Re-orientation and integration of the classical and interspecific linkage maps of the long arm of tomato chromosome 7

To obtain reliable classical and integrated interspecies maps of the long arm of chromosome 7 of tomato, detailed mapping work was undertaken and several phenotypic and molecular markers were assigned loci on both maps to provide reliable cross-reference points. To maximise the value of the new maps, pair-wise segregation data for classical genetic markers from the literature were included, based on large segregating populations with readily scorable phenotypes. In addition, to increase confidence in these maps, introgression lines were used to confirm important map locations. The revised classical map is based on two- and three-point test-cross data from a number of F₂ and BC₁ mapping populations. The integrated interspecies map is based on F₂ mapping populations derived from crosses of Lycopersicon esculentum with Lycopersicon pennellii (LA716). The genetic analyses for both maps were performed using the computer package JoinMap. The revised composite classical map indicates that some of the map positions reported in the literature are incorrect. The linear order of the classical markers common to both the revised classical and integrated interspecies maps are in complete agreement. Production of the integrated interspecies map resulted in re-orientation of the existing molecular map. Received: 13 September 2000 / Accepted: 20 December 2000     

A genetic linkage map of sweetpotato [Ipomoea batatas (L.) Lam.] based on AFLP markers

Amplified Fragment Length Polymorphism (AFLP) based genetic linkage maps were developed for hexaploid sweetpotato (Ipomoea batatas (L.) Lam., 2n = 6x = 90) using a segregating population derived from a biparental cross between the cultivars 'Tanzania' and 'Bikilamaliya'. A total of 632 ('Tanzania') and 435 ('Bikilamaliya') AFLPs could be ordered in 90 and 80 linkage groups, respectively. Total map lengths were 3655.6 cM and 3011.5 cM, respectively, with an average distance of 5.8 cM between adjacent markers. The genetic linkage analysis was performed in two steps. First a framework map was elaborated from the single dose markers. Interspersed duplex and double-simplex markers were used to detect homologous groups within and corresponding linkage groups among the parental maps. The type of polyploidy (autopolyploidy vs. allopolyploidy) was examined using the ratio of linkage in coupling phase to linkage in repulsion phase and the ratio of non-simplex to simplex markers. Our data support the predominance of polysomic inheritance with some degree of preferential pairing.     

Application of AFLP, RAPD and ISSR markers to genetic mapping of European and Japanese larch

Genetic linkage maps have been increasingly developed for a wide variety of plants, using segregating populations such as F₂s or backcrosses between inbred lines. These pedigrees are rarely available in outbred species like forest trees which have long generation times. Thus genetic mapping studies have to use peculiar pedigrees and markers in appropriate configurations. We constructed single-tree genetic linkage maps of European larch (Larix decidua Mill.) and Japanese larch [Larix kaempferi (Lamb.) Carr.] using segregation data from 112 progeny individuals of an hybrid family. A total of 266 markers (114 AFLP, 149 RAPD and 3 ISSR loci) showing a testcross configuration, i.e.heterozygous in one parent and null in the other parent, were grouped at LOD 4.0, θ=0.3. The maternal parent map (L. decidua)consisted of 117 markers partitioned within 17 linkage groups (1152 cM) and the paternal parent map (L. kaempferi) had 125 markers assembled into 21 linkage groups (1206 cM). The map distance covered by markers was determined by adding a 34.7-cM independence distance at the end of each group and unlinked marker. It reached 2537 cM and 2997 cM respectively for European larch and Japanese larch, and represented respectively a 79.6% and 80.8% coverage of the overall genome. A few 3:1 segregating markers were used to identify homologous linkage groups between the European larch and the Japanese larch genetic maps. The PCR-based molecular markers allowed the construction of genetic maps, thus ensuring a good coverage of the larch genome for further QTL detection and mapping studies. Received: 15 March 1999 / Accepted: 29 March 1999     

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     

Prevalence of segregation distortion in diploid alfalfa and its implications for genetics and breeding applications

