Mapping genetic factors controlling pollen viability in an interspecific cross in Helianthus sect. Helianthus

Segregation of 48 genetic markers, including one CMS restorer gene, one morphological character gene, six isozymes and 40 RAPD loci, was scored in a backcross progeny of an interspecific hybrid H. argophyllusxH. annuus cv RHA274. A linkage map was generated taking into account segregation distortions for 11 of the 48 loci in the frame of two different models considering locus-pair segregation in the context of either independent selection pressures or non-equilibrated parental classes. The map consists of nine linkage groups and nine isolated markers covering 390 cM. Approximately half of the plants of the BC1 were male fertile as expected for the segregation of one dominant male-fertility restorer gene; however, these displayed a large range of variation for pollen viability. About 80% of this variation was explained by three genomic regions located on linkage groups 1, 2 and 3. The observation of meiotic chromosomes revealed a significant rate of mispairing (rod bivalents and tetravalents) in tight correlation with pollen viability, indicating that chromosome rearrangements (translocations) are the preponderant factors reducing pollen viability in this progeny. Cytogenetic and mapping data suggest that the three genomic regions involved in pollen-viability variation are located close to translocation points which differentiate the parental-species karyotypes. Segregation distortion was observed for loci correlated with pollen-viability variation. These were most likely the result of two possible suggested mechanisms.     

Development of SSR markers and construction of a consensus genetic map for chicory (Cichorium intybus L.)

A consensus genetic map for chicory (2n = 2x = 18) was obtained after the integration of molecular marker data of two industrial chicory progenies (K28K59, Rubis118) and one witloof chicory progeny (BR). As a limited number of co-dominant markers was available at the beginning of this work, three different microsatellite-enriched libraries were produced from genomic DNA, resulting in 420, 719 and 1,251 sequences, respectively. The level of informative Simple Sequence Repeat (SSR) sequences from the three libraries ranged from 28 to 40%, thus defining a set of 730 SSR markers available for polymorphism screening. A subset of 81 Sequence-Tagged Sites (STS) developed from EST, cDNA, genes, and non-coding sequences was screened through Single Strand Conformational Polymorphism (SSCP) analysis, leading to 46 polymorphic loci integrated in the genetic maps. Markers were grouped and ordered on 9 homologous Linkage Groups (LG) for each of the three maps: 274 markers in K28K59, 282 markers in Rubis118, 178 markers in BR. Co-linear regions between maps were identified through 193 ‘bridge’ markers that allowed the integration of the 9 homologous LG in a consensus map containing 472 markers and covering 878 cM. Comparison across maps revealed the presence of 4 conserved regions with significant distorted markers, also defined as Segregation Distortion Regions (SDR), affected by gametic or zygotic selection factors. Marker distribution was not always uniform; 6 LG possessed homologous clustered regions in all maps. The consensus map could be the starting point for the identification and the cloning of major genes and QTL in fundamental and applied genetic areas in chicory.     

A saturated genetic linkage map of rubber tree (Hevea spp.) based on RFLP, AFLP, microsatellite, and isozyme markers

The first genetic map for Hevea spp. (2n=36) is presented here. It is based on a F₁ progeny of 106 individuals allowing the construction of a female, a male, and a synthetic map according to the pseudo-testcross strategy. Progeny were derived from an interspecific cross between PB260, a H. brasiliensis cultivated clone, and RO38, a H. brasiliensis×H. benthamiana interspecific hybrid clone. The disomic inheritance observed for all the codominant markers scattered on the 2n=36 chromosomes revealed that Hevea behaves as diploids. Homologous linkage groups between the two parental maps were merged using bridge loci. A total of 717 loci constituted the synthetic map, including 301 RFLPs, 388 AFLPs, 18 microsatellites, and 10 isozymes. The markers were assembled into 18 linkage groups, thus reflecting the basic chromosome number, and covered a total distance of 2144 cM. Nine markers were found to be unlinked. Segregation distortion was rare (1.4%). Average marker density was 1 per 3 cM. Comparison of the distance between loci in the parental maps revealed significantly less meiotic recombination in the interspecific hybrid male parent than in the female parent. Hevea origin and genome organisation are discussed. Received: 2 February 1999 / Accepted: 11 March 1999     

