A Primary Linkage Map of the Porcine Genome Reveals a Low Rate of Genetic Recombination

A comprehensive genetic linkage map of the porcine genome has been developed by typing 128 genetic markers in a cross between the European Wild Boar and a domestic breed (Large White). The marker set includes 68 polymerase chain reaction-formatted microsatellites, 60 anchored reference markers informative for comparative mapping and 47 markers which have been physically assigned by in situ hybridization. Novel multipoint assignments are provided for 54 of the markers. The map covers about 1800 cM, and the average spacing between markers is 11 cM. We used the map data to estimate the genome size in pigs, thereby addressing the total recombination distance in a third mammalian species. A sex-average genome length of 1873 +/- 139 cM was obtained by comparing the recombinational and physical distances in defined regions of the genome. This is strikingly different from the length of the human genome (3800-4000 cM) and is more similar to the mouse estimate (1600 cM). The recombination rate in females was significantly higher than in males.     

Investigation of genomic organization in switchgrass (Panicum virgatum L.) using DNA markers

We report an early investigation into genomic organization and chromosomal transmission in switchgrass based on restriction fragment length polymorphism (RFLP) markers. The segregation of 224 single dose restriction fragments (SDRF) in 85 full-sib progeny of a cross between the genotypes Alamo (AP13) and Summer (VS16) was used to determine linkage associations in each parent. In the seed parent AP13, 11 cosegregation groups were identified by 45 SDRF markers with a cumulative recombination length of 412.4 cM. In the pollen parent VS16, 57 SDRF markers were assigned to 16 cosegregation groups covering a length of 466.5 cM. SDRF markers identified by the same probes and mapping to different cosegregation groups were used to combine the two maps and identify homology groups. Eight homology groups were identified among the nine haploid linkage groups expected in switchgrass. The high incidence of repulsion phase associations indicates that preferential pairing between homologous chromosomes is predominant in switchgrass. Based on marker distribution in the paternal map (VS16), we estimated the recombinational length of switchgrass genome to be 4,617 cM. In order to link 95% of the genome to a marker at a 15-cM distance, a minimum of 459 markers will be required. Using information from the ratio of repulsion to coupling linkages, we infer that switchgrass is an autotetraploid with a high degree of preferential pairing. The information presented in this study establishes a foundation for extending genetic mapping in this crop and constitutes a framework for basic and applied genetic studies.Electronic Supplementary Material Supplementary material is available for this article at     

A genetic linkage map for azuki bean [Vigna angularis (Willd.) Ohwi & Ohashi]

To make progress in genome analysis of azuki bean (Vigna angularis) a genetic linkage map was constructed from a backcross population of (V. nepalensis x V. angularis) x V.angularis consisting of 187 individuals. A total of 486 markers—205 simple sequence repeats (SSRs), 187 amplified fragment length polymorphisms (AFLPs) and 94 restriction fragment length polymorphisms (RFLPs) —were mapped onto 11 linkage groups corresponding to the haploid chromosome number of azuki bean. This map spans a total length of 832.1 cM with an average marker distance of 1.85 cM and is the most saturated map for a Vigna species to date. In addition, RFLP markers from other legumes facilitated finding several orthologous linkage groups based on previously published RFLP linkage maps. Most SSR primers that have been developed from SSR-enriched libraries detected a single locus. The SSR loci identified are distributed throughout the azuki bean genome. This moderately dense linkage map equipped with many SSR markers will be useful for mapping a range of useful traits such as those related to domestication and stress resistance. The mapping population will be used to develop advanced backcross lines for high resolution QTL mapping of these traits. O.K. Han, A. Kaga, T. Isemura have contributed equally to this paper.     

