Linkage analysis in tetraploid potato and association of markers with quantitative resistance to late blight (Phytophthora infestans)

We have constructed a partial linkage map in tetraploid potato which integrates simplex, duplex and double-simplex AFLP markers. The map consists of 231 maternal and 106 paternal markers with total map lengths of 990.9?cM and 484.6?cM. The longer of the two cumulative map lengths represents approximately 25% coverage of the genome. In tetraploids, much of the polymorphism between parental clones is masked by `dosage' which significantly reduces the number of individual markers that can be scored in a population. Consequently, the major advantage of using AFLPs – their high multiplex ratio – is reduced to the point where the use of alternative multi-allelic marker types would be significantly more efficient. The segregation data and map information have been used in a QTL analysis of late blight resistance, and a multi-allelic locus at the proximal end of chromosome VIII has been identified which contributes significantly to the expression of resistance. No late blight resistance genes or QTLs have previously been mapped to this location.     

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     

Triticum turgidum L. 6A and 6B recombinant substitution lines: extended linkage maps and characterization of residual background alien genetic variation

  Triticum turgidum L. var ‘durum’ cv ‘Langdon’-T. t. var ‘dicoccoides’ chromosome 6A and 6B recombinant substitution lines (RSLs) and a F₂ population derived from a ‘Langdon’-T. t. var ‘dicoccoides’ disomic chromosome 6A substitution lineבLangdon’ cross were analyzed with the objective of markedly increasing the number of markers assigned to and the resolution of previously constructed 6A and 6B linkage maps. Fifty-seven markers were added to the 6A RSL-population map, which now consists of 73 markers that span 111 cM, and 40 markers were added to the 6B RSL-population map, which now consists of 56 markers that span 123 cM. With the exception of 2 6B loci, all of the loci on the two RSL-population maps were ordered at a LOD score ≥3.0. Thirty-seven orthologous markers were mapped in the two chromosomes and colinearity between them is strongly indicated. The 6A RSL-population map and the F₂-population map are highly similar, indicating that the former population, which consists of 66 lines, can be reliably used for mapping, as was previously demonstrated for the 6B RSL population. In the absence of selection and genetic drift, the lines in a RSL population, except at loci in the substituted/recombined chromosome, should be near-isogenic. An unexpected finding was that at least 26 and possibly 29 of the RFLPs detected in the RSL populations (18% of the markers analyzed) are not located in the substituted/recombined chromosomes. Linkage analysis of the markers disclosed that at least 19 of them are located in six or seven segments that span approximately 10 cM and 17 cM of the genetic lengths of 6B and 6A, respectively, in the 6A and 6B RSL populations, respectively, a finding that suggests that 40 or more alien segments spanning 8–15% of the genetic length of the 13 unsubstituted chromosomes are present in both of the RSL populations. Alien alleles are fixed in many RSLs for most of the markers, in most cases at a frequency consistent with theoretical expectations. Highly distorted segregation favoring the alien allele was detected for all of the markers in 2 of the segments, however. Nine of the markers were among those mapped in the substituted/recombined chromosomes; the linkage data obtained for the other 10 was sufficient to assign them to approximate map positions. Received: 12 June 1997 / Accepted: 6 October 1997     

A genetic linkage map of Quercus robur L. (pedunculate oak) based on RAPD, SCAR, microsatellite, minisatellite, isozyme and 5S rDNA markers

 A genetic map of Pedunculate oak (Quercus robur) was constructed based on one 5S rDNA, 271 RAPD, ten SCAR, 18 microsatellite, one minisatellite, and six isozyme markers. A total of 94 individuals from a full-sib family was genotyped. Two maps, including 307 markers, were constructed according to the “two-way pseudo-testcross” mapping strategy. Testcross markers segregating in the 1 : 1 ratio were first used to establish separate maternal (893.2 cM, 12 linkage groups) and paternal (921.7 cM, 12 linkage groups) maps. Both maps provided 85–90% genome coverage. Homologies between the male and female linkage groups were then identified based on 74 intercross markers segregating in the 3 : 1, 1 : 2 : 1 and 1 : 1 : 1 : 1 ratios (RAPDs, SCARs, SSRs, 5S rDNA and isozymes) in the hybrid progeny. In each map, approximately 18% of the studied markers showed segregation distortion. More than 60% of the skewed markers were due to an excess of heterozygote genotypes. This map will be used for: (1) studying the molecular organisation of genomic regions involved in inter- and intraspecific differentiation in oaks and (2) identification of QTLs for adaptive traits. Received: 30 January 1998 / Accepted: 12 May 1998     

