AFLP markers in a molecular linkage map of maize: codominant scoring and linkage group ditsribution

We exploited the AFLP technique to saturate a RFLP linkage map derived from a maize mapping population. By using two restriction enzyme, EcoRI and PstI, differing in methylation sensitivity, both in combination with MseI, we detected 1568 bands of which 340 where polymorphic. These were added to the exitsing RFLP marker data to study the effects of incorporation of AFLPs produced by different restriction-enzyme combinations upon genetic maps. Addition of the AFLP data resulted in greater genome coverage, both through linking previously separate groups and the extension of other groups. The increase of the total map length was mainly caused by the addition of markers to telomeric regions, where RFLP markers were poorly represented. The percentage of informative loci was significantly different between the EcoRI and PstI assays. There was also evidence that PstI AFLP markers were more randomly distributed across chromosomes and chromosome regions, while EcoRI AFLP markers clustered mainly at centomeric regions. The more-random ditsribution of PstI AFLP markers on the genetic map reported here may reflect a preferential localisation of the markers in the hypomethylated telomeric regions of the chromosomes. Received: 22 December 1998 / Accepted: 25 March 1999     

Improved linkage map and thirty new microsatellite markers for rat Chromosome 10

An improved linkage map for rat Chromosome (Chr) 10 with two F₂ populations was constructed. Thirty new microsatellite markers were generated from a Chr 10-specific, small-insert genomic library and mapped to rat Chr 10. Among them were the rat homologs for the mouse gene for light and heavy chains of myeloperoxidase and human neurofibromatosis 1. Eight newly generated markers (D10Mco62, D10Mco63, D10Mco64, D10Mco65, D10Mco67, D10Mco68, D10Mco70, and D10Mco74) were mapped to the region of the rat Chr 10 blood pressure QTL. The availability of such markers may be instrumental in the search for genes responsible for the hypertension. Received: 13 July 1998 / Accepted: 9 September 1998     

A microsatellite marker based linkage map of tobacco

We report the first linkage map of tobacco (Nicotiana tabacum L.) generated through microsatellite markers. The microsatellite markers were predominantly derived from genomic sequences of the Tobacco Genome Initiative (TGI) through bioinformatics screening for microsatellite motives. A total of 684 primer pairs were screened for functionality in a panel of 16 tobacco lines. Of those, 637 primer pairs were functional. Potential parents for mapping populations were evaluated for their polymorphism level through genetic similarity analysis. The similarity analysis revealed that the known groups of tobacco varieties (Burley, Flue-cured, Oriental and Dark) form distinct clusters. A mapping population, based on a cross between varieties Hicks Broad Leaf and Red Russian, and consisting of 186 F2 individuals, was selected for mapping. A total of 282 functional microsatellite markers were polymorphic in this population and 293 loci could be mapped together with the morphological trait flower color. Twenty-four tentative linkage groups spanning 1,920 cM could be identified. This map will provide the basis for the genetic mapping of traits in tobacco and for further analyses of the tobacco genome. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

A linkage map of common carp (Cyprinus carpio) based on AFLP and microsatellite markers

Common carp (Cyprinus carpio) is an important fish for aquaculture, but genomics of this species is still in its infancy. In this study, a linkage map of common carp based on Amplified Fragment Length Polymorphism (AFLP) and microsatellite (SSR) markers has been generated using gynogenetic haploids. Of 926 markers genotyped, 151 (149 AFLPs, two SSRs) were distorted and eliminated from the linkage analyses. A total of 699 AFLP and 20 microsatellite (SSR) markers were assigned to the map, which comprised 64 linkage groups and covered 5506.9 cM Kosambi, with an average interval distance of 7.66 cM Kosambi. The normality tests on interval map distances showed a non‐normal marker distribution. Visual inspection of the map distance distribution histogram showed a cluster of interval map distances on the left side of the chart, which suggested the occurrence of AFLP marker clusters. On the other hand, the lack of an obvious cluster on the right side showed that there were a few big gaps which need more markers to bridge. The correlation analysis showed a highly significant relatedness between the length of linkage group and the number of markers, indicating that the AFLP markers in this map were randomly distributed among different linkage groups. This study is helpful for research into the common carp genome and for further studies of genetics and marker‐assisted breeding in this species.     