Linkages between restriction fragment length,isozyme, and morphological markers in lentil

Summary A genetic linkage map of lentil comprising 333 centimorgans (cM) was constructed from 20 restriction fragment length, 8 isozyme, and 6 morphological markers segregating in a single interspecific cross (Lens culinaris × L. orientalis). Because the genotypes at marker loci were determined for about 66 F₂ plants, linkages are only reported for estimates of recombination less than 30 cM. Probes for identification of restriction fragment length polymorphisms (RFLPs) were isolated from a cDNA and EcoRI and PstI partial genomic libraries of lentil. The cDNA library gave the highest frequency of relatively low-copy-number probes. The cDNAs were about twice as efficient, relative to random genomic fragments, in RFLP detection per probe. Nine markers showed significant deviations from the expected F₂ ratios and tended to show a predominance of alleles from the cultigen. Assuming a genome size of 10 Morgans, 50% of the lentil genome could be linked within 10 cM of the 34 markers and the map is of sufficient size to attempt mapping of quantitative trait loci.     

Development of a Mesoamerican intra-genepool genetic map for quantitative trait loci detection in a drought tolerant?×?susceptible common bean (Phaseolus vulgaris L.) cross

Drought is a major constraint to common bean (Phaseolus vulgaris L.) production, especially in developing countries where irrigation for the crop is infrequent. The Mesoamerican genepool is the most widely grown subdivision of common beans that include small red, small cream and black seeded varieties. The objective of this study was to develop a reliable genetic map for a Mesoamerican × Mesoamerican drought tolerant × susceptible cross and to use this map to analyze the inheritance of yield traits under drought and fully irrigated conditions over 3 years of experiments. The source of drought tolerance used in the cross was the cream-seeded advanced line BAT477 crossed with the small red variety DOR364 and the population was made up of recombinant inbred lines in the F5 generation. Quantitative trait loci were detected by composite interval mapping for the traits of overall seed yield, yield per day, 100 seed weight, days to flowering and days to maturity for each field environment consisting of two treatments (irrigated and rainfed) and lattice design experiments with three repetitions for a total of six environments. The genetic map based on amplified fragment length polymorphism and random amplified polymorphic DNA markers was anchored with 60 simple sequence repeat (SSR) markers and had a total map length of 1,087.5 cM across 11 linkage groups covering the whole common bean genome with saturation of one marker every 5.9 cM. Gaps for the genetic map existed on linkage groups b03, b09 and b11 but overall there were only nine gaps larger than 15 cM. All traits were inherited quantitatively, with the greatest number for seed weight followed by yield per day, yield per se, days to flowering and days to maturity. The relevance of these results for breeding common beans is discussed in particular in the light of crop improvement for drought tolerance in the Mesoamerican genepool.     

A consensus linkage map for sugi (Cryptomeria japonica) from two pedigrees, based on microsatellites and expressed sequence tags

A consensus map for sugi (Cryptomeria japonica) was constructed by integrating linkage data from two unrelated third-generation pedigrees, one derived from a full-sib cross and the other by self-pollination of F1 individuals. The progeny segregation data of the first pedigree were derived from cleaved amplified polymorphic sequences, microsatellites, restriction fragment length polymorphisms, and single nucleotide polymorphisms. The data of the second pedigree were derived from cleaved amplified polymorphic sequences, isozyme markers, morphological traits, random amplified polymorphic DNA markers, and restriction fragment length polymorphisms. Linkage analyses were done for the first pedigree with JoinMap 3.0, using its parameter set for progeny derived by cross-pollination, and for the second pedigree with the parameter set for progeny derived from selfing of F1 individuals. The 11 chromosomes of C. japonica are represented in the consensus map. A total of 438 markers were assigned to 11 large linkage groups, 1 small linkage group, and 1 nonintegrated linkage group from the second pedigree; their total length was 1372.2 cM. On average, the consensus map showed 1 marker every 3.0 cM. PCR-based codominant DNA markers such as cleaved amplified polymorphic sequences and microsatellite markers were distributed in all linkage groups and occupied about half of mapped loci. These markers are very useful for integration of different linkage maps, QTL mapping, and comparative mapping for evolutional study, especially for species with a large genome size such as conifers.     