Segregation distortion (SD) is often observed in plant populations; its presence can affect mapping and breeding applications. To investigate the prevalence of SD in diploid alfalfa (Medicago sativa L.), we developed two unrelated segregating F₁ populations and one F₂ population. We genotyped all populations with SSR markers and assessed SD at each locus in each population. The three maps were syntenic and largely colinear with the Medicago truncatula genome sequence. We found genotypic SD for 24 and 34% of markers in the F₁ populations and 68% of markers in the F₂ population; distorted markers were identified on every linkage group. The smaller percentage of genotypic SD in the F₁ populations could be because they were non-inbred and/or due to non-fully informative markers. For the F₂ population, 60 of 90 mapped markers were distorted, and they clustered into eight segregation distortion regions (SDR). Most SDR identified in the F₁ populations were also identified in the F₂ population. Genotypic SD was primarily due to zygotic rather than allelic distortion, suggesting zygotic not gametic selection is the main cause of SD. On the F₂ linkage map, distorted markers in all SDR except two showed heterozygote excess. The severe SD in the F₂ population likely biased genetic distances among markers and possibly also marker ordering and could affect QTL mapping of agronomic traits. To reduce the effects of SD and non-fully informative markers, we suggest constructing linkage maps and conducting QTL mapping in advanced generation populations.     

RFLP genetic linkage maps from four F(2.3) populations and a joinmap of Gossypium hirsutum L

An RFLP genetic linkage joinmap was constructed from four different mapping populations of cotton (Gossypium hirsutum L.). Genetic maps from two of the four populations have been previously reported. The third genetic map was constructed from 199 bulk-sampled plots of an F_2.3 (HQ95–6×’MD51ne’) population. The map comprises 83 loci mapped to 24 linkage groups with an average distance between markers of 10.0 centiMorgan (cM), covering 830.1 cM or approximately 18% of the genome. The fourth genetic map was developed from 155 bulk-sampled plots of an F_2.3 (119– 5 sub-okra×’MD51ne’) population. This map comprises 56 loci mapped to 16 linkage groups with an average distance between markers of 9.3 cM, covering 520.4 cM or approximately 11% of the cotton genome. A core of 104 cDNA probes was shared between populations, yielding 111 RFLP loci. The constructed genetic linkage joinmap from the above four populations comprises 284 loci mapped to 47 linkage groups with the average distance between markers of 5.3 cM, covering 1,502.6 cM or approximately 31% of the total recombinational length of the cotton genome. The linkage groups contained from 2 to 54 loci each and ranged in distance from 1.0 to 142.6 cM. The joinmap provided further knowledge of competitive chromosome arrangement, parental relationships, gene order, and increased the potential to map genes for the improvement of the cotton crop. This is the first genetic linkage joinmap assembled in G. hirsutum with a core of RFLP markers assayed on different genetic backgrounds of cotton populations (Acala, Delta, and Texas plain). Research is ongoing for the identification of quantitative trait loci for agronomic, physiological and fiber quality traits on these maps, and the identification of RFLP loci lineage for G. hirsutum from its diploid progenitors (the A and D genomes). Received: 23 February 2001 / Accepted: 8 June 2001     

Comparison off genetic distance and order off DNA markers in five populations of rice.

A group of about 300 evenly distributed DNA markers from a high density RFLP linkage map of rice constructed using an F2 population derived from a japonica variety, Nipponbare, and an indica variety, Kasalath, were used to evaluate gene order and genetic distance in four other rice mapping populations. The purpose of this study was to determine the degree to which information gained from the high density linkage map could be applied to other mapping populations, particularly with regard to its utility in bridging quantitative traits and molecular and physical mapping information. The mapping populations consisted of two F2 populations derived from Dao Ren Qiao/Fl-1084 and Kinandangputi/Fl-1007, recombinant inbred lines from Asominori/IR24, and a backcross population from Sasanishiki/Habataki//Sasanishiki. All DNA markers commonly mapped in the four populations showed the same linkage groups as in the Nipponbare/Kasalath linkage map with conserved linkage order. The genetic distance between markers among the different populations did not vary to a significant level in any of the 12 chromosomes. The differences in some markers could be attributed to the size of the population used in the construction of the linkage maps. Furthermore, the conservation of linkage order found in the distal region of chromosomes 11 and 12 was also confirmed in the RFLP maps based on the four populations of rice. These suggest that any major genetic information from the Nipponbare/Kasalath map can be expected to be approximately the same in other crosses or populations. This high density RFLP linkage map, which is being utilized in constructing a physical map of rice, can be very useful in interpreting genome structure with great accuracy in other populations. Key words : linkage map, japonica, indica, gene order, genetic distance.     

An expressed sequence tag SSR map of tetraploid alfalfa (Medicago sativa L.)

