Aligning male and female linkage maps of apple (Malus pumila Mill.) using multi-allelic markers

 Linkage maps for the apple cultivars ‘Prima’ and ‘Fiesta’ were constructed using RFLP, RAPD, isozyme, AFLP, SCAR and microsatellite markers in a ‘Prima’בFiesta’ progeny of 152 individuals. Seventeen linkage groups, putatively corresponding to the seventeen haploid apple chromosomes, were obtained for each parent. These maps were aligned using 67 multi-allelic markers that were heterozygous in both parents. A large number of duplicate RFLP loci was observed and, in several instances, linked RFLP markers in one linkage group showed corresponding linkage in another linkage group. Distorted segregation was observed mainly in two regions of the genome, especially in the male parent alleles. Map positions were provided for resistance genes to scab and rosy leaf curling aphid (Vf and Sd ₁, respectively) for the fruit acidity gene Ma and for the self-incompatibility locus S. The high marker density and large number of mapped codominant RFLPs and some microsatellite markers make this map an ideal reference map for use in other progenies also and a valuable tool for the mapping of quantitative trait loci. Received: 17 November 1997 / Accepted: 9 December 1997     

RFLP and RAPD markers linked to the rosy leaf curling aphid resistance gene (Sd1) in apple

 Sd ₁ is a dominant gene for resistance to biotypes 1 and 2 of the rosy leaf curling aphid, Dysaphis devecta Wlk., which can cause economic damage to apple trees. This report describes the identification of three RFLP and four RAPD markers linked to Sd ₁ in a cross between the D. devecta susceptible variety ‘Prima’ (sd ₁ sd ₁) and the resistant variety ‘Fiesta’ (Sd ₁ sd ₁). Potted trees were artificially infested in the glasshouse, and the ratio of resistant:susceptible plants supported the hypothesis that the resistance was under the control of a single dominant gene. The position of the gene was mapped to a single locus on a ‘Fiesta’ chromosome, within 2 cM of three tightly linked RFLP markers (MC064a, 2B12a and MC029b); the four RAPD markers were located further away (between 13 and 46 cM). This is the first report of molecular markers for an aphid resistance gene in tree fruit crops. The potential application of these markers in a marker-assisted resistance breeding programme is discussed. Received: 1 July 1996/Accepted: 23 August 1996     

Rust mite resistance in apple assessed by quantitative trait loci analysis

The aim of this study was to assess the genetic basis of rust mite (Aculus schlechtendali) resistance in apple (Malus × domestica). A. schlechtendali infestation of apple trees has increased as a consequence of reduced side effects of modern fungicides on rust mites. An analysis of quantitative trait loci (QTLs) was carried out using linkage map data available for F₁ progeny plants of the cultivars ‘Fiesta’ × ‘Discovery’. Apple trees representing 160 different genotypes were surveyed for rust mite infestation, each at three different sites in two consecutive years. The distribution of rust mites on the individual apple genotypes was aggregated and significantly affected by apple genotype and site. We identified two QTLs for A. schlechtendali resistance on linkage group 7 of ‘Fiesta’. The AFLP marker E35M42-0146 (20.2 cM) and the RAPD marker AE10-400 (45.8 cM) were closest positioned to the QTLs and explained between 11.0% and 16.6% of the phenotypic variability. Additionally, putative QTLs on the ‘Discovery’ chromosomes 4, 5 and 8 were detected. The SSR marker Hi03a10 identified to be associated to one of the QTLs (AFLP marker E35M42-0146) was traced back in the ‘Fiesta’ pedigree to the apple cultivar ‘Wagener’. This marker may facilitate the breeding of resistant apple cultivars by marker assisted selection. Furthermore, the genetic background of rust mite resistance in existing cultivars can be evaluated by testing them for the identified SSR marker. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