A first generation microsatellite- and SNP-based linkage map of Jatropha

Jatropha curcas is a potential plant species for biodiesel production. However, its seed yield is too low for profitable production of biodiesel. To improve the productivity, genetic improvement through breeding is essential. A linkage map is an important component in molecular breeding. We established a first-generation linkage map using a mapping panel containing two backcross populations with 93 progeny. We mapped 506 markers (216 microsatellites and 290 SNPs from ESTs) onto 11 linkage groups. The total length of the map was 1440.9 cM with an average marker space of 2.8 cM. Blasting of 222 Jatropha ESTs containing polymorphic SSR or SNP markers against EST-databases revealed that 91.0%, 86.5% and 79.2% of Jatropha ESTs were homologous to counterparts in castor bean, poplar and Arabidopsis respectively. Mapping 192 orthologous markers to the assembled whole genome sequence of Arabidopsis thaliana identified 38 syntenic blocks and revealed that small linkage blocks were well conserved, but often shuffled. The first generation linkage map and the data of comparative mapping could lay a solid foundation for QTL mapping of agronomic traits, marker-assisted breeding and cloning genes responsible for phenotypic variation.     

RFLP linkage map of the Ethiopian cereal tef [Eragrostis tef (Zucc) Trotter]

Tef [Eragrostis tef (Zucc)Trotter] is one of the most important cereal crops in Ethiopia. It is an allotetraploid species with a genome size of 720 Mbp. In this paper we report results of genetic linkage-map construction for E. tef using tef and heterologous cDNA probes for the first time. One hundred and sixteen F₈ recombinant inbred lines (RILs) from the cross E. tef cv Kaye Murri×Eragrostis pilosa (accession30–5) were used for mapping. Parental lines were digested with nine restriction enzymes and screened using 159 tef cDNA and 162 heterologous probes including the grass genome anchor probes. The polymorphism level between parental lines was 66.9%. One hundred and thirty nine polymorphic probes were hybridized against 116 RILs. Both the tef and the heterologous probes hybridized well against tef genomic DNA. The linkage map defined 1,489 cM of the tef genome comprising 149 marker loci distributed among 20 linkage groups. The average interval between markers was 9.99 cM. A fraction (14.8%) of the markers deviated significantly from the expected segregation. Such a genetic linkage map is useful for tagging economically useful genes in tef because a wide range of agronomically important traits is segregating within this population. This would enable the use of a marker assisted breeding strategy which, in turn, will enhance breeding efficiency. Alignment of the tef RFLP map with the rice RFLP map indicates that a number of syntenic chromosomal fragments exist between tef and rice in which the gene order was for the most part collinear. The comparative mapping information should enable tef scientists to take advantage of whatever genetic progress is made on the cereal model species rice. Received: 9 June 2000 / Accepted: 31 August 2000     

A genetic linkage map construction for sesame (Sesamum indicum L.)

Sesame (Sesamum indicum L.) is one of the oldest oilseed crops with high seed oil quality. The first sesame genetic linkage map based on F2 segregating population of an intraspecific cross between two cultivars was constructed. Using three types of PCR-based markers, 284 polymorphic loci including 10 EST-SSR marker, 30 AFLP marker and 244 RSAMPL marker, respectively, had been screened. Subsequently, a total of 220 molecular markers were mapped in 30 linkage groups covering a genetic length of 936.72 cM, and the average distance between markers was 4.93 cM. In this map, the linkage groups contained from 2 to 33 loci each and ranged in distance from 6.44 cM to 74.52 cM. Based on map information, sesame genome length was estimated to be approximately 1,232.53 cM, and genome coverage of this map was about 76.0%. As a starting point of sesame genome study, the genetic linkage map will be hopeful to tag traits of breeding interest and further aid in the sesame molecular breeding. Furthermore, RSAMPL marker had been also appreciated in this paper, for its first usage in genetic map construction and higher utilization potential in some crop species lacking much genome information.     

Towards an integrated linkage map of common bean 2. Development of an RFLP-based linkage map