A genetic map constructed from a population segregating for a trait of interest is required for QTL identification. The goal of this study was to construct a molecular map of tetraploid alfalfa (Medicago sativa.) using simple sequence repeat (SSR) markers derived primarily from expressed sequence tags (ESTs) and bacterial artificial chromosome (BAC) inserts of M. truncatula. This map will be used for the identification of drought tolerance QTL in alfalfa. Two first generation backcross populations were constructed from a cross between a water-use efficient, M. sativa subsp. falcata genotype and a low water-use efficient M. sativa subsp. sativa genotype. The two parents and their F₁ were screened with 1680 primer pairs designed to amplify SSRs, and 605 single dose alleles (SDAs) were amplified. In the F₁, 351 SDAs from 256 loci were mapped to 41 linkage groups. SDAs not inherited by the F₁, but transmitted through the recurrent parents and segregating in the backcross populations, were mapped to 43 linkage groups, and 44 of these loci were incorporated into the composite maps. Homologous linkage groups were joined to form eight composite linkage groups representing the eight chromosomes of M. sativa. The composite maps consist of eight composite linkage groups with 243 SDAs from M. truncatula EST sequences, 38 SDAs from M. truncatula BAC clone sequences, and five SDAs from alfalfa genomic SSRs. The total composite map length is 624 cM, with average marker density per composite linkage group ranging from 1.5 to 4.4 cM, and an overall average density of 2.2 cM. Segregation distortion was 10%, and distorted loci tended to cluster on individual homologues of several linkage groups. Electronic Supplementary Material Supplementary material is available for this article at     

The first genetic map of pigeon pea based on diversity arrays technology (DArT) markers

With an objective to develop a genetic map in pigeon pea (Cajanus spp.), a total of 554 diversity arrays technology (DArT) markers showed polymorphism in a pigeon pea F₂ mapping population of 72 progenies derived from an interspecific cross of ICP 28 (Cajanus cajan) and ICPW 94 (Cajanus scarabaeoides). Approximately 13% of markers did not conform to expected segregation ratio. The total number of DArT marker loci segregating in Mendelian manner was 405 with 73.1% (P > 0.001) of DArT markers having unique segregation patterns. Two groups of genetic maps were generated using DArT markers. While the maternal genetic linkage map had 122 unique DArT maternal marker loci, the paternal genetic linkage map has a total of 172 unique DArT paternal marker loci. The length of these two maps covered 270.0 cM and 451.6 cM, respectively. These are the first genetic linkage maps developed for pigeon pea, and this is the first report of genetic mapping in any grain legume using diversity arrays technology.     

A Preliminary Genetic Linkage Map of the Pacific Abalone Haliotis discus hannai Ino

Preliminary genetic linkage maps were constructed for the Pacific abalone (Haliotis discus hannai Ino) using amplified fragment length polymorphism (AFLP), randomly amplified polymorphic DNA (RAPD), and microsatellite markers segregating in a F₁ family. Nine microsatellite loci, 41 RAPD, and 2688 AFLP markers were genotyped in the parents and 86 progeny of the mapping family. Among the 2738 markers, 384 (including 365 AFLP markers, 10 RAPD markers, and 9 microsatellite loci) were polymorphic and segregated in one or both parents: 241 in the female and 146 in the male. The majority of these markers, 232 in the female and 134 in the male, segregated according to the expected 1:1 Mendelian ratio (α = 0.05). Two genetic linkage maps were constructed using markers segregating in the female or the male parent. The female framework map consisted of 119 markers in 22 linkage groups, covering 1773.6 cM with an average intermarker space of 18.3 cM. The male framework map contained 94 markers in 19 linkage groups, spanning 1365.9 cM with an average intermarker space of 18.2 cM. The sex determination locus was mapped to the male map but not to the female map, suggesting a XY-male determination mechanism. Distorted markers showing excess of homozygotes were mapped in clusters, probably because of their linkage to a gene that is incompatible between two parental populations.     

Comparative mapping in Pinus: sugar pine (Pinus lambertiana Dougl.) and loblolly pine (Pinus taeda L.)

The majority of genomic research in conifers has been conducted in the Pinus subgenus Pinus mostly due to the high economic importance of the species within this taxon. Genetic maps have been constructed for several of these pines and comparative mapping analyses have consistently revealed notable synteny. In contrast, little genomic research has been conducted on the Pinus subgenus Strobus, even though these pines have strong ecological relevance. We report a consensus genetic linkage map for sugar pine (Pinus lambertiana Dougl.) constructed with 399 single nucleotide polymorphisms markers derived from annotated genes. The map is 1,231 cM in length and organized into 19 linkage groups. Two of the mapping populations were derived from trees that were segregating for the major gene of resistance (Cr1) to Cronartium ribicola, the fungal pathogen responsible for white pine blister rust. The third mapping population was derived from a full-sib cross segregating for partial resistance to white pine blister rust. In addition, we report the first comparative mapping study between subgenera Strobus and Pinus. Sixty mapped markers were found in common between sugar pine and the loblolly pine reference map with 56 of them (93%) showing collinearity. All 19 linkage groups of the sugar pine consensus map coaligned to the 12 linkage groups of the loblolly pine reference map. The syntenic relationship observed between these two clades of pines provides a foundation for advancing genomic research and genetic resources in subgenus Strobus.     