Construction of an RFLP map of barley

Summary In order to construct an RFLP map of barley, two populations were analyzed using 251 genomic and cDNA markers: one population comprised 71 F₁ antherderived double haploid (DH) individuals of an intraspecific cross (IGRI x FRANKA), and the other 135 individuals of an interspecific F₂/F₃ progeny (VADA x H. spontaneum). The distribution of nonrepetitive clones over the seven barley chromosomes revealed a maximum for chromosome 2H and a minimum for 6H. The polymorphism of the interspecific progeny (76%) clearly exceeded that of the intraspecific progeny (26%) although, based on their pedigrees, IGRI and FRANKA are only distantly related. The contribution of individual chromosomes of the DH parents to the overall polymorphism varied between 8% and 50%. A significant portion (44% versus 10% of the interspecific progeny) of the markers mapped on the DH offspring showed distorted segregation, caused mainly by the prevalence of variants originating from the parent that better responded to in vitro culture (IGRI). In contrast to the interspecific map, probes displaying skewed segregation were clustered on the DH map on discrete segments. The colinear arrangement of both maps covers a distance of 1,453 cM and identifies regions of varying map distances.     

Molecular mapping of the Oregon Wolfe Barleys: a phenotypically polymorphic doubled-haploid population

A phenotypically polymorphic barley (Hordeum vulgare L.) mapping population was developed using morphological marker stocks as parents. Ninety-four doubled-haploid lines were derived for genetic mapping from an F₁ using the Hordeum bulbosum system. A linkage map was constructed using 12 morphological markers, 87 restriction fragment length polymorphism (RFLP), five random amplified polymorphic DNA (RAPD), one sequence-tagged site (STS), one intron fragment length polymorphism (IFLP), 33 simple sequence repeat (SSR), and 586 amplified fragment length polymorphism (AFLP) markers. The genetic map spanned 1,387 cM with an average density of one marker every 1.9 cM. AFLP markers tended to cluster on centromeric regions and were more abundant on chromosome 1 (7H). RAPD markers showed a high level of segregation distortion, 54% compared with the 26% observed for AFLP markers, 27% for SSR markers, and 18% for RFLP markers. Three major regions of segregation distortion, based on RFLP and morphological markers, were located on chromosomes 2 (2H), 3 (3H), and 7 (5H). Segregation distortion may indicate that preferential gametic selection occurred during the development of the doubled-haploid lines. This may be due to the extreme phenotypes determined by alleles at morphological trait loci of the dominant and recessive parental stocks. Several molecular markers were found to be closely linked to morphological loci. The linkage map reported herein will be useful in integrating data on quantitative traits with morphological variants and should aid in map-based cloning of genes controlling morphological traits. Received: 23 August 2000 / Accepted: 15 December 2000     

Development and Characterization of a Genetic Linkage Map of Cryptococcus neoformans var. neoformans Using Amplified Fragment Length Polymorphisms and Other Markers

A segregating population of single basidiospore isolates from a sexual cross was used to generate the first moderately dense genetic linkage map of Cryptococcus neoformans var. neoformans (Serotype D). Polymorphic DNA markers were developed using amplified fragment length polymorphisms, random amplified polymorphic DNA, and gene-encoding sequences. These markers were used to analyze 100 meiotic progeny. All markers were tested for distorted segregation with a goodness of fit test. Of the total of 181 markers, 148 showed balanced (1:1) segregation ratios. Segregation distortion was observed for 33 markers. Based on all the markers, a linkage map was generated that consists of 14 major linkage groups with 127 markers, several small linkage groups, and 2 linkage groups that consist only of highly skewed markers. The genetic distance of the linkage map is 1356.3 cM. The estimated total haploid genome size for C. neoformans var. neoformans was calculated using Hulberts method and yielded a map size of 1917 cM. The number of major linkage groups correlates well with the proposed number of 13 chromosomes for C. neoformans var. neoformans. Several genes, including CAP64, CnLAC, and the mating-type locus, were mapped, and their associations were consistent with published data. To date, 6 linkage groups have been assigned to their corresponding chromosomes. This linkage map should provide a framework for the ongoing genome sequencing project and will be a useful tool for studying the genetics and pathogenicity of this important medical yeast.     