Summary A restriction fragment length polymorphism (RFLP)-based linkage map for common bean (Phaseolus vulgaris L.) covering 827 centiMorgans (cM) was developed based on a F₂ mapping population derived from a cross between BAT93 and Jalo EEP558. The parental genotypes were chosen because they exhibited differences in evolutionary origin, allozymes, phaseolin type, and for several agronomic traits. The segregation of 152 markers was analyzed, including 115 RFLP loci, 7 isozyme loci, 8 random amplified polymorphic DNA (RAPD) marker loci, and 19 loci corresponding to 15 clones of known genes, 1 virus resistance gene, 1 flower color gene, and 1 seed color pattern gene. Using MAPMAKER and LINKAGE-1, we were able to assign 143 markers to 15 linkage groups, whereas 9 markers remained unassigned. The average interval between markers was 6.5 cM; only one interval was larger than 30 cM. A small fraction (9%) of the markers deviated significantly from the expected Mendelian ratios (121 or 31) and mapped into four clusters. Probes of known genes belonged to three categories: seed proteins, pathogen response genes, and Rhizobium response genes. Within each category, sequences homologous to the various probes were unlinked. The I gene for bean common mosaic virus resistance is the first disease resistance gene to be located on the common bean genetic linkage map.     

Construction of an SSR and RAD Marker-Based Genetic Linkage Map for Carnation (<Emphasis Type="Italic">Dianthus caryophyllus</Emphasis> L.)

A high-density genetic map, an essential tool for comparative genomic studies and quantitative trait locus fine mapping, can also facilitate genome sequence assembly. The sequence-based marker technology known as restriction site-associated DNA (RAD) enables synchronous, single nucleotide polymorphism marker discovery, and genotyping using massively parallel sequencing. We constructed a high-density linkage map for carnation (Dianthus caryophyllus L.) based on simple sequence repeat (SSR) markers in combination with RAD markers developed by double-digest RAD sequencing (ddRAD-seq). A total of 2404 (285 SSR and 2119 RAD) markers could be assigned to 15 linkage groups spanning 971.5 cM, with an average marker interval of 0.4 cM. The total length of scaffolds with identified map positions was 95.6 Mb, which is equivalent to 15.4 % of the estimated genome size. The generated map is the first SSR and RAD marker-based high-density linkage map reported for carnation. The ddRAD-seq pipeline developed in this study should also help accelerate genetic and genomics analyses and molecular breeding of carnation and other non-model crops.     

Development of a black gram [Vigna mungo (L.) Hepper] linkage map and its comparison with an azuki bean [Vigna angularis (Willd.) Ohwi and Ohashi] linkage map

The Asian Vigna group of grain legumes consists of six domesticated species, among them black gram is widely grown in South Asia and to a lesser extent in Southeast Asia. We report the first genetic linkage map of black gram [Vigna mungo (L.) Hepper], constructed using a BC₁F₁ population consisting of 180 individuals. The BC₁F₁ population was analyzed in 61 SSR primer pairs, 56 RFLP probes, 27 AFLP loci and 1 morphological marker. About 148 marker loci could be assigned to the 11 linkage groups, which correspond to the haploid chromosome number of black gram. The linkage groups cover a total of 783 cM of the black gram genome. The number of markers per linkage group ranges from 6 to 23. The average distance between adjacent markers varied from 3.5 to 9.3 cM. The results of comparative genome mapping between black gram and azuki bean show that the linkage order of markers is highly conserved. However, inversions, insertions, deletions/duplications and a translocation were detected between the black gram and azuki bean linkage maps. The marker order on parts of linkage groups 1, 2 and 5 is reversed between the two species. One region on black gram linkage group 10 appears to correspond to part of azuki bean linkage group 1. The present study suggests that the azuki bean SSR markers can be widely used for Asian Vigna species and the black gram genetic linkage map will assist in improvement of this crop.Electronic Supplementary Material Supplementary material is available in the online version of this article at and is accessible for authorized users.The first three authors contributed equally to this research     

Construction of a genetic linkage map for silver carp (Hypophthalmichthys molitrix)

For silver carp (Hypophthalmichthys molitrix), a combined microsatellite (or simple sequence repeat) and amplified fragment length polymorphism (AFLP) sex average linkage map was constructed. A total of 483 markers (245 microsatellites and 238 AFLPs) were assigned to 33 linkage groups. The map spanned 1352.2 cM, covering 86.4% of the estimated genome size of silver carp. The maximum and average spaces between 420 loci were 21.5 cM and 3.2 cM, respectively. The length of linkage groups ranged from 3.6 cM to 98.5 cM with an average of 41.0 cM. The number of markers per group varied from 2 to 44 with an average of 14.6. The AFLP markers significantly improved the integrity of microsatellite-based linkage groups and increased the genome coverage and marker evenness. A genome-wide recombination suppression was observed in male. In an extreme case, six microsatellites co-segregated in male, but spanned a 45.1 cM region in female.     