QTL analysis for aphid resistance and growth traits in apple

The rosy apple aphid (Dysaphis plantaginea), the leaf-curling aphid (Dysaphis cf. devecta) and the green apple aphid (Aphis pomi) are widespread pest insects that reduce growth of leaves, fruits and shoots in apple (Malus × domestica). Aphid control in apple orchards is generally achieved by insecticides, but alternative management options like growing resistant cultivars are needed for a more sustainable integrated pest management (IPM). A linkage map available for a segregating F₁-cross of the apple cultivars ‘Fiesta’ and ‘Discovery’ was used to investigate the genetic basis of resistance to aphids. Aphid infestation and plant growth characteristics were repeatedly assessed for the same 160 apple genotypes in three different environments and 2 consecutive years. We identified amplified fragment length polymorphism (AFLP) markers linked to quantitative trait loci (QTLs) for resistance to D. plantaginea (‘Fiesta’ linkage group 17, locus 57.7, marker E33M35–0269; heritability: 28.3%), and to D. cf. devecta (‘Fiesta’ linkage group 7, locus 4.5, marker E32M39–0195; heritability: 50.2%). Interactions between aphid species, differences in climatic conditions and the spatial distribution of aphid infestation were identified as possible factors impeding the detection of QTLs. A pedigree analysis of simple sequence repeat (SSR) marker alleles closely associated with the QTL markers revealed the presence of the alleles in other apple cultivars with reported aphid resistance (‘Wagener’, ‘Cox’s Orange Pippin’), highlighting the genetic basis and also the potential for gene pyramiding of aphid resistance in apple. Finally, significant QTLs for shoot length and stem diameter were identified, while there was no relationship between aphid resistance and plant trait QTLs. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Development of an STS map of an interspecific progeny of Malus

Simple sequence repeat (SSR) markers developed from Malus, as well as Prunus, Pyrus and Sorbus, and some other sequence-tagged site (STS) loci were analysed in an interspecific F₁ apple progeny from the cross ‘Fiesta’ × ‘Totem’ that segregated for several agronomic characters. A linkage map was constructed using 259 STS loci (247 SSRs, four SCARs and eight known-function genes) and five genes for agronomic traits—scab resistance (Vf), mildew resistance (Pl-2), columnar growth habit (Co), red tissues (Rt) and green flesh background colour (Gfc). Ninety SSR loci and three genes (ETR1, Rt and Gfc) were mapped for the first time in apple. The transferability of markers from other Maloideae to Malus was found to be around 44%. The loci are spread across 17 linkage groups, corresponding to the basic chromosome number of Malus and cover 1,208 cM, approximately 85% of the estimated length of the apple genome. Interestingly, we have extended the top of LG15 with eight markers covering 25 cM. The average map density is 4.7 cM per marker; however, marker density varies greatly between linkage groups, from 2.5 in LG14 to 8.9 in LG7, with some areas of the genome still in need of further STS markers for saturation. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. An erratum to this article can be found at     

QTL mapping of aroma compounds analysed by headspace solid-phase microextraction gas chromatography in the apple progeny ‘Discovery’ × ‘Prima’

Improving fruit quality of apple varieties is an important but complex breeding goal. Flavour is among the key factors of apple fruit quality but in spite of the analytical and biochemical knowledge about volatiles little is known about the genetic and molecular bases of apple aroma. The aim of this study was to use a saturated molecular linkage map of apple to identify QTLs for aroma compounds such as alcohols, esters and terpenes, but also for a number of unidentified volatile compounds (non-targeted analysis approach). Two parental genetic maps were constructed for the apple cultivars ‘Discovery’ and ‘Prima’ by using mainly AFLP and SSR markers. ‘Discovery’ and ‘Prima’ showed very different volatile patterns, and ‘Discovery’ mostly had the higher volatile concentrations in comparison with the Vf-scab resistant ‘Prima’ which has its origin in the small-fruited apple species Malus floribunda. About 50 putative QTLs for a total of 27 different apple fruit volatiles were detected through interval mapping by using genotypic data of 150 F₁ individuals of the mapping population ‘C3’ together with phenotypic data obtained by head-space solid phase microextraction gas chromatography. QTLs for volatile compounds putatively involved in apple aroma were found on 12 out of the 17 apple chromosomes, but they were not evenly dispersed. QTLs were mainly clustered on linkage groups LG 2, 3 and 9. In a first attempt, a LOX (lipoxygenase) candidate gene, putatively involved in volatile metabolism, was mapped on LG 9, genetically associated with a cluster of QTLs for ester-type volatiles. Implications for aroma breeding in apple are discussed.     