Segregation distortion at marker loci: variation during microspore embryogenesis in maize

In the present study, we analyzed the segregation distortions of markers during in vitro androgenesis in maize. This was based on four segregating populations derived from the A188×DH7 one-way-cross. These populations consisted of very young androgenetic embryos, well-developed calluses, haploid regenerated plantlets and spontaneous diploid plantlets. These structures all represented different developmental stages, from that of microspores to the regenerated plantlets. This study complemented a previous one by Murigneux et al. 1994, where distorted segregations of RFLP markers were detected in a single-seed-descent population and in a doubled-haploid population derived from the same cross. The weakly biased SSD maize genetic map was used as a reference to locate 145 AFLP loci whose allelic segregations were also analyzed in the androgenetic segregating populations. Segregation distortions were determined based on chi-square analysis (P<0.01 and P<0.001). Regions on chromosomes 2 and 8 showed distortions from the beginning of embryo formation, with large effects throughout the process. Regions on chromosomes 3, 4, 6 and 10 could control callus formation from microspores. Other deviations of marker genotypes on chromosomes 1, 4, 6 and 10 could be associated with the regeneration phase. Moreover, the statistical method of Cheng et al. for mapping a lethal factor locus inside segments of linked distorted markers was used to estimate the position of seven partial lethal androgenetic factors on chromosomes 1, 2, 8 and 10. These factors could represent selective genes actively involved in maize androgenesis. Received: 31 July 2000 / Accepted: 2 January 2001     

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.     

Genetic linkage map of the interspecific grape rootstock cross Ramsey (Vitis champinii) × Riparia Gloire (Vitis riparia)

The first genetic linkage map of grape derived from rootstock parents was constructed using 188 progeny from a cross of Ramsey (Vitis champinii) × Riparia Gloire (V. riparia). Of 354 simple sequence repeat markers tested, 205 were polymorphic for at least one parent, and 57.6% were fully informative. Maps of Ramsey, Riparia Gloire, and the F1 population were created using JoinMap software, following a pseudotestcross strategy. The set of 205 SSRs allowed for the identification of all 19 Vitis linkage groups (2n=38), with a total combined map length of 1,304.7 cM, averaging 6.8 cM between markers. The maternal map consists of 172 markers aligned into 19 linkage groups (1,244.9 cM) while 126 markers on the paternal map cover 18 linkage groups (1,095.5 cM). The expected genome coverage is over 92%. Segregation distortion occurred in the Ramsey, Riparia Gloire, and consensus maps for 10, 13, and 16% of the markers, respectively. These distorted markers clustered primarily on the linkage groups 3, 5, 14 and 17. No genome-wide difference in recombination rate was observed between Ramsey and Riparia Gloire based on 315 common marker intervals. Fifty-four new Vitis-EST-derived SSR markers were mapped, and were distributed evenly across the genome on 16 of the 19 linkage groups. These dense linkage maps of two phenotypically diverse North American Vitis species are valuable tools for studying the genetics of many rootstock traits including nematode resistance, lime and salt tolerance, and ability to induce vigor.     

RFLP and RAPD mapping in flax (Linum usitatissimum)

A map of flax (Linum usitatissimum) using restriction fragment length polymorphisms (RFLPs) and random amplified polymorphic DNAs (RAPDs), and comprising 15 linkage groups containing 94 markers, has been developed covering about 1000 cM. The mapping populations were the F₂ populations from two crosses between diverse cultivars. From one cross, CI1303 and Stormont Cirrus, 20 RFLP and 520 RAPD markers were analyzed. Thirteen RFLP and 80 RAPD markers were on the 15 linkage groups, in addition to one sequence-tagged site (STS). Seven polymorphic RAPD markers were found to have unusual segregation patterns. RAPDs were expressed as dominant markers, but for these markers a prevalence of the progeny lacked a band rather than the expected one-fourth ratio. However, these exceptions may be related to the instability of the genome of Stormont Cirrus in which stable and heritable genomic changes can be induced by environmental factors. The current map could be used for the identification of markers linked to loci controlling the ability to generate heritable changes in response to environmental growth conditions, and to develop anchor loci with STSs for a more general application. Received: 20 March 1999 / Accepted: 16 December 1999     