A genetic linkage map of Cryptococcus neoformans variety neoformans serotype D (Filobasidiella neoformans)

To construct a genetic linkage map of the heterothallic yeast, Cryptococcus neoformans (Filobasidiella neoformans), we crossed two mating-compatible strains and analyzed 94 progeny for the segregation of 301 polymorphic markers, consisting of 228 restriction site polymorphisms, 63 microsatellites, two indels, and eight mating-type (MAT)-associated markers. All but six markers showed no significant (P < 0.05) segregation distortion. At a minimum LOD score of 6.0 and a maximum recombination frequency of 0.30, 20 linkage groups were resolved, resulting in a map length of approximately 1500 cM. Average marker density is 5.4 cM (range 1-28.7 cM). Hybridization of selected markers to blots of electrophoretic karyotypes unambiguously assigned all linkage groups to chromosomes and led us to conclude that the C. neoformans genome is approximately 20.2 Mb, comprising 14 chromosomes ranging in size from 0.8 to 2.3 Mb, with a ratio of approximately 13.2 kb/cM averaged across the genome. However, only 2 of 12 ungrouped markers hybridized to chromosome 10. The hybridizations revealed at least one possible reciprocal translocation involving chromosomes 8, 9, and 12. This map has been critical to genome sequence assembly and will be essential for future studies of quantitative trait inheritance.     

A Restriction Fragment Length Polymorphism Map and Electrophoretic Karyotype of the Fungal Maize Pathogen Cochliobolus Heterostrophus

A restriction fragment length polymorphism (RFLP) map has been constructed of the nuclear genome of the plant pathogenic ascomycete Cochliobolus heterostrophus. The segregation of 128 RFLP and 4 phenotypic markers was analyzed among 91 random progeny of a single cross; linkages were detected among 126 of the markers. The intact chromosomal DNAs of the parents and certain progeny were separated using pulsed field gel electrophoresis and hybridized with probes used to detect the RFLPs. In this way, 125 markers were assigned to specific chromosomes and linkages among 120 of the markers were confirmed. These linkages totalled 941 centimorgans (cM). Several RFLPs and a reciprocal translocation were identified tightly linked to Tox1, a locus controlling host-specific virulence. Other differences in chromosome arrangement between the parents were also detected. Fourteen gaps of at least 40 cM were identified between linkage groups on the same chromosomes; the total map length was therefore estimated to be, at a minimum, 1501 cM. Fifteen A chromosomes ranging from about 1.3 megabases (Mb) to about 3.7 Mb were identified; one of the strains also has an apparent B chromosome. This chromosome appears to be completely dispensable; in some progeny, all of 15 markers that mapped to this chromosome were absent. The total genome size was estimated to be roughly 35 Mb. Based on these estimates of map length and physical genome size, the average kb/cM ratio in this cross was calculated to be approximately 23. This low ratio of physical length to map distance should make this RFLP map a useful tool for cloning genes.     

A Genetic linkage map of Pinyon pine (Pinus edulis) based on amplified fragment length polymorphisms

 Amplified fragment length polymorphisms (AFLP) were used to rapidly generate a dense linkage map for pinyon pine (Pinus edulis). The map population consisted of 40 megagametophytes derived from one tree at Sunset Crater, Arizona. A total of 78 primer combinations, each with three to five selective nucleotides, amplified 542 polymorphic markers. Of these, 33 markers showed significant deviation from the expected Mendelian genotypic segregation ratio of 1 : 1, and 164 showed complete linkage with another marker. This resulted in 338 unique markers mapping to 25 linkage groups, each of which ranged from 2 to 22 markers, averaging 80 centiMorgans (cM) in size and covering 2,012 cM (2,200 cM with the inclusion of 25 cM for each of 7 unlinked markers). Pairwise linkage values gave a genome size estimate of 2,390 cM, suggesting comprehensive coverage of the genome. A search for subsets of primer combinations giving the best map coverage found 10 primer combinations which together marked 72% of the linkage map to within 10 cM; an additional 10 primer combinations increased this percentage to 85%. Our map represents an initial step towards the identification of quantitative trait loci associated with pest resistance and water stress in pinyons and will further allow us to examine introgression rates between P. edulis and P. californiarum. Received: 14 October 1997 / Accepted: 29 April 1998     

