morphological markers defining all 12 tomato chromosomes. Depending on the tomato genotype, mono, -di- or trigenic ratios were observed. It is concluded that a dominant L. peruvianum allele at a locus (Rg-1) near the middle of chromosome 3 determines efficient shoot regeneration on root explants in tomato in combination with dominant alleles at one or two other loci of either L. peruvianum or L. esculentum origin. The map location of the Rg-1 locus was refined further using a number of chromosome-3-specific RFLPs. The addition of new classical and RFLP linkage data to existing literature data and subsequent processing resulted in a revised and integrated map of tomato chromosome 3. From a morphological and physiological analysis of genotypes differing in Rg phenotype, it is concluded that the genetic component associated with regeneration determines the maintenance of morphogenetic competence and not the sensitivity to hormones.     

Linkage Arrangement of RFLP loci in progenies from crosses between doubled haploid Asparagus officinalis L. clones

A preliminary genetic map of the dioecious species Asparagus officinalis L. (2n = 20) has been constructed on the basis of restriction fragment length polymorphism (RFLP) and isozyme marker data. With DNA samples digested with either EcoRI or HindIII 61 out of 148 probes (41%) identified RFLPs in six families of doubled haploid lines obtained through anther culture. A higher level of polymorphism (65%) was observed when a single family was screened for RFLPs using six distinct restriction enzymes. Segregation analysis of the BC progenies (40–80 individuals) resulted in a 418-cM extended map comprising 43 markers: 39 RFLPs, three isozymes and one morphological (sex). These markers are clustered in 12 linkage groups and four of them exhibited significant deviations from the expected 11 ratio. One isozyme and three RFLP markers were assigned to the sex chromosome.     

An RFLP linkage map of Lycopersicon peruvianum

In order to map genes determining resistance to bacterial canker in tomato, backcrosses were made between a resistant and a susceptible Lycopersicon peruvianum accession. The linkage study with RFLP markers yielded a genetic map of L. Peruvianum. This map was compared to that derived from a L. esculentum x L. pennellii F₂ population, based on 70 shared RFLP markers. The maps showed a good resemblance in both the order of markers and the length of the chromosomes, with the exception of just one relocated marker on chromosome 9. Because backcrosses were made with the F₁, either as the pollen parent or as the pistil parent, linkage maps from male and female meioses could be estimated. It was concluded that recombination at male meiosis was reduced, and that gametophytic selection for parental genotypes at more than one locus per chromosome might be partly responsible for the reduction of the estimated male map length.     

Lycopersicon esculentum lines containing small overlapping introgressions from L. pennellii

Summary The objective of this project was to introgress small overlapping chromosome segments which cover the genome of L. pennellii into Lycopersicon esculentum lines. The interspecific hybrid was backcrossed to L. esculentum, and a map of 981 cM, based on 146 molecular markers covering the entire genome, was produced. A similar backcross 1 population was selfed for six generations, under strong selection for cultivated tomato phenotypes, to produce 120 introgression lines. The introgression lines were assayed for the above-mentioned molecular markers, and 21 lines covering 936 cM of L. pennellii, with an average introgression of 86 cM, were selected to provide a resource for the mapping of new DNA clones. The rest of the lines have shorter introgressions consisting of specific regions with an average size of 38 cM. The proportion of the L. pennellii genome in the introgression lines was lower than expected (252 cM) because of strong selection against the wild-parent phenotype. The mean introgression rate for ends of linkage groups in the 120 lines was 3 times higher than for other regions of the genome. The introgression lines can assist in RFLP-based gene cloning by allowing the rapid selection of DNA markers that map to specific chromosome segments. The introgression lines also provide a base population for the mapping and breeding for quantitative traits such as salt and drought tolerance that characterize the wild species L. pennellii.