Genetic linkage maps of white birches (<Emphasis Type="Italic">Betula platyphylla</Emphasis> Suk. and <Emphasis Type="Italic">B. pendula</Emphasis> Roth) based on RAPD and AFLP markers

A pseudo-testcross mapping strategy was used in combination with the random amplified polymorphism DNA (RAPD) and amplified fragment length polymorphism (AFLP) genotyping methods to develop two moderately dense genetic linkage maps for Betula platyphylla Suk. (Asian white birch) and B. pendula Roth (European white birch). Eighty F₁ progenies were screened with 291 RAPD markers and 451 AFLP markers. We selected 230 RAPD and 362 AFLP markers with 1:1 segregation and used them for constructing the parent-specific linkage maps. The resultant map for B. platyphylla was composed of 226 markers in 24 linkage groups (LGs), and spanned 2864.5 cM with an average of 14.3 cM between adjacent markers. The linkage map for B. pendula was composed of 226 markers in 23 LGs, covering 2489.7 cM. The average map distance between adjacent markers was 13.1 cM. Clustering of AFLP markers was observed on several LGs. The availability of these white birch linkage maps will contribute to the molecular genetics and the implementation of marker-assisted selection in these important forest species.     

Genetic Linkage Maps of <Emphasis Type="Italic">Betula platyphylla</Emphasis> Suk Based on ISSR and AFLP Markers

Two separate genetic linkage maps for Chinese silver birch based on inter-simple sequence repeat (ISSR) and amplified fragment-length polymorphism (AFLP) were constructed by a pseudo-testcross mapping strategy. Eighty F₁ progenies were obtained from the cross between two parental trees with desirable traits (the paternal one selected from ‘Qinghai’ and the maternal one from ‘Wangqing’). A total of 46 ISSR primers and 31 AFLP primers were employed to generate 102 ISSR and 355 AFLP polymorphic markers in the F₁ progenies. About 5.7% of all the markers displayed high segregation distortion with a P value below 0.01 and such markers were not used for map constructions. The paternal map consisted of 137 loci, spread over 13 groups and spanned 694.2 cM at an average distance of 5.1 cM between the markers, while in the maternal map, 147 loci were distributed in 14 groups covering a map distance about 949.62 cM at an average distance of 6.5 cM. These initial maps can serve as the basis for developing a more detailed genetic map.     

A genetic linkage map of Pacific white shrimp (<Emphasis Type="Italic">Litopenaeus vannamei</Emphasis>): sex-linked microsatellite markers and high recombination rates

Pacific white shrimp (Litopenaeus vannamei) is the leading species farmed in the Western Hemisphere and an economically important aquaculture species in China. In this project, a genetic linkage map was constructed using amplified fragment length polymorphism (AFLP) and microsatellite markers. One hundred and eight select AFLP primer combinations and 30 polymorphic microsatellite markers produced 2071 markers that were polymorphic in either of the parents and segregated in the progeny. Of these segregating markers, 319 were mapped to 45 linkage groups of the female framework map, covering a total of 4134.4 cM; and 267 markers were assigned to 45 linkage groups of the male map, covering a total of 3220.9 cM. High recombination rates were found in both parental maps. A sex-linked microsatellite marker was mapped on the female map with 6.6 cM to sex and a LOD of 17.8, two other microsatellite markers were also linked with both 8.6 cM to sex and LOD score of 14.3 and 16.4. The genetic maps presented here will serve as a basis for the construction of a high-resolution genetic map, quantitative trait loci (QTLs) detection, marker-assisted selection (MAS) and comparative genome mapping.     