An integrated genetic map of Populus deltoides based on amplified fragment length polymorphisms

Amplified fragment length polymorphism (AFLP) is an efficient molecular technique for generating a large number of DNA-based genetic markers in Populus. We have constructed an integrated genetic map for a Populus backcross population derived from two selected P. deltoides clones using AFLP markers. A traditional strategy for genetic mapping in outcrossing species, such as forest trees, is based on two-way pseudo-testcross configurations of the markers (testcross markers) heterozygous in one parent and null in the other. By using the markers segregating in both parents (intercross markers) as bridges, the two parent-specific genetic maps can be aligned. In this study, we detected a number of non-parental heteroduplex markers resulting from the PCR amplification of two DNA segments that have a high degree of homology to one another but differ in their nucleotide sequences. These heteroduplex markers detected have served as bridges to generate an integrated map which includes 19 major linkage groups equal to the Populus haploid chromosome number and 24 minor groups. The 19 major linkage groups cover a total of 2,927 cM, with an average spacing between two markers of 23. 3 cM. The map developed in this study provides a first step in producing a highly saturated linkage map of the Populus deltoides genome. Received: 10 September 1999 / Accepted: 3 November 1999     

Development of an AFLP based linkage map of Ler, Col and Cvi Arabidopsis thaliana ecotypes and construction of a Ler/Cvi recombinant inbred line population

An amplified fragment polymorphism (AFLP) based linkage map has been generated for a new Landsberg erecta/Cape Verde Islands (Ler/Cvi) recombinant inbred line (RIL) population. A total of 321 molecular PCR based markers and the erecta mutation were mapped. AFLP markers were also analysed in the Landsberg erecta/Columbia (Ler/Col) RIL population ( Lister & Dean 1993) and 395 AFLP markers have been integrated into the previous Arabidopsis molecular map of 122 RFLPs, CAPSs and SSLPs. This enabled the evaluation of the efficiency and robustness of AFLP technology for linkage analyses in Arabidopsis. AFLP markers were found throughout the linkage map. The two RIL maps could be integrated through 49 common markers which all mapped at similar positions. Comparison of both maps led to the conclusion that segregating bands from a common parent can be compared between different populations, and that AFLP bands of similar molecular size, amplified with the same primer combination in two different ecotypes, are likely to correspond to the same locus. AFLPs were found clustering around the centromeric regions, and the authors have established the map position of the centromere of chromosome 3 by a quantitative analysis of AFLP bands using trisomic plants. AFLP markers were also used to estimate the polymorphism rate among the three ecotypes. The larger polymorphism rate found between Ler and Cvi compared to Ler and Col will mean that the new RIL population will provide a useful material to map DNA polymorphisms and quantitative trait loci.     

A genetic linkage map of European chestnut (Castanea sativa Mill.) based on RAPD, ISSR and isozyme markers

A genetic linkage map of European chestnut (Castanea sativa Mill.) based on RAPD, ISSR and isozyme markers was constructed using the two-way pseudo-testcross strategy. A total of 96 individuals from a F₁ full-sib family was genotyped with 381 molecular markers (311 RAPDs, 65 ISSRs, 5 isozymes). Markers in testcross configuration, segregating 1:1, were used to establish two separate maternal and paternal maps including 187 and 148 markers, respectively. The markers identified 12 linkage groups based on the haploid number of chestnut. The female and male framework maps reached a total length of 720 and 721 cM (Kosambi), respectively, representing a 76% and 68% coverage of the overall genome. A total of 46 markers, found in intercross configuration, segregating 3:1 and 1:2:1, were used to identify homologous linkage groups between parental maps; out of 12 linkage groups 11 could be joined. RAPD and ISSR markers showed a good and comparable reliability, allowing for the first time the establishment of a saturated linkage map for European chestnut. These maps will be a starting point for studies on the structure, evolution and function of the chestnut genome. Identification of QTLs for adaptive traits in chestnut will be the primary target. Received: 3 July 2000 / Accepted: 16 October 2000     

Regulation of Arabidopsis defense responses against Spodoptera littoralis by CPK-mediated calcium signaling

Background

Genetic markers and linkage mapping are basic prerequisites for marker-assisted selection and map-based cloning. In the case of the key grassland species Lolium spp., numerous mapping populations have been developed and characterised for various traits. Although some genetic linkage maps of these populations have been aligned with each other using publicly available DNA markers, the number of common markers among genetic maps is still low, limiting the ability to compare candidate gene and QTL locations across germplasm.