AFLP genetic maps of Eucalyptus globulus and E. tereticornis

 Amplified fragment length polymorphism (AFLP) analysis is a rapid and efficient technique for detecting large numbers of DNA markers in eucalypts. We have used AFLP markers in a two-way pseudo-testcross strategy to generate genetic maps of two clones of different Eucalyptus species (E. tereticornis and E. globulus). Of 606 polymorphic fragments scored, 487 segregated in a 1 : 1 ratio, corresponding to DNA polymorphisms heterozygous in one parent and null in the other. In the maternal E. tereticornis map, 268 markers were ordered in 14 linkage groups (919 cM); the paternal E. globulus map had 200 markers in 16 linkage groups (967 cM). Results from PGRI software were compared with MAPMAKER. The average density of markers was approximately 1 per 3.9 cM. Framework markers were ordered with an average confidence level of 90%, covering 80–100% of the estimated Eucalyptus genome size. In order to investigate the homologies between the E. tereticornis and the E. globulus genetic linkage maps, we included 19 markers segregating 3 : 1 in the analysis. Some homeologous linkage groups were recognized. The linkage data developed in these maps will be used to detect loci controlling commercially important traits. Received: 17 July 1997 / Accepted: 13 October 1997     

Towards second-generation STS (sequence-tagged sites) linkage maps in conifers: a genetic map of Norway spruce ( Picea abies K.)

Genetic linkage maps have been produced for a wide range of organisms during the last decade, thanks to the increasing availability of molecular markers. The use of microsatellites (or Simple Sequence Repeats, SSRs) as genetic markers has led to the construction of “second-generation” genetic maps for humans, mouse and other organisms of major importance. We constructed a second-generation single-tree genetic linkage map of Norway spruce (Picea abies K.) using a panel of 72 haploid megagametophytes with a total of 447 segregating bands [366 Amplified Fragment Length Polymorphisms (AFLPs), 20 Selective Amplification of Microsatellite Polymorphic Loci (SAMPLs) and 61 SSRs, each single band being treated initially as a dominant marker]. Four hundred and thirteen markers were mapped in 29 linkage groups (including triplets and doublets) covering a genetic length of 2198.3 cM, which represents 77.4% of the estimated genome length of Picea abies (approximately 2839 cM). The map is still far from coalescing into the expected 12 chromosomal linkage groups of Norway spruce (2n = 2x = 24). A possible explanation for this comes from the observed non-random distribution of markers in the framework map. Thirty-eight SSR marker loci could be mapped onto 19 linkage groups. This set of highly informative Sequence Tagged Sites (STSs) can be used in many aspects of genetic analysis of forest trees, such as marker-assisted selection, QTL mapping, positional cloning, gene flow analysis, mating system analysis and genetic diversity studies. Received: 5 November 1997 / Accepted: 16 March 1998     

Genomic mapping and testing for quantitative trait loci in tea (<Emphasis Type="Italic">Camellia sinensis</Emphasis> (L.) O. Kuntze)

The tea industry is significant in the economies of tea-growing countries. Prospects of improving yield of made tea genomic information were explored using clones from a cross between clones TRFCA SFS150 and AHP S15/10. The 42 clones were tested in two distinct tea-growing regions in Kenya. Bulk segregant analysis was performed followed by complete genotyping. Out of 260 informative markers, 100 markers that showed 1:1 segregation were used to construct a linkage map. The map contained 30 (19 maternal and 11 paternal) linkage groups that spanned 1,411.5 cM with mean interval of 14.1 cM between loci. Based on the map, quantitative trait loci (QTL) analysis was done on yield data over 2003–2007 across the two sites, Timbilil and Kangaita. Twenty-three putative QTLs were detected, 16 in five different linkage groups for Timbilil, two in two groups for Kangaita, and the rest were associated with unassigned markers. No QTL was detected at both sites, which showed strong genotype × site interaction (G × E) but highly effective within-site heritability ([^(h)]² {\hat{h}^2} generally > 0.7). Problems of overestimated and spurious QTL effects arising from the smallness of the population should be mitigated by generally high within-site heritability. At least two unassigned markers associated with yield at Kangaita over the whole study period, suggesting potential as candidate markers for site-specific marker-assisted selections. Implications of the results with respect to mapping population, G × E, and marker-assisted selection are discussed.     