A genetic linkage map of willow ( Salix viminalis) based on AFLP and microsatellite markers

The genus Salix (willow) contains a number of species of great value as biomass crops. Efforts to breed varieties with improved biomass yields and resistances to pests and diseases are limited by the lack of knowledge on the genetic basis of the traits. We have used AFLP and microsatellite markers to construct a genetic map of willow from a full-sib cross of the diploid species Salix viminalis (2n = 38). In accordance with a double pseudo-testcross approach, separate parental maps were constructed and merged to produce a consensus map comprising 291 AFLP and 39 willow microsatellite markers. Nineteen poplar microsatellites were also tested in willow. Five of these amplified loci, of which two were mapped. Linkage groups of the consensus map that could be identified in the parental maps are presented here and spanned 1,256.5 cM with an average interval between markers of 4.4 cM.     

A genetic linkage map for Pinus radiata based on RFLP, RAPD, and microsatellite markers

A genetic linkage map for radiata pine (Pinus radiata D. Don) has been constructed using segregation data from a three-generation outbred pedigree. A total of 208 loci were analyzed including 165 restriction fragment length polymorphism (RFLP), 41 random amplified polymorphic DNA (RAPD) and 2 microsatellite markers. The markers were assembled into 22 linkage groups of 2 or more loci and covered a total distance of 1382 cM. Thirteen loci were unlinked to any other marker. Of the RFLP loci that were mapped, 93 were detected by loblolly pine (P. taeda L.) cDNA probes that had been previously mapped or evaluated in that species. The remaining 72 RFLP loci were detected by radiata pine probes from a PstI genomic DNA library. Two hundred and eighty RAPD primers were evaluated, and 41 loci which were segregating in a 11 ratio were mapped. Two microsatellite markers were also placed on the map. This map and the markers derived from it will have wide applicability to genetic studies in P. radiata and other pine species.     

Preliminary linkage map of the turkey (Meleagris gallopavo) based on microsatellite markers

The turkey is an agriculturally important species for which, until now, there is no published genetic linkage map based on microsatellite markers--still the markers most used in the chicken and other farm animals. In order to increase the number of markers on a turkey genetic linkage map we decided to map new microsatellite sequences obtained from a GT-enriched turkey genomic library. In different chicken populations more than 35-55% of microsatellites are polymorphic. In the turkey populations tested here, 43% of all turkey primers tested were found to be polymorphic, in both commercial and wild type turkeys. Twenty linkage groups (including the Z chromosome) containing 74 markers have been established, along with 37 other unassigned markers. This map will lay the foundations for further genetic mapping and the identification of genes and quantitative trait loci in this economically important species. Genome comparisons, based on genetic maps, with related species such as the chicken would then also be possible. All primer information, polymerase chain reaction (PCR) conditions, allele sizes and genetic linkage maps can be viewed at http://roslin.thearkdb.org/. The DNA is also available on request through the Roslin Institute.     

A chicken linkage map based on microsatellite markers genotyped on a Japanese Large Game and White Leghorn cross

A detailed linkage map is necessary for efficient detection of quantitative trait loci (QTL) in chicken resource populations. In this study, microsatellite markers isolated from a (CA)n-enriched library (designated as ABR Markers) were mapped using a population developed from a cross between Japanese Game and White Leghorn chickens. In total, 296 markers including 193 ABR, 43 MCW, 31 ADL, 22 LEI, 3 HUJ, 2 GCT, 1 UMA and 1 ROS were mapped by linkage to chicken chromosomes 1-14, 17-21, 23, 24, 26-28 and Z. In addition, five markers were assigned to the map based on the chicken draft genomic sequence, bringing the total number of markers on the map to 301. The resulting linkage map will contribute to QTL mapping in chicken.     

A genetic linkage map of Theobroma cacao L.

A linkage map of the cocoa genome comprising 193 loci has been constructed. These loci consist of 5 isozymes, 101 cDNA/RFLPs, 4 loci from genes of known function, 55 genomic DNA/RFLPs and 28 RAPDs. A population of 100 individuals derived from a cross between two heterozygous genotypes was used. Segregation analyses were performed with the JoinMap program. Ten linkage groups, which putatively correspond to the ten gametic chromosomes of cocoa, were identified. The map covers a total length of 759 cM with a 3.9 cM average distance between 2 markers. A small fraction (9%) of the markers deviated significantly from the expected Mendelian ratios.     