An AFLP-based linkage map of Japanese red pine (Pinus densiflora) using haploid DNA samples of megagametophytes from a single maternal tree

We have constructed an AFLP-based linkage map of Japanese red pine (Pinus densiflora Siebold et Zucc.) using haploid DNA samples of 96 megagametophytes from a single maternal tree, selection clone Kyungbuk 4. Twenty-eight primer pairs generated a total of 5,780 AFLP fragments. Five hundreds and thirteen fragments were verified as genetic markers with two alleles by their Mendelian segregation. At the linkage criteria LOD 4.0 and maximum recombination fraction 0.25(theta), a total of 152 markers constituted 25 framework maps for 19 major linkage groups. The maps spanned a total length of 2,341 cM with an average framework marker spacing of 18.4 cM. The estimated genome size was 2,662 cM. With an assumption of equal marker density, 82.2% of the estimated genome would be within 10 cM of one of the 230 linked markers, and 68.1% would be within 10 cM of one of the 152 framework markers. We evaluated map completeness in terms of LOD value, marker density, genome length, and map coverage. The resulting map will provide crucial information for future genomic studies of the Japanese red pine, in particular for QTL mapping of economically important breeding target traits.     

A genetic linkage map of the soybean cyst nematode <Emphasis Type="Italic">Heterodera glycines</Emphasis>

A genetic linkage map of the soybean cyst nematode (SCN) Heterodera glycines was constructed using a population of F2 individuals obtained from matings between two highly inbred SCN lines, TN16 and TN20. The AFLP fingerprinting technique was used to genotype 63 F2 progeny with two restriction enzyme combinations (EcoRI/MseI and PstI/TaqI) and 38 primer combinations. The same F2 population was also genotyped for Hg-cm-1 (H. glycines chorismate mutase-1), a putative virulence gene, using real-time quantitative PCR. Some of the markers were found to be distributed non-randomly. Even so, of the 230 markers analyzed, 131 could be mapped onto ten linkage groups at a minimum LOD of 3.0, for a total map distance of 539 cM. The Hg-cm-1 locus mapped to linkage group III together with 16 other markers. The size of the H. glycines genome was estimated to be in the range of 630-743 cM, indicating that the current map represents 73-86% of the genome, with a marker density of one per 4.5 cM, and a physical/genetic distance ratio of between 124 kb/cM and 147 kb/cM. This genetic map will be of great assistance in mapping H. glycines markers to genes of interest, such as nematode virulence genes and genes that control aspects of nematode parasitism.     

Genetic mapping of a new set of microsatellite markers in a reference common bean (Phaseolus vulgaris) population BAT93 x Jalo EEP558

The present study describes a new set of 61 polymorphic microsatellite markers for beans and the construction of a genetic map using the BAT93 x Jalo EEP558 (BJ) population for the purpose of developing a reference linkage map for common bean (Phaseolus vulgaris). The main objectives were to integrate new microsatellites on the existing framework map of the BJ population, and to develop the first linkage map for the BJ population based exclusively on microsatellites. Of the total of 264 microsatellites evaluated for polymorphism, 42.8% showed polymorphism between the genitors. An integrated map was created totaling 199 mapped markers in 13 linkage groups, with an observed length of 1358 cM and a mean distance between markers of 7.23 cM. For the map constructed exclusively with microsatellites, 106 markers were placed in 12 groups with a total length of 606.8 cM and average distance of 6.8 cM. Linkage group designation and marker order for BM microsatellites generally agreed with previous mapping, while the new microsatellites were well distributed across the genome, corroborating the utility of the BJ population for a reference map. The extensive use of the microsatellites and the availability of a reference map can help in the development of other genetic maps for common bean through the transfer of information of marker order and linkage, which will allow comparative analysis and map integration, especially for future quantitative trait loci and association mapping studies.