Results

A set of 204 expressed sequence tag (EST)-derived simple sequence repeat (SSR) markers has been assigned to map positions using eight different ryegrass mapping populations. Marker properties of a subset of 64 EST-SSRs were assessed in six to eight individuals of each mapping population and revealed 83% of the markers to be polymorphic in at least one population and an average number of alleles of 4.88. EST-SSR markers polymorphic in multiple populations served as anchor markers and allowed the construction of the first comprehensive consensus map for ryegrass. The integrated map was complemented with 97 SSRs from previously published linkage maps and finally contained 284 EST-derived and genomic SSR markers. The total map length was 742 centiMorgan (cM), ranging for individual chromosomes from 70 cM of linkage group (LG) 6 to 171 cM of LG 2.

Conclusions

The consensus linkage map for ryegrass based on eight mapping populations and constructed using a large set of publicly available Lolium EST-SSRs mapped for the first time together with previously mapped SSR markers will allow for consolidating existing mapping and QTL information in ryegrass. Map and markers presented here will prove to be an asset in the development for both molecular breeding of ryegrass as well as comparative genetics and genomics within grass species.     

RFLP markers for sugar beet breeding: chromosomal linkage maps and location of major genes for rhizomania resistance, monogermy and hypocotyl colour

An RFLP linkage map for the nine chromosomes of sugar beet (Beta vulgaris L. ssp. vulgaris var. altissima Doell) was constructed by using a segregating population from a cross between two plants which were heterozygous for several agronomically interesting characters. One hundred and eleven RFLP loci have been mapped to nine linkage groups using 92 genomic markers. The current RFLP map covers a total length of 540 cM. Evidence for the existence of a major gene for rhizomania resistance (Rr1) is given, together with its map position on linkage group IV in the interval between loci GS44 and GS28a. The presence of an RFLP fragment at the GS3d locus is, until now, the best molecular marker for rhizomania-resistant genotypes in segregating populations of sugar beet; GS3d is linked to Rr1 with 6.7 cM. The gene MM, controlling the polygerm/monogerm seed type, has been mapped on linkage group IX in a distal position at 4.2 cM from the locus GS7. The gene R controlling the hypocotyl colour maps to linkage group VII and does not recombine with the RFLP locus GS42. The inheritance of a group of RFLP loci revealed the possible presence of a translocation in the population used to establish the map. The data presented are discussed in relation to the possibility of using RFLP markers in sugar beet breeding.     

Comparative genome analysis of mungbean (Vigna radiata L. Wilczek) and cowpea (V. unguiculata L. Walpers) using RFLP mapping data

Genome relationships between mungbean (Vigna tradiata) and cowpea (V. Unguiculata) based on the linkage arrangement of random genomic restriction fragment length polymorphism (RFLP) markers have been investigated. A common set of probes derived from cowpea, common bean (Phaseolus vulgaris), mungbean, and soybean (Glycine max) PstI genomic libraries were used to construct the genetic linkage maps. In both species, a single F₂ population from a cross between an improved cultivar and a putative wild progenitor species was used to follow the segregation of the RFLP markers. Approximately 90% of the probes hybridized to both mungbean and cowpea DNA, indicating a high degree of similarity in the nucleotide sequences among these species. A higher level of polymorphism was detected in the mungbean population (75.7%) than in the cowpea population (41.2%). Loci exhibiting duplications, null phenotypes, and distorted segregation ratios were detected in both populations. Random genomic DNA RFLP loci account for about 89% of the currently mapped markers with a few cDNA and RAPD markers added. The current mungbean map is comprised of 171 loci/loci clusters distributed in 14 linkage groups spanning a total of 1570cM. On the other hand, 97 markers covered 684 cM and defined 10 linkage groups in the current cowpea map. The mungbean and cowpea genomes were compared on the basis of the copy number and linkage arrangement of 53 markers mapped in common between the two species. Results indicate that nucleotide sequences are conserved, but variation in copy number were detected and several rearrangements in linkage orders appeared to have occurred since the divergence of the two species. Entire linkage groups were not conserved, but several large linkage blocks were maintained in both genomes.     