A consensus map for <Emphasis Type="Italic">Cucurbita pepo</Emphasis>

Using random amplified polymorphic DNA (RAPD), amplified fragment length polymorphism (AFLP), simple sequence repeats (SSR), and morphological traits, the first genetic maps for Cucurbita pepo (2n=2x=40) were constructed and compared. The two mapping populations consisted of 92 F₂ individuals each. One map was developed from a cross between an oil-seed pumpkin breeding line and a zucchini accession, into which genes for resistance to Zucchini Yellow Mosaic Virus (ZYMV) from a related species, C. moschata, had been introgressed. The other map was developed from a cross between an oil-seed pumpkin and a crookneck variety. A total of 332 and 323 markers were mapped in the two populations. Markers were distributed in each map over 21 linkage groups and covered an average of 2,200 cM of the C. pepo genome. The two maps had 62 loci in common, which enabled identification of 14 homologous linkage groups. Polyacrylamide gel analyses allowed detection of a high number of markers suitable for mapping, 10% of which were co-dominant RAPD loci. In the Pumpkin-Zucchini population, bulked segregant analysis (BSA) identified seven markers less than 7 cM distant from the locus n, affecting lignification of the seed coat. One of these markers, linked to the recessive hull-less allele (AW11-420), was also found in the Pumpkin-Crookneck population, 4 cM from n. In the Pumpkin-Zucchini population, 24 RAPD markers, previously introduced into C. pepo from C. moschata, were mapped in two linkage groups (13 and 11 markers in LGpz1 and LGpz2, respectively), together with two sequence characterized amplified region (SCAR) markers linked to genes for resistance to ZYMV.     

Construction of an intra-specific sweet cherry (<Emphasis Type="Italic">Prunus avium</Emphasis> L.) genetic linkage map and synteny analysis with the <Emphasis Type="Italic">Prunus</Emphasis> reference map

Linkage maps of the sweet cherry cultivar ‘Emperor Francis’ (EF) and the wild forest cherry ‘New York 54’ (NY) were constructed using primarily simple sequence repeat (SSR) markers and gene-derived markers with known positions on the Prunus reference map. The success rate for identifying SSR markers that could be placed on either the EF or NY maps was only 26% due to two factors: a reduced transferability of other Prunus-species-derived markers and a low level of polymorphism in the mapping parents. To increase marker density, we developed four cleaved amplified polymorphic sequence markers (CAPS), 19 derived CAPS markers, and four insertion–deletion markers for cherry based on 101 Prunus expressed sequence tags. In addition, four gene-derived markers representing orthologs of a tomato vacuolar invertase and fruit size gene and two sour cherry sorbitol transporters were developed. To complete the linkage analysis, 61 amplified fragment length polymorphism and seven sequence-related amplified polymorphism markers were also used for map construction. This analysis resulted in the expected eight linkage groups for both parents. The EF and NY maps were 711.1 cM and 565.8 cM, respectively, with the average distance between markers of 4.94 cM and 6.22 cM. A total of 82 shared markers between the EF and NY maps and the Prunus reference map showed that the majority of the marker orders were the same with the Prunus reference map suggesting that the cherry genome is colinear with that of the other diploid Prunus species. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

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.     

AFLP-Based Genetic Linkage Maps of the Pacific Oyster <Emphasis Type="Italic">Crassostrea gigas</Emphasis> Thunberg

Amplified fragment length polymorphisms (AFLPs) were used for genome mapping in the Pacific oyster Crassostrea gigas Thunberg. Seventeen selected primer combinations produced 1106 peaks, of which 384 (34.7%) were polymorphic in a backcross family. Among the polymorphic markers, 349 were segregating through either the female or the male parent. Chi-square analysis indicated that 255 (73.1%) of the markers segregated in a Mendelian ratio, and 94 (26.9%) showed significant (P < 0.05) segregation distortion. Separate genetic linkage maps were constructed for the female and male parents. The female framework map consisted of 119 markers in 11 linkage groups, spanning 1030.7 cM, with an average interval of 9.5 cM per marker. The male map contained 96 markers in 10 linkage groups, covering 758.4 cM, with 8.8 cM per marker. The estimated genome length of the Pacific oyster was 1258 cM for the female and 933 cM for the male, and the observed coverage was 82.0% for the female map and 81.3% for the male map. Most distorted markers were deficient for homozygotes and closely linked to each other on the genetic map, suggesting the presence of major recessive deleterious genes in the Pacific oyster.     

Mapping of qualitative and quantitative phenotypic traits in <Emphasis Type="Italic">Rosa</Emphasis> using AFLP markers

A segregating population of 91 hybrids issued from a cross between a dihaploid rose, derived from the haploidisation of a modern cultivar, and a diploid species was used to construct linkage maps of the parental genomes. As in other recent genetic studies in Rosa, AFLPs were used as molecular markers. Two segregating qualitative traits, recurrent blooming and double corolla, already known to be inherited as single recessive and dominant genes, respectively, were recorded in the mapping population. A quantitative trait, thorn density of the shoots, was also evaluated in this population. Sixty eight and 108 AFLP markers located on 8 and 6 linkage groups could be analysed in the female and male parent, respectively. The two recorded qualitative phenotypic markers were mapped as well as the quantitative one, after having performed QTL analyses on the parental maps in the latter case. It appears that thorn quantity is controlled by a major and a minor QTL which are located on the same linkage group at 36.5 and 3.2 cM from the single seasonal-blooming gene, respectively.     