Genetic Mapping and QTL Analysis of Growth-Related Traits in the Pacific Oyster

The Pacific oyster (Crassostrea gigas) is one of the most important oysters cultured worldwide. To analyze the oyster genome and dissect growth-related traits, we constructed a sex-averaged linkage map by combining 64 genomic simple sequence repeats, 42 expressed sequence tag-derived SSRs, and 320 amplified fragment length polymorphism markers in an F₁ full-sib family. A total of 426 markers were assigned to 11 linkage groups, spanning 558.2 cM with an average interval of 1.3 cM and 94.7% of genome coverage. Segregation distortion was significant for 18.8% of the markers (P < 0.05), and distorted markers tended to occur on some genetic regions or linkage groups. Most growth-related quantitative traits were highly significantly (P < 0.01) correlated, and principal component analysis obtained four principal components. Quantitative trait locus (QTL) analysis identified three significant QTLs for two principal components, which explained 0.6–13.9% of the phenotypic variation. One QTL for sex was detected on linkage group 6, and the inheritabilities of sex for parental alleles and maternal alleles on that locus C15 are 39.8% and 0.01%, respectively. The constructed linkage map and determined QTLs can provide a tool for further genetic analysis of the traits and be potential for marker-assisted selection in C. gigas breeding.     

Linkage map of Japanese black pine based on AFLP and RAPD markers including markers linked to resistance against the pine needle gall midge

Macrogametophytes derived from the seeds of a tree resistant to pine needle gall midge (PGM) were analyzed using amplified fragment length polymorphism (AFLP). A total of 244 segregating loci were detected among 71 macrogametophytes. Combining the AFLP results with previously reported segregation data for 127 random amplified polymorphic DNA (RAPD) markers, 157 AFLP and 50 RAPD markers with confirmed map positions were assigned to 20 linkage groups and three pairs covering 2085.5 cM with an average distance of 10.1 cM. The total map distance covers about 77.1–78.4% of the total genome, estimated to be approximately 2665–2719 cM in length. Thus, using AFLP markers, the previous RAPD map of this tree was improved in terms of the average distance between markers, the total map distance, and coverage of the genome. Three RAPD markers linked to a gene associated with resistance to PGM were also located on this map. Rceived: 14 April 2000 / Accepted: 21 August 2000     

QTL analysis of late blight resistance in a diploid potato family of Solanum phureja × S. stenotomum

Field resistance to Phytophthora infestans (Mont.) de Bary, the causal agent of late blight in potatoes, has been characterized in a potato segregating family of 230 full-sib progenies derived from a cross between two hybrid Solanum phureja × S. stenotomum clones. The distribution of area under the disease progress curve values, measured in different years and locations, was consistent with the inheritance of multigenic resistance. Relatively high levels of resistance and transgressive segregations were also observed within this family. A genetic linkage map of this population was constructed with the intent of mapping quantitative trait loci (QTLs) associated with this late blight field resistance. A total of 132 clones from this family were genotyped based on 162 restriction fragment length polymorphism (RFLP) markers. The genome coverage by the map (855.2 cM) is estimated to be at least 70% and includes 112 segregating RFLP markers and two phenotypic markers, with an average distance of 7.7 cM between two markers. Two methods were employed to determine trait–marker association, the non-parametric Kruskal–Wallis test and interval mapping analysis. Three major QTLs were detected on linkage group III, V, and XI, explaining 23, 17, and 10%, respectively, of the total phenotypic variation. The present study revealed the presence of potentially new genetic loci in this diploid potato family contributing to general resistance against late blight. The identification of these QTLs represents the first step toward their introgression into cultivated tetraploid potato cultivars through marker-assisted selection.     

Linkage mapping and genome length in eastern white pine (Pinus strobus L.)