Towards a genus-wide reference linkage map for Eucalyptus based exclusively on highly informative microsatellite markers

A novel set of 50 highly polymorphic microsatellite markers were developed and mapped on existing RAPD framework maps of Eucalyptus grandis and E. urophylla. Together with the twenty previously developed microsatellite markers, these were used to align the existing maps for the two most commercially important Eucalyptus species in the tropics. Sixty-three microsatellite markers were placed on the E. grandis map in 11 linkage groups, and 53 on the E. urophylla map distributed in 10 linkage groups. Approximately 66% of the microsatellite markers segregated in a fully informative fashion, allowing the establishment of colinear syntenic linkage groups between the two maps. The 50 new microsatellite markers were highly informative, with an average of 14 alleles per locus, and average expected heterozygosity between 0.82 and 0.87. Furthermore, within the subgenus Symphyomyrtus, to which the vast majority of commercially important Eucalyptus species belong, these markers display on average 90% transportability. This set of 70 mapped microsatellite markers represents a significant step toward the development of a genus-wide reference linkage map for Eucalyptus. These highly multiallelic and transportable markers constitute a powerful tool for QTL discovery and validation, and can be used in directed searches for QTL allele variation across Eucalyptus pedigrees.     

A high-density linkage map of Theobroma cacao L.

The first linkage map established by Lanaud et al. (1995) was used as a starting point to produce a high-density molecular linkage map. A mapping population of 181 progenies resulting from a cross between two heterozygous genotypes, a Forastero and a Trinitario (hybrid between Forastero and Criollo), was used for the linkage analysis. A new DNA isolation protocol was established, which allows enough good quality DNA to construct a genetic map with PCR-based markers. The map comprises 424 markers with an average spacing between markers of 2.1 cM. The marker types used were five isozymes, six loci from known function genes, 65 genomic RFLPs, 104 cDNA RFLPs, three telomeric probes, 30 RAPDs, 191 AFLPs and 20 microsatellites. The use of new marker types, AFLP and microsatellites, did not disturb the original order of the RFLP loci used on the previous map. The genetic markers were distributed over ten linkage groups and cover 885.4 cM. The maximum distance observed between adjacent markers was 16.2 cM, and 9.4% of all loci showed skewed segregation. Received: 2 January 2000 / Accepted: 12 February 2000     

A microsatellite linkage map of Drosophila mojavensis

Background

Drosophila mojavensishas been a model system for genetic studies of ecological adaptation and speciation. However, despite its use for over half a century, no linkage map has been produced for this species or its close relatives.

Results

We have developed and mapped 90 microsatellites in D. mojavensis, and we present a detailed recombinational linkage map of 34 of these microsatellites. A slight excess of repetitive sequence was observed on the X-chromosome relative to the autosomes, and the linkage groups have a greater recombinational length than the homologous D. melanogaster chromosome arms. We also confirmed the conservation of Muller's elements in 23 sequences between D. melanogaster and D. mojavensis.

Conclusions

The microsatellite primer sequences and localizations are presented here and made available to the public. This map will facilitate future quantitative trait locus mapping studies of phenotypes involved in adaptation or reproductive isolation using this species.     

A microsatellite genetic linkage map for Xiphophorus

Interspecies hybrids between distinct species of the genus Xiphophorus are often used in varied research investigations to identify genomic regions associated with the inheritance of complex traits. There are 24 described Xiphophorus species and a greater number of pedigreed strains; thus, the number of potential interspecies hybrid cross combinations is quite large. Previously, select Xiphophorus experimental crosses have been shown to exhibit differing characteristics between parental species and among the hybrid fishes derived from crossing them, such as widely differing susceptibilities to chemical or physical agents. For instance, genomic regions harboring tumor suppressor and oncogenes have been identified via linkage association of these loci with a small set of established genetic markers. The power of this experimental strategy is related to the number of genetic markers available in the Xiphophorus interspecies cross of interest. Thus, we have undertaken the task of expanding the suite of easily scored markers by characterization of Xiphophorus microsatellite sequences. Using a cross between Xiphophorus maculatus and X. andersi, we report a linkage map predominantly composed of microsatellite markers. All 24 acrocentric chromosome sets of Xiphophorus are represented in the assembled linkage map with an average intergenomic distance of 7.5 cM. Since both male and female F1 hybrids were used to produce backcross progeny, these recombination rates were compared between "male" and "female" maps. Although several genomic regions exhibit differences in map length, male- and female-derived maps are similar. Thus Xiphophorus, in contrast to zebrafish, Danio rerio, and several other vertebrate species, does not show sex-specific differences in recombination. The microsatellite markers we report can be easily adapted to any Xiphophorus interspecies and some intraspecies crosses, and thus provide a means to directly compare results derived from independent experiments.     