Comparing EST-based genetic maps between <Emphasis Type="BoldItalic">Pinus sylvestris</Emphasis> and <Emphasis Type="BoldItalic">Pinus taeda</Emphasis>

A genetic map of Pinus sylvestris was constructed using ESTP (expressed sequence tag polymorphism) markers and other gene-based markers, AFLP markers and microsatellites. Part of the ESTP markers (40) were developed and mapped earlier in Pinus taeda, and additional markers were generated based on P. sylvestris sequences or sequences from other pine species. The mapping in P. sylvestris was based on 94 F₁ progeny from a cross between plus-tree parents E635C and E1101. AFLP framework maps for the parent trees were first constructed. The ESTP and other gene sequence-based markers were added to the framework maps, as well as five published microsatellite loci. The separate maps were then integrated with the aid of AFLPs segregating in both trees (dominant segregation ratios 3:1) as well as gene markers and microsatellites segregating in both parent trees (segregation ratios 1:1:1:1 or 1:2:1). The integrated map consisted of 12 groups corresponding to the P. taeda linkage groups, and additionally three and six smaller groups for E1101 and E635C, respectively. The number of framework AFLP markers in the integrated map is altogether 194 and the number of gene markers 61. The total length of the integrated map was 1,314 cM. The set of markers developed for P. sylvestris was also added to existing maps of two P. taeda pedigrees. Starting with a mapped marker from one pedigree in the source species resulted in a mapped marker in a pedigree of the other species in more than 40% of the cases, with about equal success in both directions. The maps of the two species are largely colinear, even if the species have diverged more than 70 MYA. Most cases of different locations were probably due to problems in identifying the orthologous members of gene families. These data provide a first ESTP-containing map of P. sylvestris, which can also be used for comparing this species to additional species mapped with the same markers.Communicated by C. Möllers     

Development of transcript-associated microsatellite markers for diversity and linkage mapping studies in hop (<Emphasis Type="Italic">Humulus lupulus</Emphasis> L.)

Data mining of gene sequences available from various projects dealing with the development of expressed sequence tags (ESTs) can contribute to the discovery of new microsatellite markers. Our aim was to develop new microsatellite markers in hop isolated from an enriched cDNA library and from coding GenBank sequences and to test their suitability in hop diversity studies and for construction of a linkage map. In a set of 614 coding GenBank sequences, 72 containing microsatellites were found (11.7%); the most frequent were trinucleotide repeats (54.0%) followed by dinucleotide repeats (34.5%). Additionally, 11 sequences containing microsatellites were isolated from an enriched cDNA library. A total of 34 primer pairs were designed, 29 based on GenBank sequences and five on sequences from the cDNA enriched library. Twenty-seven (79.4%) coding microsatellites were successfully amplified and used in diversity and linkage mapping studies. Eleven primer pairs amplified 12 coding microsatellite loci suitable for mapping and were placed on female and male linkage maps. We were able to extend previous simple sequence repeat (SSR) female, male and integral maps by 38.8, 25.8 and 40.0 cM, respectively. In the diversity study, 36 diverse hop genotypes were analyzed. Twenty-four coding microsatellites were polymorphic, 17 showing co-dominant behavior and 7 primer pairs amplifying three or more bands in some hop genotypes. Altogether, 143 microsatellite DNA fragments were amplified and they revealed a clear separation of hop genotypes according to geographical region, use or breeding history. In addition, a discussion and comparison of results with other plant coding/EST SSR studies is presented. Our results showed that these microsatellite markers can enhance hop diversity and linkage mapping studies and are a comparable marker system to non-coding SSRs.     

An integrative genetic linkage map of winter wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.)