 Haploid linkage analysis of eastern white pine, Pinus strobus L., was carried out using mainly RAPD markers and microsatellite, or simple-sequence-repeat, markers. Ninety one loci mapped to 12 linkage groups of three or more markers. The resulting framework genome map, the first for a soft pine species, contained 69 markers. The map covered 58% of the estimated genome length of 2071 cM(K), with a 95% confidence interval of 1828–2242 cM(K). A systematic comparison of linkage data from eastern white pine, longleaf pine (P. palustris Mill.) and maritime pine (P. pinaster Ait.), gave genome-length estimates for all three species very close to either 2000 cM(K) or 2600 cM(H), depending on whether the Kosambi(K) or Haldane(H) map functions, respectively, were employed. Differences among previous pine genome-length estimates were attributed to the divergent criteria used in the methods of estimation, and indicate the need for the adoption of uniform criteria when performing genome-length estimates. Current data suggest that members of the two pine subgenera, which diverged during the late Mesozoic era, have highly conserved rates of recombination. Received: 5 January 1997/Accepted: 24 January 1997     

Construction of a genetic linkage map for roses using RAPD and AFLP markers

A segregating population of diploid rose hybrids (2n = 2x = 14) was used to construct the first linkage maps of the rose genome. A total of 305 RAPD and AFLP markers were analysed in a population of 60 F₁ plants based on a so-called ”double-pseudotestcross” design. Of these markers 278 could be located on the 14 linkage groups of the two maps, covering total map lengths of 326 and 370 cM, respectively. The average distances between markers in the maps for 93/1–117 and 93/1–119 is 2.4 and 2.6 cM, respectively. In addition to the molecular markers, genes controlling two phenotypic characters, petal number (double versus single flowers) and flower colour (pink versus white), were mapped on linkage groups 3 and 2, respectively. The markers closest to the gene for double flowers, Blfo, and to the gene for pink flower colour, Blfa, cosegregated without recombinants. The maps provide a tool for further genetic analyses of horticulturally important genes as, for example, resistance genes and a starting point for marker-assisted breeding in roses. Received: 22 September 1998 / Accepted: 12 March 1999     

Identification of AFLP markers linked to resistance of cowpea (Vigna unguiculata L.) to parasitism by Striga gesnerioides

AFLP and bulked segregant analysis were used to identify molecular markers linked to resistance of cowpea [Vigna ungiculata (L.) Walp.] to parasitism by Striga gesnerioides (Willd.) Vatke. Segregation analysis of F₂ progeny from a cross of Tvx3236, a Striga-susceptible line, with IT82D-849, a resistant cultivar, showed that resistance to S. gesnerioides race 1 from Burkina Faso was controlled by a single dominant gene, designated Rsg2–1. Three AFLP markers were identified that are tightly linked to Rsg2–1: E-AAC/M-CAA₃₀₀ (2.6 cM), E-ACT/M-CAA₅₂₄ (0.9 cM), and E-ACA/M-CAT_140/150 (0.9 cM), which appears to be codominant. Segregation analysis of a different F₂ population resulting from a cross of the Striga-susceptible line IT84S-2246–4 with Tvu 14676, a S. gesnerioides race 3 resistant line, showed that resistance to S. gesnerioides race 3 was also controlled by a single dominant gene, designated Rsg4–3. Six AFLP markers linked to Rsg4–3 were identified: E-ACA/M-CAG₁₂₀ (10.1 cM), E-AGC/M-CAT₈₀ (4.1 cM), E-ACA/M-CAT₁₅₀ (2.7 cM), E-AGC/M-CAT₁₅₀ (3.6 cM), E-AAC/M-CAA₃₀₀ (3.6 cM), and E-AGC/M-CAT₇₀ (5.1 cM). Segregation analysis of the E-AAC/M-CAA₃₀₀ and E-ACA/M-CAG₁₂₀ markers in recombinant inbred lines derived from IT84S-2049×524B determined that both are located within linkage group 1 of the cowpea genetic map. The identification of AFLP markers linked to Striga resistance provides a stepping stone for a marker-assisted selection program and the eventual cloning and characterization of the gene(s) encoding resistance to this noxious parasitic weed. Received: 24 April 2000 / Accepted: 21 August 2000     

First report on the presence of fire blight resistance in linkage group 11 ofPyrus ussuriensis Maxim