A genetic linkage map of the sea cucumber, Apostichopus japonicus (Selenka), based on AFLP and microsatellite markers

We present the first genetic maps of the sea cucumber ( Apostichopus japonicus ), constructed with an F₁ pseudo-testcross strategy. The 37 amplified fragment length polymorphism (AFLP) primer combinations chosen identified 484 polymorphic markers. Of the 21 microsatellite primer pairs tested, 16 identified heterozygous loci in one or other parent, and six were fully informative, as they segregated in both parents. The female map comprised 163 loci, spread over 20 linkage groups (which equals the haploid chromosome number), and spanned 1522.0 cM, with a mean marker density of 9.3 cM. The equivalent figures for the male map were 162 loci, 21 linkage groups, 1276.9 and 7.9 cM. About 2.5% of the AFLP markers displayed segregation distortion and were not used for map construction. The estimated coverage of the genome was 84.8% for the female map and 83.4% for the male map. The maps generated will serve as a basis for the construction of a high-resolution genetic map and mapping of the functional genes and quantitative trait loci, which will then open the way for the application of a marker-assisted selection breeding strategy in this species.     

A genetic linkage map of human chromosome 20 composed entirely of microsatellite markers.

Twenty-six (CA)n polymorphic microsatellites were isolated from a flow-sorted chromosome 20 library. To reduce the number of sequencing gels, these microsatellites were genotyped on 15 CEPH families using a procedure derived from the multiplex sequencing technique of G. M. Church and S. Kieffer-Higgins (1988, Science 240:185-188). A primary map with a strongly supported order was constructed with 15 (CA)n markers. Regional localizations for the 11 other microsatellites were obtained with regard to the primary map. The 26 loci span approximately 160 cM. Regional localizations for a set of index markers (D20S4, D20S6, D20S16, and D20S19) and for other markers from the CEPH Public Database (D20S5, D20S15, D20S17, and D20S18) have also been determined. The total map spans a genetic distance of approximately 195 cM extending from the (CA)n marker IP20M7 on 20p to D20S19 on 20q. The density of microsatellite markers is markedly higher in the pericentromeric region, with an average distance of 3 to 4 cM between adjacent markers over a distance of 43 cM. Two-thirds of these randomly isolated microsatellites are clustered in that region between D20S5 and D20S16 representing approximately one-fourth of the linkage map. This might suggest a nonrandom distribution of (CA)n simple sequence repeats on this chromosome.     

Genetics of screwworm: new genetic markers and preliminary linkage map

Eight new genetic markers for Cochliomyia hominivorax (Diptera: Calliphoridae), the screwworm, are characterized. The markers include three eye mutants, lemon-eye (le), cherry-eye (ch), and red-eye (re); one wing mutant, curly-wing (cw); and four allozyme markers, amylase (Amy-1), glycerol-3-phosphate dehydrogenase (Gpd), phosphoglucomutase (Pgm), and octanol dehydrogenase (Odh). The markers are associated into four linkage groups. Radiation-induced translocations were used to correlate the linkage groups with their respective chromosomes. A preliminary genetic linkage map with these and three previously characterized loci is presented.     

A second‐generation genetic linkage map for bighead carp (Aristichthys nobilis) based on microsatellite markers

Bighead carp (Aristichthys nobilis) is an important aquaculture fish worldwide. Genetic linkage maps for the species were previously reported, but map resolution remained to be improved. In this study, a second‐generation genetic linkage map was constructed for bighead carp through a pseudo‐testcross strategy using interspecific hybrids between bighead carp and silver carp. Of the 754 microsatellites genotyped in two interspecific mapping families (with 77 progenies for each family), 659 markers were assigned to 24 linkage groups, which were equal to the chromosome numbers of the haploid genome. The consensus map spanned 1917.3 cM covering 92.8% of the estimated bighead carp genome with an average marker interval of 2.9 cM. The length of linkage groups ranged from 52.2 to 133.5 cM with an average of 79.9 cM. The number of markers per linkage group varied from 11 to 55 with an average of 27.5 per linkage group. Normality tests on interval distances of the map showed a non‐normal marker distribution; however, significant correlation was found between the length of linkage group and the number of markers below the 0.01 significance level (two‐tailed). The length of the female map was 1.12 times that of the male map, and the average recombination ratio of female to male was 1.10:1. Visual inspection showed that distorted markers gathered in some linkage groups and in certain regions of the male and female maps. This well‐defined genetic linkage map will provide a basic framework for further genome mapping of quantitative traits, comparative mapping and marker‐assisted breeding in bighead carp.