Construction of a 550 kb BAC contig spanning the genomic region containing the apple scab resistance gene Vf

A positional cloning project was started in apple with the aim of isolating the Vf resistance gene of Malus floribunda 821. Vf confers resistance against apple scab, the most important disease in apple orchards. A chromosome walk starting from two molecular markers (M18-CAPS and AM19-SCAR) flanking Vf was performed, using a bacterial artificial chromosome (BAC) library containing inserts of the cultivar Florina, which is heterozygous for Vf. Thirteen BAC clones spanning the region between the two markers were identified in nine chromosome walking steps. The size of the resulting contig is approximately 550 kb. In order to map the Vf region in more detail, we analyzed over 2000 plants from different populations segregating for Vf with markers produced from BAC end sequences. In this way, we were able to restrict the possible location of the Vf gene to a minimum of five clones spanning an interval of approximately 350 kb. Received: 4 July 1999 / Accepted: 16 September 1999     

Genome mapping of three major resistance genes to woolly apple aphid (Eriosoma lanigerum Hausm.)

Woolly apple aphid (WAA; Eriosoma lanigerum Hausm.) can be a major economic problem to apple growers in most parts of the world, and resistance breeding provides a sustainable means to control this pest. We report molecular markers for three genes conferring WAA resistance and placing them on two linkage groups (LG) on the genetic map of apple. The Er1 and Er2 genes derived from ‘Northern Spy’ and ‘Robusta 5,’ respectively, are the two major genes that breeders have used to date to improve the resistance of apple rootstocks to this pest. The gene Er3, from ‘Aotea 1’ (an accession classified as Malus sieboldii), is a new major gene for WAA resistance. Genetic markers linked to the Er1 and Er3 genes were identified by screening random amplification of polymorphic deoxyribonucleic acid (DNA; RAPD) markers across DNA bulks from resistant and susceptible plants from populations segregating for these genes. The closest RAPD markers were converted into sequence-characterized amplified region markers and the genome location of these two genes was assigned to LG 08 by aligning the maps around the genes with a reference map of ‘Discovery’ using microsatellite markers. The Er2 gene was located on LG 17 of ‘Robusta 5’ using a genetic map developed in a M.9 × ‘Robusta 5’ progeny. Markers for each of the genes were validated for their usefulness for marker-assisted selection in separate populations. The potential use of the genetic markers for these genes in the breeding of apple cultivars with durable resistance to WAA is discussed.     

<Emphasis Type="Italic">Vr</Emphasis><Subscript><Emphasis Type="Italic">2</Emphasis></Subscript>: a new apple scab resistance gene

Reports from several European countries of the breakdown of the Vf resistance, the most frequently used source of resistance in breeding programs against apple scab, emphasize the urgency of diversifying the basis of apple scab resistance and pyramiding different apple scab resistances with the use of their associated molecular markers. GMAL 2473 is an apple scab resistant selection thought to carry the resistance gene Vr. We report the identification by BSA of three AFLP markers and one RAPD marker associated with the GMAL 2473 resistance gene. SSRs associated with the resistance gene were found by (1) identifying the linkage group carrying the apple scab resistance and (2) testing the SSRs previously mapped in the same region. One such SSR, CH02c02a, mapped on linkage group 2, co-segregates with the resistance gene. GMAL 2473 was tested with molecular markers associated with other apple scab resistance genes, and accessions carrying known apple scab resistance genes were tested with the SSR linked to the resistance gene found in GMAL 2473. The results indicate that GMAL 2473 does not carry Vr, and that a new apple scab resistance gene, named Vr ₂, has been identified.     

A major resistance gene from Russian apple ‘Antonovka’ conferring field immunity against apple scab is closely linked to the <Emphasis Type="Italic">Vf</Emphasis> locus

A major scab resistance gene called Va1 was identified in the Russian apple cultivar ‘Antonovka’ (accession APF22) conferring scab resistance under conditions of natural scab infection in the field. After scab scorings over a period of 3 years, a 1:1 segregation was observed in the mapping population 04/214 (‘Golden Delicious’ × ‘Antonovka’). The Va1 resistance gene provides sufficient broad spectrum resistance that is of use in apple resistance breeding and has been assigned Rvi17 according the proposal for a new scab nomenclature (Bus et al., Acta Horticulturae 814:739–746, 2009). Analysis of simple sequence repeats (SSRs) located on the apple linkage group (LG) 1 showed that the Va1 locus is closely linked (1 cM) to SSR CH-Vf1 known to cosegregate with the Vf locus. A tight genetic association was also observed between a specific cleaved amplified polymorphic sequence marker (ARD-CAPS) developed from the HcrVf paralog Vf2ARD present in ‘Antonovka’, but there is no indication yet for a causal relationship with Vf2ARD. Although the whole race spectrum of Va1 is still unknown, it was obvious that it acts against the scab races 6 and 7 which are able to overcome the resistance of Malus floribunda 821. A second resistance factor (named Va2) was studied by race 1-specific scab tests based on grafted 04/214 clones. A 1:1-segregation ratio was observed, too, but 18 “phenotypic recombinants” were found after comparisons with the field scab data of the same genotypes. Va2 was mapped on LG 1 with a genetic distance of about 15 cM above CH-Vf1. The positions of the newly identified ‘Antonovka’ scab resistance factors are compared with previously reported Va mapping approaches and published results from quantitative trait loci analyses performed with different ‘Antonovka’ genotypes.     