Fire blight, caused by the gram-negative bacteriumErwinia amylovora (Burrill) Winslow et al., is a dangerous disease of pome fruits, including pear. A pear breeding program for fire blight resistance was initiated in 2003 at the Department of Pomology, Warsaw University of Life Sciences, Poland. Since several Asian species are considered to be potential sources of resistance to fire blight, the susceptiblePyrus communis ‘Doyenne du Comice’ was crossed with the resistantP. ussuriensis. The F₁ full-sib progeny composed of 155 seedlings was tested for susceptibility to fire blight by artificial shoot inoculation. A framework linkage map of both parents was constructed based on 48 AFLP and 32 SSR markers and covered a length of 595 cM and 680 cM in ‘Doyenne du Comice’ andP. ussuriensis, respectively. For the first time a putative QTL for fire blight resistance inP. ussuriensis linkage group 11 was identified. Another putative QTL in linkage group 4 of ‘Doyenne du Comice’ seems to indicate that sources of fire blight resistance can be identified also in the susceptible cultivars.     

Linkage maps of grapevine displaying the chromosomal locations of 420 microsatellite markers and 82 markers for <Emphasis Type="Italic">R</Emphasis>-gene candidates

Genetic maps functionally oriented towards disease resistance have been constructed in grapevine by analysing with a simultaneous maximum-likelihood estimation of linkage 502 markers including microsatellites and resistance gene analogs (RGAs). Mapping material consisted of two pseudo-testcrosses, ‘Chardonnay’ × ‘Bianca’ and ‘Cabernet Sauvignon’ × ‘20/3’ where the seed parents were Vitis vinifera genotypes and the male parents were Vitis hybrids carrying resistance to mildew diseases. Individual maps included 320–364 markers each. The simultaneous use of two mapping crosses made with two pairs of distantly related parents allowed mapping as much as 91% of the markers tested. The integrated map included 420 Simple Sequence Repeat (SSR) markers that identified 536 SSR loci and 82 RGA markers that identified 173 RGA loci. This map consisted of 19 linkage groups (LGs) corresponding to the grape haploid chromosome number, had a total length of 1,676 cM and a mean distance between adjacent loci of 3.6 cM. Single-locus SSR markers were randomly distributed over the map (CD = 1.12). RGA markers were found in 18 of the 19 LGs but most of them (83%) were clustered on seven LGs, namely groups 3, 7, 9, 12, 13, 18 and 19. Several RGA clusters mapped to chromosomal regions where phenotypic traits of resistance to fungal diseases such as downy mildew and powdery mildew, bacterial diseases such as Pierce’s disease, and pests such as dagger and root-knot nematode, were previously mapped in different segregating populations. The high number of RGA markers integrated into this new map will help find markers linked to genetic determinants of different pest and disease resistances in grape. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Mapping QTLs for submergence tolerance in rice by AFLP analysis and selective genotyping

By combining the amplified fragment length polymorphism (AFLP) technique with selective genotyping, we constructed a linkage map for rice and assigned each linkage group to a corresponding chromosome. The AFLP map, consisting of 202 AFLP markers, was generated from 74 recombinant inbred lines (RIL) which were selected from both extremes of the population (250 lines) with respect to the response to complete submergence. Map length was 1756 cM, with an average interval size of 8.5 cM. To assign linkage groups to chromosomes, we used 50 previously mapped AFLP markers as anchor markers distributed over the 12 chromosomes. Other AFLP markers were then assigned to specific chromosomes based on their linkage to anchor markers. This AFLP map is equivalent to the RFLP/AFLP map constructed previously as the anchors were in the same order in both maps. Furthermore, tests with two restriction fragment length polymorphism (RFLP) markers and two sequence-tagged site (STS) markers showed that they mapped in the expected positions. Using this AFLP map, a major gene for submergence tolerance was localized on chromosome 9. Quantitative trait loci (QTL) associated with submergence tolerance were detected on chromosomes 6, 7, 11, and 12. We conclude that the combination of AFLP mapping and selective genotyping provides a much faster and easier approach to QTL identification than the use of RFLP markers. Received: 20 December 1996 / Accepted: 21 January 1997     

Genetic loci associated with field resistance to late blight in offspring of Solanum phureja and S.tuberosum grown under short-day conditions