A linkage map of the chickpea (Cicer arietinum L.) genome based on recombinant inbred lines from a C. arietinum×C. reticulatum cross: localization of resistance genes for fusarium wilt races 4 and 5

An integrated molecular marker map of the chickpea genome was established using 130 recombinant inbred lines from a wide cross between a cultivar resistant to fusarium wilt caused by Fusarium oxysporum Schlecht. emend. Snyd. &. Hans f. sp. ciceri (Padwick) Snyd & Hans, and an accession of Cicer reticulatum (PI 489777), the wild progenitor of chickpea. A total of 354 markers were mapped on the RILs including 118 STMSs, 96 DAFs, 70 AFLPs, 37 ISSRs, 17 RAPDs, eight isozymes, three cDNAs, two SCARs and three loci that confer resistance against different races of fusarium wilt. At a LOD-score of 4.0, 303 markers cover 2077.9 cM in eight large and eight small linkage groups at an average distance of 6.8 cM between markers. Fifty one markers (14.4%) were unlinked. A clustering of markers in central regions of linkage groups was observed. Markers of the same class, except for ISSR and RAPD markers, tended to generate subclusters. Also, genes for resistance to races 4 and 5 of fusarium wilt map to the same linkage group that includes an STMS and a SCAR marker previously shown to be linked to fusarium wilt race 1, indicating a clustering of several fusarium-wilt resistance genes around this locus. Significant deviation from the expected 1 : 1 segregation ratio was observed for 136 markers (38.4%, P<0.05). Segregation was biased towards the wild progenitor in 68% of the cases. Segregation distortion was similar for all marker types except for ISSRs that showed only 28.5% aberrant segregation. The map is the most extended genetic map of chickpea currently available. It may serve as a basis for marker-assisted selection and map-based cloning of fusarium wilt resistance genes and other agronomically important genes in future. Received: 17 November 1999 / Accepted: 4 June 2000     

A high-density molecular map for ryegrass (Lolium perenne) using AFLP markers

AFLP markers have been successfully employed for the development of a high-density linkage map of ryegrass (Lolium perenne L.) using a progeny set of 95 plants from a testcross involving a doubled-haploid tester. This genetic map covered 930 cM in seven linkage groups and was based on 463 amplified fragment length polymorphism (AFLP) markers using 17 primer pairs, three isozymes and five EST markers. The average density of markers was approximately 1 per 2.0 cM. However, strong clustering of AFLP markers was observed at putative centromeric regions. Around these regions, 272 markers covered about 137 cM whereas the remaining 199 markers covered approximately 793 cM. Most genetic distances between consecutive pairs of markers were smaller than 20 cM except for five gaps on groups A, C, D, F and G. A skeletal map with a uniform distribution of markers can be extracted from this high-density map, and can be applied to detect and map QTLs. We report here the application of AFLP markers to genome mapping, in Lolium as a prelude to quantitative trait locus (QTL) identification for diverse agronomic traits in ryegrass and for marker-assisted plant breeding. Received: 4 November 1998 / Accepted:15 March 1999     

Genes and traits associated with chromosome 2H and 5H regions controlling sensitivity of reproductive tissues to frost in barley

Frost at flowering can cause significant damage to cereal crops. QTL for low temperature tolerance in reproductive tissues (LTR tolerance) were previously described on barley 2HL and 5HL chromosome arms. With the aim of identifying potential LTR tolerance mechanisms, barley Amagi Nijo × WI2585 and Haruna Nijo × Galleon populations were examined for flowering time and spike morphology traits associated with the LTR tolerance loci. In spring-type progeny of both crosses, winter alleles at the Vrn-H1 vernalization response locus on 5H were linked in coupling with LTR tolerance and were unexpectedly associated with earlier flowering. In contrast, tolerance on 2HL was coupled with late flowering alleles at a locus we named Flt-2L. Both chromosome regions influenced chasmogamy/cleistogamy (open/closed florets), although tolerance was associated with cleistogamy at the 2HL locus and chasmogamy at the 5HL locus. LTR tolerance controlled by both loci was accompanied by shorter spikes, which were due to fewer florets per spike on 5HL, but shorter rachis internodes on 2HL. The Eps-2S locus also segregated in both crosses and influenced spike length and flowering time but not LTR tolerance. Thus, none of the traits was consistently correlated with LTR tolerance, suggesting that the tolerance may be due to some other visible trait or an intrinsic (biochemical) property. Winter alleles at the Vrn-H1 locus and short rachis internodes may be of potential use in barley breeding, as markers for selection of LTR tolerance at 5HL and 2HL loci, respectively. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