Genetic mapping of the pear scab resistance gene Vnk of Japanese pear cultivar Kinchaku

Pear scab (caused by Venturia nashicola) is one of the most harmful diseases of pears, especially Japanese and Chinese pear species. The molecular identification and early selection of resistant plants could greatly improve pear breeding. We have identified the position of the scab resistance gene, designated Vnk in an indigenous Japanese pear cultivar Kinchaku, within the pear genome by using simple sequence repeat (SSR) markers derived from pear and apple. The position of Vnk was identified in the central region of linkage group 1 of Kinchaku. Several amplified fragment length polymorphism (AFLP) markers linked to Vnk were obtained by bulked segregant analysis. Among them, the AFLP marker closest to Vnk was converted into a sequence tagged site (STS) marker. Four random amplified polymorphic DNA (RAPD) markers previously found to be loosely associated with Vnk (Iketani et al. 2001) were successfully converted into STS markers. Six markers (one SSR Hi02c07 and five STSs converted from AFLP and RAPD) showed tight linkages to Vnk, being mapped with distances ranging from 2.4 to 12.4 cM. The SSR CH-Vf2, which was isolated from a BAC clone of the contig containing the apple scab gene Vf, was mapped at the bottom of linkage group 1 in Kinchaku, suggesting that the Vnk and Vf loci are located in different genomic regions of the same homologous linkage group.     

Assessment of fire blight tolerance in apple based on plant inoculations with Erwinia amylovora and DNA markers

Fire blight (Erwinia amylovora) causes serious damage to pome fruit orchards, and identification of germplasm with heritable disease resistance is therefore crucial. Two dominant SCAR (sequence characterised amplified region) marker alleles (AE10-375 and GE-8019), flanking a previously identified QTL (quantitative trait locus) for resistance to fire blight on ‘Fiesta’ linkage group 7 in apple cultivars related to ‘Cox’s Orange Pippin’, were screened on 205 apple cultivars. Both marker alleles were present in 22% of the cultivars, indicating presence of the QTL allele for tolerance, and both were lacking in 25%, indicating homozygosity for absence of the QTL tolerance allele. However, 33% had only the marker allele AE10-375, while 20% had only GE-8019, suggesting that some cultivars with the dominant alleles for both of the flanking markers can carry these on separate chromosomes and may lack the QTL allele for tolerance. In 2009 and 2010, terminal shoots of greenhouse-grown grafted trees of 21 cultivars (only 20 in 2010) were inoculated with Erwinia amylovora. ‘Idared’ (susceptible) and ‘Enterprise’ (tolerant) were included as controls. Disease severity for each cultivar was expressed as percentage of necrosis in relation to entire length of shoot, and the ranking of cultivars in 2009 and 2010 was compared with a Spearman rank correlation test, P < 0.01. A relationship between presence of both flanking marker alleles for tolerance and level of fire blight tolerance was confirmed with a Mann–Whitney U-test, P < 0.01 in 2009, and P < 0.05 in 2010. A PCO (principal coordinate) analysis based on band profiles obtained with 12 SSR (simple sequence repeat) loci produced three loose clusters, two of which contained known offspring of ‘Cox’s Orange Pippin’, and one with cultivars that were either unrelated or had an unknown origin. Cases where DNA markers did not predict level of fire blight damage as expected, were, however, as common among descendants of ‘Cox’s Orange Pippin’ as among apparently unrelated cultivars. Obviously the ‘Fiesta’ LG 7 QTL has some predictive value, both for known ‘Cox’ relatives and others, but more efficient markers would be desirable for marker-assisted selection.     