Field resistance to late blight – a fungal disease caused by Phytophthora infestans – has been genetically characterized by analyzing trait-marker association in a Solanum phureja (phu)×dihaploid Solanum tuberosum (dih-tbr) population. Trait data were developed at three locations over a 3-year period under natural infection pressure. RAPD (random amplified polymorphic DNA) and AFLP (amplified fragment length polymorphism)markers were used to develop anonymous genetic linkage groups subsequently anchored to potato chromosomes using mapped RFLP (restriction fragment length polymorphism), SSR (single sequence repeats) and AFLP markers. RFLP and SSR markers achieved the most-accurate anchoring. Two genetic maps were obtained, with 987.4 cM for phu and 773.7 cM for dih-tbr. Trait-marker association was revealed by single-marker and interval mapping analyses. Two important QTLs (quantitative trait loci) were detected on chromosomes VII and XII as a contribution from both parents, totalling up to 16% and 43%, respectively, of the phenotypic variation (PH). One additional QTL was detected on chromosome XI (up to 11% of the PH) as a contribution from the phu parent, and three others were detected on chromosome III (up to 13% of the PH), chromosome V (up to 11% of the PH) and chromosome VIII (up to 11% of the PH) as a contribution from the dih-tbr parent. Our results reveal new genetic loci of the potato genome that contribute to resistance to late blight. We postulate that some of these loci could be related to plant growth under short-day conditions. Received: 5 July 2000 / Accepted: 17 November 2000     

An interspecific (Capsicum annuum ×C. chinese) F2 linkage map in pepper using RFLP and AFLP markers

We have constructed a molecular linkage map of pepper (Capsicum spp.) in an interspecific F₂ population of 107 plants with 150 RFLP and 430 AFLP markers. The resulting linkage map consists of 11 large (206–60.3 cM) and 5 small (32.6–10.3 cM) linkage groups covering 1,320 cM with an average map distance between framework markers of 7.5 cM. Most (80%) of the RFLP markers were pepper-derived clones, and these markers were evenly distributed across the genome. By using 30 primer combinations, we were able to generate 444 AFLP markers in the F₂ population. The majority of the AFLP markers clustered in each linkage group, although PstI/MseI markers were more evenly distributed than EcoRI/MseI markers within the linkage groups. Genes for the biosynthesis of carotenoids and capsaicinoids were mapped on our linkage map. This map will provide the basis of studying secondary metabolites in pepper. Received: 20 October 1999 / Accepted: 3 July 2000     

An EST-SSR-based linkage map for Persea americana Mill. (avocado)

Recent enhancement of the pool of known molecular markers for avocado has allowed the construction of the first moderately dense genetic map for this species. Over 300 SSR markers have been characterized and 163 of these were used to construct a map from the reciprocal cross of two Florida cultivars 'Simmonds' and 'Tonnage'. One hundred thirty-five primer pairs amplified 163 usable loci with 20 primer pairs amplifying more than one locus. 'Tonnage' was heterozygous for 152 (93%) loci, whereas 'Simmonds' was heterozygous for 64 (39%). Null alleles were identified at several loci. Linkage maps were produced for both reciprocal crosses and combined to generate a composite linkage map for the F₁ population of 715 individuals. The composite map contains 12 linkage groups. Linkage groups ranged in size from 157.3 cM (LG2) to 2.4 cM (LG12) and the number of loci mapped per group ranged from 29 (LG1) to two (LG12). The total map length was 1,087.4 cM. Only seven markers were observed to have segregation distortion (α ≤ 0.05) across both sub-composite (reciprocal) maps. Phenotypic data from traits of horticultural interest are currently being collected on this population with the ultimate goal of identifying useful quantitative trait loci and the development of a marker-assisted selection program.     

Construction of RFLP-based maps of foxtail millet, Setaria italica (L.) P. Beauv.

 An RFLP-based map consisting of 160 loci was constructed in an intervarietal cross of foxtail millet [Setaria italica (L.) P. Beauv.], Longgu 25×Pagoda Flower Green. The map comprises nine linkage groups, which were aligned with the nine foxtail millet chromosomes using trisomic lines, and spans 964 cM. The intraspecific map was compared to an interspecific map, constructed in a S. italica×S. viridis cross. Both the order of the markers and the genetic distances between the loci were highly conserved. Deviations from the expected 1 : 2 : 1 Mendelian segregation ratios were observed in both the intra- and inter-specific populations. The segregation data indicate that chromosome VIII in the Longgu 25×Pagoda Flower Green cross carries a gene that strongly affects gamete fertility. Received: 18 December 1996 / Accepted: 4 August 1997