Mapping 245 SSR markers on the <Emphasis Type="Italic">Vitis vinifera</Emphasis> genome: a tool for grape genetics

The aim of the present work was to develop a microsatellite marker-based map of the Vitis vinifera genome (n=19), useful for genetic studies in this perennial heterozygous species, as SSR markers are highly transferable co-dominant markers. A total of 346 primer pairs were tested on the two parents (Syrah and Grenache) of a full sib population of 96 individuals (S × G population), successfully amplifying 310 markers. Of these, 88.4% markers were heterozygous for at least one of the two parents. A total of 292 primer pairs were then tested on Riesling, the parent of the RS₁ population derived from selfing (96 individuals), successfully amplifying 299 markers among which 207 (62.9%) were heterozygous. Only 6.7% of the markers were homozygous in all three genotypes, stressing the interest of such markers in grape genetics. Four maps were constructed based on the segregation of 245 SSR markers in the two populations. The Syrah map was constructed from the segregations of 177 markers that could be ordered into 19 linkage groups (total length 1,172.2 cM). The Grenache map was constructed with the segregations of 178 markers that could be ordered into 18 linkage groups (total length 1,360.6 cM). The consensus S × G map was constructed with the segregations of 220 markers that were ordered into 19 linkage groups (total length 1,406.1 cM). One hundred and eleven markers were scored on the RS₁ population, among them 27 that were not mapped using the S × G map. Out of these 111 markers, 110 allowed to us to construct a map of a total length of 1,191.7 cM. Using these four maps, the genome length of V. vinifera was estimated to be around 2,200 cM. The present work allowed us to map 123 new SSR markers on the V. vinifera genome that had not been ordered in a previous SSR-based map (Riaz et al. 2004), representing an average of 6.5 new markers per linkage group. Any new SSR marker mapped is of great potential usefulness for many applications such as the transfer of well-scattered markers to other maps for QTL detection, the use of markers in specific regions for the fine mapping of genes/QTL, or for the choice of markers for MAS.     

RFLP mapping of Brassica napus using doubled haploid lines

The combined use of doubled haploid lines and molecular markers can provide new genetic information for use in breeding programs. An F₁-derived doubled haploid (DH) population of Brassica napus obtained from a cross between an annual canola cultivar (Stellar) and a biennial rapeseed (Major) was used to construct a linkage map of 132 restriction fragment length polymorphism loci. The marker loci were arranged into 22 linkage groups and six pairs of linked loci covering 1016 cM. The DH map was compared to a partial map constructed with a common set of markers for an F₂ population derived from the same F₁ plant, and the overall maps were not significantly different. Comparisons of maps in Brassica species suggest that less recombination occurs in B. napus (n = 19) than expected from the combined map distances of the two hypothesized diploid progenitors, B. oleracea (n = 9) and B. rapa (n=10). A high percentage (32%) of segregating marker loci were duplicated in the DH map, and conserved linkage arrangements of some duplicated loci indicated possible intergenome homoeology in the amphidiploid or intragenome duplications from the diploid progenitors. Deviation from Mendelian segregation ratios (P < 0.05) was observed for 30% of the marker loci in the DH population and for 24% in the F₂ population. Deviation towards each parent occurred at equal frequencies in both populations and marker loci that showed deviation clustered in specific linkage groups. The DH lines and molecular marker map generated for this study can be used to map loci for agronomic traits segregating in this population. Present address Embrapa/Cenargen, C.P. 0.2372, CEP 70.770, Brasilia DF, Brazil     

A Primary Genetic Linkage Map of 14 Polymorphic Loci for the Short Arm of Human Chromosome 8

A genetic linkage map of markers for the short arm of human chromosome 8 has been constructed with 14 polymorphic DNA markers on the basis of genotypes obtained in 40 CEPH reference families. This unbroken map spans 45 cM in males and 79 cM in females. The 14 markers include three genes, MSR, LPL, and NEFL, and one anonymous DNA segment that were previously assigned to chromosome 8. The other 10 marker had been isolated from a chromosome 8-specific cosmid library and physically localized to chromosomal bands by fluorescence in situ hybridization. The order of loci determined by genetic linkage was consistent with their physical locations. This map will facilitate efficient linkage studies of human genetic diseases that may be segregating on chromosome 8p and will provide anchor points for development of high-resolution maps for this chromosomal region.