We constructed a genetic linkage map based on a cross between two Swiss winter wheat (Triticum aestivum L.) varieties, Arina and Forno. Two-hundred and forty F₅ single-seed descent (SSD)-derived lines were analysed with 112 restriction fragment length polymorphism (RFLP) anonymous probes, 18 wheat cDNA clones coding for putative stress or defence-related proteins and 179 simple-sequence repeat (SSR) primer-pairs. The 309 markers revealed 396 segregating loci. Linkage analysis defined 27 linkage groups that could all be assigned to chromosomes or chromosome arms. The resulting genetic map comprises 380 loci and spans 3,086 cM with 1,131 cM for the A genome, 920 cM for the B genome and 1,036 cM for the D genome. Seventeen percent of the loci showed a significant (P < 0.05) deviation from a 1:1 ratio, most of them in favour of the Arina alleles. This map enabled the mapping of QTLs for resistance against several fungal diseases such as Stagonospora glume blotch, leaf rust and Fusarium head blight. It will also be very useful for wheat genetic mapping, as it combines RFLP and SSR markers that were previously located on separate maps. S. Paillard and T. Schnurbusch contributed equally to the work     

Genetic mapping of local adaptation along the altitudinal gradient in <Emphasis Type="Italic">Abies sachalinensis</Emphasis>

Understanding the genetic bases of local adaptation in dominant conifer species is critical in predicting the impacts of rapid climate change on forest ecosystems. However, the genetic basis of adaptation is not yet fully understood due to the huge and complex genomes of conifers and the unavailability to date of suitable crossing material. In this study, we constructed a linkage map for Abies sachalinensis (2n = 24) and investigated quantitative trait loci (QTLs) associated with local adaptation along an altitudinal gradient. A segregating population of 239 seedlings was produced from a cross between two F₁ hybrids (high-altitude × low-altitude genotypes). QTL mapping of phenological and growth traits was performed using a pseudo-testcross strategy with linkage maps based on 1251 single-nucleotide polymorphism (SNP) and three simple sequence repeat (SSR) markers. Two maps consisting of 12 linkage groups with an average marker interval of ca. 3 cM were constructed for each parent. The total lengths of the maps were 1861 and 1949 cM. A permutation test identified four significant QTLs and 11 additional suggestive QTLs, with high logarithm of odds (LOD) scores (> 3.0). This is the first highly saturated linkage map produced for Abies taxa. Our results suggest that spring bud phenology is controlled by several QTLs with moderate effects. The use of the mapping population created by crossing two hybrids (high × low altitude genotypes) and numerous SNP markers enabled us to investigate the genetic basis of adaptive traits in conifer species.     

Mapping of a locus for adult plant resistance to downy mildew in broccoli (<Emphasis Type="Italic">Brassica oleracea</Emphasis> convar. <Emphasis Type="Italic">italica</Emphasis>)

The identification of the gene Pp523, conferring downy mildew resistance to adult plants of broccoli (Brassica oleracea convar. italica), led to the construction of a genetic map that included this resistance locus, 301 amplified fragment length polymorphisms, 55 random amplified polymorphic DNAs, 46 inter-simple sequence repeats, three simple sequence repeats, four other PCR markers and a flower colour locus, all gathered into nine major linkage groups. Nineteen additional molecular markers were clustered into one group of four markers, one group of three markers and six pairs of markers. The map spans over 731.9 cM, corresponding to 89.5% of the 818 cM estimated to be the total genome length. A significant number of the mapped markers, 19.3%, showed distorted segregation. The average distance between mapped adjacent markers is 1.64 cM, which places this map among the densest published to date for this species. Using bulked segregant analysis, we identified a group of molecular markers flanking and closely linked in coupling to the resistance gene and included these in the map. Two markers linked in coupling, OPK17_980 and AT.CTA_133/134, are located at 3.1 cM and 3.6 cM, respectively, at each side from the resistance gene. These markers can be used for marker-assisted selection in breeding programs aiming at the introgression of this gene in susceptible B. oleracea genotypes. The fine mapping of the genomic region surrounding the Pp523 resistance gene is currently being carried out, a basic condition for its isolation via positional cloning.     