Saturation mapping of the apple scab resistance gene Vf using AFLP markers

Using the amplified fragment length polymorphism (AFLP) technique combined with a ”narrow-down” bulk segregant strategy enabled us to quickly identify 15 tightly linked AFLP markers to the Vf gene that confers resistance to the apple scab disease. High-resolution mapping placed all 15 AFLP markers within an interval of 0.6 cM around the Vf region; 7 of them were found to be inseparable from the Vf gene, 1 was localized left of, and the remaining 7 located right of the Vf gene. In addition, eight previously identified RAPD markers were also mapped, but only three, including M18, AM19, and AL07, were localized within this short interval, and none co-segregated with the Vf gene. The saturation of the Vf region with AFLP markers will accelerate both marker-assisted selection and map-based cloning. The advantages of this ”narrow-down” strategy, estimation of physical distances among AFLP markers, and their potential application are also discussed. Received: 22 December 1999 / Accepted: 25 March 2000     

QTL mapping of fire blight resistance in apple

Fire blight caused by the bacterium Erwinia amylovora is a severe threat to apple and pear orchards worldwide. Apple varieties exhibit a wide range of relative susceptibility/tolerance to fire blight. Although, no monogenic resistance against fire blight has been identified yet, recent evidence indicates the existence of quantitative resistance. Potential sources of fire blight resistance include several wild Malus species and some apple cultivars. F1 progenies of ‘Fiesta’בDiscovery’ were inoculated with the Swiss strain Ea 610 and studied under controlled conditions to identify quantitative trait loci (QTLs) for fire blight resistance. Disease was evaluated at four time points after inoculation. Shoot lesion length and the area under disease progress curve (AUDPC) values were used for QTL analysis. One significant (LOD score of 7.5–8.1, p<0.001) QTL was identified on the linkage group 7 of ‘Fiesta’ (F7). The F7 QTL explained about 37.5–38.6% of the phenotypic variation.     

Linkage maps of the apple ( Malus × domestica Borkh.) cvs ‘Ralls Janet’ and ‘Delicious’ include newly developed EST markers

Two apple genetic linkage maps were constructed using amplified fragment length polymorphisms (AFLPs), simple sequence repeats (SSRs), random amplified polymorphic DNAs (RAPDs), and expressed sequence tag (EST)-derived markers in combination with a pseudo-testcross mapping strategy in which the cultivars ‘Ralls Janet’ and ‘Delicious’ were used as the respective seed parents. Mitsubakaido (Malus sieboldii) was used as the pollen parent for each of the segregating F₁ populations. Expressed sequence tag data were obtained from the random sequencing of cDNA libraries constructed from in vitro cultured shoots and maturing fruits of cv ‘Fuji’, which is the offspring of a cross between ‘Ralls Janet’ and ‘Delicious’. In addition, a number of published gene sequences were used to develop markers for mapping. The ‘Ralls Janet’ map consisted of 346 markers (178 AFLPs, 95 RAPDs, 54 SSRs, 18 ESTs, and the S locus) in 17 linkage groups, with a total length of 1082 cM, while that of ‘Delicious’ comprised 300 markers (120 AFLPs, 81 RAPDs, 64 SSRs, 32 ESTs, and the S, Rf, and MdACS-1 loci) on 17 linkage groups spanning 1031 cM. These maps are amenable to comparisons with previously published maps of ‘Fiesta’ and ‘Discovery’ (Liebhard et al., Mol Breed 10:217–241, 2002; Liebhard et al., Theor Appl Genet 106:1497–1508, 2003a) because several of the SSRs (one to three markers per linkage group) were used in all of the maps. Distorted marker segregation was observed in three and two regions of the ‘Ralls Janet’ and ‘Delicious’ maps, respectively. These regions were localized in different parts of the genome from those in previously reported apple linkage maps. This marker distortion may be dependent on the combinations of cultivars used for map construction.     

A new pear scab resistance gene Rvp1 from the European pear cultivar ‘Navara’ maps in a genomic region syntenic to an apple scab resistance gene cluster on linkage group 2

Scab, caused by the ascomycete fungus Venturia pirina, leads to severe damage on European pear varieties resulting in a loss of commercial value and requiring frequent use of fungicides. Identifying scab resistance genes, developing molecular markers linked to these genes and establishing marker-assisted selection would be an effective way to improve European pear breeding for scab resistance. Most of the European pear cultivars (Pyrus communis) are currently reported to be sensitive. The pear cultivar ‘Navara’ was shown to carry a major scab resistance gene whose phenotypic expression in seedling progenies was a typical stellate necrosis symptom. The resistance gene was called Rvp1, for resistance to V. pirina, and was mapped on linkage group 2 of the pear genome close to microsatellite marker CH02b10. This genomic region is known to carry a cluster of scab resistance genes in apple indicating a first functional synteny for scab resistance between apple and pear.     