High-resolution mapping of loci conferring resistance to sugarcane mosaic virus in maize using RFLP, SSR, and AFLP markers

Sugarcane mosaic virus (SCMV) is one of the most important virus diseases of maize in Europe. Genetic analysis on backcross five (BC5) progeny derived from the cross FAP1360A (resistant) × F7 (susceptible) confirmed that at least two dominant genes, Scm1 and Scm2, are required for resistance to SCMV in the progeny of this cross. With the aid of RFLP and SSR marker analyses, Scm1 was mapped in the region of 8.7 cM – between the nucleolus organizer region (nor) and RFLP marker bnl6.29 on the short arm of chromosome 6, while Scm2 was mapped to an interval of 26.8 cM flanked by the RFLP markers umc92 and umc102 near the centromere region of chromosome 3. Both chromosome regions were further enriched for AFLP markers by successful application of a bulked segregant analysis to this oligogenic trait. A total of 23 linked AFLP markers were identified, clustered in chromosome regions adjacent to either Scm1 or Scm2. Seven AFLP markers linked to Scm1 resided within the nor-bnl6.29 interval, and one of them, E3M8-1, showed no recombination with Scm1. Three AFLP markers linked to Scm2 are located between umc92 and umc102. Received: 13 October 1998 / Accepted: 18 January 1999     

Segregation distortion caused by weak hybrid necrosis in recombinant inbred lines of common wheat

Segregation distortion of molecular markers is closely related to hybrid incompatibility in progeny from intraspecific crosses. Recent reports in higher plants have demonstrated that hybrid sterility results in segregation distortion at the causal gene regions in progeny of intraspecific crosses. Ne1 and Ne2 complementary loci are known to control hybrid necrosis in intraspecific crosses of common wheat cultivars. Here, we examine the effect of a weak necrosis allele Ne1 ^w on the segregation ratio of molecular markers in recombinant inbred lines (RILs) of common wheat. Some RILs showed accelerated cell death in the leaves at the heading stage due to the epistatic interaction between two quantitative trait loci (QTL) on chromosomes 5B and 2B. Chromosomal localization of these QTL corresponding to Ne1 ^w and Ne2 showed distorted segregation ratios of assigned markers having oppositely biased direction. Although the Ne1 ^w and Ne2 interaction had no obvious effect on seed fertility, Ne1 ^w reduced completion of grain development under the Ne2-homozygous background. This reduction might be one of causes that induces segregation distortion in the 5B and 2B chromosomal regions of RILs. The present study demonstrated that weak hybrid necrosis has limited phenotypic effects; it causes segregation distortion in progeny from intraspecific crosses.     

A genetic map of an interspecific cross in Allium based on amplified fragment length polymorphism (AFLPTM) markers

Segregation of 692 polymorphic AFLP^TM (amplified fragment length polymorphism) fragments was determined in an F₂ of the interspecific cross A. roylei x A. cepa. Two different enzyme combinations were used, PstI/MseIand EcoRI/MseI; in the latter one extra selective nucleotide was added to the MseI primer. The map based on A. cepa markers consisted of eight linkage groups with 262 markers covering 694 cM of the expected 800 cM. The map based on A. roylei markers comprised 15 linkage groups with 243 markers and had a length of 626 cM. The two maps were not integrated, and 25% of the markers remained unlinked. One of the alliinase genes and a SCAR marker linked to the disease resistance gene to downy mildew are present on this map. Of the AFLP markers, 50—80% were polymorphic between A. cepa and A. roylei; the level of polymorphic markers between different A. cepa accessions was4-8%. Received: 28 August 1998 / Accepted: 31 March 1999     

The development of lettuce backcross inbred lines (BILs) for exploitation of the<Emphasis Type="Italic"> Lactuca saligna</Emphasis> (wild lettuce) germplasm