Construction of a genetic linkage map in <Emphasis Type="Italic">Fenneropenaeus chinensis</Emphasis> (Osbeck) using RAPD and SSR markers

The primary genetic linkage maps of Fenneropenaeus chinensis (Osbeck) were constructed by using the “two-way pseudo-testcross” strategy with RAPD and SSR markers. Parents and F1 progeny were used as segregating populations. Sixty-one RAPD primers and 20 pairs of SSR primers were screened from 460 RAPD primers and 44 pairs of SSR primers. These primers were used to analyze the parents and 82 progeny of the mapping family. About 146 primers (128 RAPDs, 18 microsatellites) in the female and 127 primers (109 RAPDs, 18 microsatellites) in the male were segregating markers. The female linkage map included eight linkage groups, nine triplets and 14 doublets, spanning 1,173 cM with the average marker density of 11.28 cM, and the observed coverage was 59.36%. The male linkage map included 10 linkage groups, 12 triplets and seven doublets, spanning 1,144.6 cM with the average marker density of 12.05 cM, and the observed coverage was 62.01%. The construction of the F. chinensis genetic linkage maps here opened a new prospect for marker-assisted selection program, comparative genomics and quantitative trait loci (QTL) gene location and cloning.     

An integrated SSR based linkage map for <Emphasis Type="Italic">Zoysia matrella</Emphasis> L. and <Emphasis Type="Italic">Z. japonica</Emphasis> Steud.

Japanese lawngrass (Zoysia japonica) and Manila grass (Z. matrella) are the two most important and commonly used Zoysia species. A consensus based SSR linkage map was developed for the genus by combining maps from each species. This used previously constructed maps for two Z. japonica populations and a new map from Z. matrella. The new SSR linkage map for Z. matrella was based on 86 F₂ individuals and contained 213 loci and covered a map distance of 1,351.2 cM in 32 linkage groups. Comparison of the three linkage maps constructed from populations with different genetic backgrounds indicated that most markers exhibited a consensus order, although some intervals or regions displayed discrepancy in marker orders or positions. The integrated map comprises 507 loci with a mean interval of 4.1 cM, covering a map distance of 2,066.6 cM in 22 linkage groups. The SSR-based map will allow marker-assisted selection and be useful for the mapping and cloning of economically important genes or quantitative trait loci.     

Development of SSR markers derived from SSR-enriched genomic library of eggplant (<Emphasis Type="Italic">Solanum melongena</Emphasis> L.)

Eggplant (Solanum melongena L.), also known as aubergine or brinjal, is an important vegetable in many countries. Few useful molecular markers have been reported for eggplant. We constructed simple sequence repeat (SSR)-enriched genomic libraries in order to develop SSR markers, and sequenced more than 14,000 clones. From these sequences, we designed 2,265 primer pairs to flank SSR motifs. We identified 1,054 SSR markers from amplification of 1,399 randomly selected primer pairs. The markers have an average polymorphic information content of 0.27 among eight lines of S. melongena. Of the 1,054 SSR markers, 214 segregated in an intraspecific mapping population. We constructed cDNA libraries from several eggplant tissues and obtained 6,144 expressed sequence tag (EST) sequences. From these sequences, we designed 209 primer pairs, 7 of which segregated in the mapping population. On the basis of the segregation data, we constructed a linkage map, and mapped the 236 segregating markers to 14 linkage groups. The linkage map spans a total length of 959.1 cM, with an average marker distance of 4.3 cM. The markers should be a useful resource for qualitative and quantitative trait mapping and for marker-assisted selection in eggplant breeding.     

<Emphasis Type="Italic">In silico</Emphasis> genotyping of the maize nested association mapping population

Nested Association Mapping (NAM) has been proposed as a means to combine the power of linkage mapping with the resolution of association mapping. It is enabled through sequencing or array genotyping of parental inbred lines while using low-cost, low-density genotyping technologies for their segregating progenies. For purposes of data analyses of NAM populations, parental genotypes at a large number of Single Nucleotide Polymorphic (SNP) loci need to be projected to their segregating progeny. Herein we demonstrate how approximately 0.5 million SNPs that have been genotyped in 26 parental lines of the publicly available maize NAM population can be projected onto their segregating progeny using only 1,106 SNP loci that have been genotyped in both the parents and their 5,000 progeny. The challenge is to estimate both the genotype and genetic location of the parental SNP genotypes in segregating progeny. Both challenges were met by estimating their expected genotypic values conditional on observed flanking markers through the use of both physical and linkage maps. About 90%, of 500,000 genotyped SNPs from the maize HapMap project, were assigned linkage map positions using linear interpolation between the maize Accessioned Gold Path (AGP) and NAM linkage maps. Of these, almost 70% provided high probability estimates of genotypes in almost 5,000 recombinant inbred lines.     