Functional analysis and expression profiling of <Emphasis Type="Italic">HcrVf1</Emphasis> and <Emphasis Type="Italic">HcrVf2</Emphasis> for development of scab resistant cisgenic and intragenic apples

Apple scab resistance genes, HcrVf1 and HcrVf2, were isolated including their native promoter, coding and terminator sequences. Two fragment lengths (short and long) of the native gene promoters and the strong apple rubisco gene promoter (P_MdRbc) were used for both HcrVf genes to test their effect on expression and phenotype. The scab susceptible cultivar ‘Gala’ was used for plant transformations and after selection of transformants, they were micrografted onto apple seedling rootstocks for scab disease tests. Apple transformants were also tested for HcrVf expression by quantitative RT-PCR (qRT-PCR). For HcrVf1 the long native promoter gave significantly higher expression that the short one; in case of HcrVf2 the difference between the two was not significant. The apple rubisco gene promoter proved to give the highest expression of both HcrVf1 and HcrVf2. The top four expanding leaves were used initially for inoculation with monoconidial isolate EU-B05 which belongs to race 1 of V. inaequalis. Later six other V. inaequalis isolates were used to study the resistance spectra of the individual HcrVf genes. The scab disease assays showed that HcrVf1 did not give resistance against any of the isolates tested regardless of the expression level. The HcrVf2 gene appeared to be the only functional gene for resistance against Vf avirulent isolates of V. inaequalis. HcrVf2 did not provide any resistance to Vf virulent strains, even not in case of overexpression. In conclusion, transformants carrying the apple-derived HcrVf2 gene in a cisgenic as well as in an intragenic configuration were able to reach scab resistance levels comparable to the Vf resistant control cultivar obtained by classical breeding, cv. ‘Santana’.     

Localization of the rice stripe disease resistance gene, Stv-bi, by graphical genotyping and linkage analyses with molecular markers

 We used graphical genotyping and linkage analyses with molecular markers to determine the chromosomal location of the rice stripe disease resistance gene, Stv-b ⁱ . The stripe resistance gene from the indica rice (Oryza sativa) cv ‘Modan’ was introgressed into several Japanese rice varieties. We found 4 RFLP markers in ‘Modan’, five susceptible parental rice varieties (‘Norin No. 8’, ‘Sachihikari’, ‘Kanto No. 98’, ‘Hokuriku No.103’ and ‘Koganebare’) and four resistant progeny varieties (‘St. No. 1’, ‘Aichi No. 6’, ‘Aoisora’ and ‘Asanohikari’). Graphical genotyping of the resistant progeny revealed a chromosomal segment ascribable to ‘Modan’ and associated with stripe resistance. The chromosomal segment from ‘Modan’ was located at 35.85 cM on chromosome 11. Linkage analysis using 120 F₂ individuals from a cross between ‘Koshihikari’ (susceptible) and ‘Asanohikari’ (resistant) revealed another 8 RFLP markers in the same chromosome. We performed a bioassay for rice stripe resistance in F₃ lines of the F₂ individuals using infective small brown planthoppers and identified an 1.8-cM segment harboring the rice stripe disease resistance gene, Stv-b ⁱ , between XNpb220 and XNpb257/ XNpb254. Furthermore, Stv-b ⁱ was linked by 0.0 cM to a RFLP marker, ST10, which was developed on the basis of the results of RAPD analysis. These DNA markers near the Stv-b ⁱ locus may be useful in marker-assisted selection and map-based cloning of the Stv-b ⁱ gene. Received: 26 September 1997 / Accepted: 4 November 1997     

Molecular selection in apple for resistance to scab caused by Venturia inaequalis

Large-scale marker-assisted selection requires highly reproducible, consistent and simple markers. The use of genetic markers is important in woody plant breeding in general, and in apple in particular, because of the high level of heterozygosity present in Malus species. We present here the transformation of two RAPD markers, which we found previously to be linked to the major scab resistance gene Vf, into more reliable and reproducible markers that can be applied directly to apple breeding. We give an example of how the use of such markers can speed up selection for the introduction of scab resistance genes into the same plant, reducing labour and avoiding time-consuming test crosses. We discuss the nature and relationship of the scab resistance gene Vf to the one present in Nova Easygro, thought to be Vr.