Backcross inbred lines (BILs) were developed in which chromosome segments of Lactuca saligna (wild lettuce) were introgressed into L. sativa (lettuce). These lines were developed by four to five backcrosses and one generation of selfing. The first three generations of backcrossing were random. Marker-assisted selection began in the BC₄ generation and continued until the final set of BILs was reached. A set of 28 lines was selected that together contained 96% of the L. saligna genome. Of these lines, 20 had a single homozygous introgression (BILs), four had two homozygous introgressions (doubleBILs) and four lines had a heterozygous single introgression (preBILs). Segregation ratios in backcross generations were compared to distorted segregation ratios in an F₂ population, and the results indicated that most of the distorted segregations can be explained by genetic effects on pollen- or egg-cell fitness. By means of BIL association mapping we were able to map 12 morphological traits and hundreds of additional amplified fragment length polymorphic (AFLP) markers. The total AFLP map now comprises 757 markers. This set of BILs is very useful for future genetic studies.Communicated by F. Salamini     

两例水稻极端异常分离现象的遗传分析

遗传异常分离既是自然界非常普遍的现象, 也是生物进化的动力之一。产生异常分离的原因可能与配子体或孢子体的选择有关。利用6个以类病变(lmi)和矮杆突变体(d6)为亲本的杂交组合(F2或F3), 对该类病变和矮杆基因的遗传规律及异常分离现象作初步的分析。结果显示, lmi×02428和d6×93-11的F2群体以及F3株系中存在极端异常分离的现象; LMI基因附近的分子标记ST8-1和D6基因附近的ST7-1、ST7-2、RM5490的带型分离同样也极显著偏离期望比; 偏分离因子与类病斑LMI和矮杆基因D6紧密连锁, 分别位于第8染色体分子标记ST8和ST8-2之间以及第7染色体分子标记ST7-1和ST7-3之间。异常分离现象还与杂交的组合有直接的关系。     

Interspecific genetic linkage map, segregation distortion and genetic conversion in coffee (Coffea sp.)

An interspecific partial genetic linkage map of Coffea sp. based on 62 backcross hybrids is presented. F₁ hybrids were generated by a cross between the wild C. pseudozanguebariae and the anciently cultivated C. liberica var. dewevrei (DEW); progeny were then derived from a backcross between F₁ hybrid and DEW. The map construction consisted of a two-step strategy using 5.5 and 3.1 LOD scores revealed by simulation file. The map consisted of 181 loci: 167 amplified fragment length polymorphism (AFLP) and 13 random fragment length polymorphism (RFLP) loci. The markers were assembled into 14 linkage groups, each with 4–31 markers covering 1,144 cM. Segregation distortion was observed for 30% of all loci, in particular 3:1 and 1:3 ratios equally favouring each of the two parents. The existence of such ratios suggests genetic conversion events. This map also represents an initial step towards the detection of quantitative trait loci. Received: 4 Janaury 2000 / Accepted: 17 January 2000     

Inheritance of resistance to Yam mosaic virus, genus Potyvirus, in white yam (Dioscorea rotundata)

Yam mosaic virus (YMV) causes the most-widespread and economically important viral disease affecting white yam (Dioscorea rotundata) in West Africa. The genetic basis of resistance in white yam to a Nigerian isolate of YMV was investigated in three tetraploid D. rotundata genotypes: TDr 93–1, TDr 93–2 and TDr 89/01444. F₁ progeny were produced using TDr 87/00571 and TDr 87/00211 as the susceptible parents. Segregation ratios indicated that a single dominant gene in a simplex condition governs the resistance in TDr 89/01444, while the resistance in TDr 93–2 is associated with the presence of a major recessive gene in duplex configuration. Segregation of progeny of the cross TDr 93–1×TDr 87/00211 fitted a genetic ratio of 2.48:1 resistant:susceptible, which can be expected when two simplex heterozygotes are crossed, indicating the possible modifying effect of the susceptible parent. A triple antibody immunosorbent assay (TAS-ELISA) was used for virus detection in inoculated plants. Slight mosaic symptoms appeared on most resistant individuals, while asymptomatic resistant genotypes with high ELISA (A₄₀₅) values were observed in all crosses. Such a heterogeneous response suggests the influence of additional modifier genes that segregate in the progeny. The finding that resistance can be inherited as a dominant or recessive character has important implications for YMV resistance breeding. Received: 15 August 2000 / Accepted: 12 April 2001