Molecular markers linked to the apple scab resistance gene <Emphasis Type="Italic">Vbj</Emphasis> derived from <Emphasis Type="Italic">Malus baccata jackii</Emphasis>

Breeding for scab-resistant apple cultivars by pyramiding several resistance genes in the same genetic background is a promising way to control apple scab caused by the fungus Venturia inaequalis. To achieve this goal, DNA markers linked to the genes of interest are required in order to select seedlings with the desired resistance allele combinations. For several apple scab resistance genes, molecular markers are already available; but until now, none existed for the apple scab resistance gene Vbj originating from the crab apple Malus baccata jackii. Using bulk segregant analysis, three RAPD markers linked to Vbj were first identified. These markers were transformed into more reliable sequence-characterised amplified region (SCAR) markers that proved to be co-dominant. In addition, three SSR markers and one SCAR were identified by comparing homologous linkage groups of existing genetic maps. Discarding plants showing genotype–phenotype incongruence (GPI plants) plants, a linkage map was calculated. Vbj mapped between the markers CH05e03 (SSR) and T6-SCAR, at 0.6 cM from CH05e03 and at 3.9 cM from T6-SCAR. Without the removal of the GPI plants, Vbj was placed 15 cM away from the closest markers. Problems and pitfalls due to GPI plants and the consequences for mapping the resistance gene accurately are discussed. Finally, the usefulness of co-dominant markers for pedigree analysis is also demonstrated.     

Alignment of a <Emphasis Type="Italic">Salix</Emphasis> linkage map to the <Emphasis Type="Italic">Populus</Emphasis> genomic sequence reveals macrosynteny between willow and poplar genomes

Poplars (genus Populus) and willows (genus Salix) are members of the Salicaceae, a family of catkin-bearing trees, shrubs and sub-shrubs. Poplar is considered the model system for biological studies in trees and considerable genetic and genomic resources have become available in recent years. The transfer of information to research studies in willow, for which fewer resources are currently available, would be highly beneficial. However, the extent of conservation between poplar and willow genomes has not yet been extensively studied. To address this, we have constructed a linkage map of willow based on a large mapping population derived from a cross between two Salix viminalis × (S. viminalis × S. schwerinii) hybrid sibs, and aligned this to the publicly available poplar genome sequence. A set of genome-wide, expressed poplar sequences was selected and used to design primer sets that efficiently amplified homeologous regions in willow. Direct sequencing of the willow products confirmed homology with the poplar target in the majority of instances and allowed identification of single nucleotide polymorphisms (SNPs) that were used to map these loci. In total, 202 amplified fragment length polymorphisms (AFLPs), 75 microsatellites and 79 SNPs were used to construct a willow consensus map that spanned 1,856.7 cM with an average interval between markers of 6.3 cM. Poplar sequences homologous to those of the mapped willow microsatellite loci were identified and used in addition to the SNP markers to putatively align all but two minor linkage groups to the poplar genome sequence. A high degree of macrosynteny was revealed.     

A full saturated linkage map of<Emphasis Type="Italic"> Picea abies</Emphasis> including AFLP,SSR, ESTP, 5S rDNA and morphological markers

Based on an F₁ progeny of 73 individuals, two parental maps were constructed according to the double pseudo-test cross strategy. The paternal map contained 16 linkage groups for a total genetic length of 1,792 cM. The maternal map covered 1,920 cM, and consisted of 12 linkage groups. These parental maps were then integrated using 66 intercross markers. The resulting consensus map covered 2,035 cM and included 755 markers (661 AFLPs, 74 SSRs, 18 ESTPs, the 5S rDNA and the early cone formation trait) on 12 linkage groups, reflecting the haploid number of chromosomes of Picea abies. The average spacing between two adjacent markers was 2.6 cM. The presence of 39 of the SSR and/or ESTP markers from this consensus map on other published maps of different Picea and Pinus species allowed us to establish partial linkage group homologies across three P. abies maps (up to five common markers per linkage group). This first saturated linkage map of P. abies could be therefore used as a support for developing comparative genome mapping in conifers.Communicated by O. Savolainen