Genetic analyses of correlated solids, flavor, and health-enhancing traits in onion (Allium cepa L.)

Onion possesses organosulfur compounds and carbohydrates that provide unique flavor and health-enhancing characteristics. Significant phenotypic correlations have been reported among soluble solids content (SSC), total dry matter, pungency, and onion-induced in vitro antiplatelet activity. A genetic map and segregating F3M families derived from a cross between two inbred populations were used to identify and estimate the effects of quantitative trait loci (QTLs) controlling these traits at 30 and 90 days postharvest. In vitro antiplatelet activities among different onion populations were consistent across six human blood donors. Most of the populations showed in vitro antiplatelet activities; however, for some donors, one of the parental lines and two F3M families had pro-aggregatory effects under our experimental conditions. SSC, dry matter, pungency, and in vitro antiplatelet activity showed significant positive phenotypic and genetic correlations. A chromosome region on linkage group E accounted for a significant amount of the phenotypic variation for all of these traits. The correlations among these traits may be due to linkage or pleiotropy of genes controlling solids content. Our results indicate that it will be difficult to develop onion populations with lower pungency and high in vitro antiplatelet activity; however, the strong genetic and phenotypic correlations between high in vitro antiplatelet activity and high SSC are beneficial for the health functionality of onion.     

Genetic mapping of a major gene affecting onion bulb fructan content

The non-structural dry matter content of onion bulbs consists principally of fructose, glucose, sucrose and fructans. The objective of this study was to understand the genetic basis for the wide variation observed in the relative amounts of these carbohydrates. Bulb carbohydrate composition was evaluated in progeny from crosses between high dry matter storage onion varieties and sweet, low dry matter varieties. When samples were analysed on a dry weight basis, reducing sugar and fructan content exhibited high negative correlations and bimodal segregation suggestive of the action of a major gene. A polymorphic SSR marker, ACM235, was identified which exhibited strong disequilibrium with bulb fructan content in F_2:3 families from the ‘W202A’ × ‘Texas Grano 438’ mapping population evaluated in two environments. This marker was mapped to chromosome 8 in the interspecific population ‘Allium cepa × A. roylei’. Mapping in the ‘Colossal Grano PVP’ × ‘Early Longkeeper P12’ F₂ population showed that a dominant major gene conditioning high-fructan content lay in the same genomic region. QTL analysis of total bulb fructan content in the intraspecific mapping population ‘BYG15-23’ × ‘AC43’ using a complete molecular marker map revealed only one significant QTL in the same chromosomal region. This locus, provisionally named Frc, may account for the major phenotypic differences in bulb carbohydrate content between storage and sweet onion varieties.     

Next Generation Mapping of Enological Traits in an F2 Interspecific Grapevine Hybrid Family

In winegrapes (Vitis spp.), fruit quality traits such as berry color, total soluble solids content (SS), malic acid content (MA), and yeast assimilable nitrogen (YAN) affect fermentation or wine quality, and are important traits in selecting new hybrid winegrape cultivars. Given the high genetic diversity and heterozygosity of Vitis species and their tendency to exhibit inbreeding depression, linkage map construction and quantitative trait locus (QTL) mapping has relied on F₁ families with the use of simple sequence repeat (SSR) and other markers. This study presents the construction of a genetic map by single nucleotide polymorphisms identified through genotyping-by-sequencing (GBS) technology in an F₂ mapping family of 424 progeny derived from a cross between the wild species V. riparia Michx. and the interspecific hybrid winegrape cultivar, ‘Seyval’. The resulting map has 1449 markers spanning 2424 cM in genetic length across 19 linkage groups, covering 95% of the genome with an average distance between markers of 1.67 cM. Compared to an SSR map previously developed for this F₂ family, these results represent an improved map covering a greater portion of the genome with higher marker density. The accuracy of the map was validated using the well-studied trait berry color. QTL affecting YAN, MA and SS related traits were detected. A joint MA and SS QTL spans a region with candidate genes involved in the malate metabolism pathway. We present an analytical pipeline for calling intercross GBS markers and a high-density linkage map for a large F₂ family of the highly heterozygous Vitis genus. This study serves as a model for further genetic investigations of the molecular basis of additional unique characters of North American hybrid wine cultivars and to enhance the breeding process by marker-assisted selection. The GBS protocols for identifying intercross markers developed in this study can be adapted for other heterozygous species.     

Construction of an improved linkage map of diploid alfalfa (Medicago sativa)

An improved genetic map of diploid (2n=2x=16) alfalfa has been developed by analyzing the inheritance of more than 800 genetic markers on the F₂ population of 137 plant individuals. The F₂ segregating population derived from a self-pollinated F₁ hybrid individual of the cross Medicago sativa ssp. quasifalcata ×Medicago sativa ssp. coerulea. This mapping population was the same one which had been used for the construction of our previous alfalfa genetic map. The genetic analyses were performed by using maximum-likelihood equations and related computer programs. The improved genetic map of alfalfa in its present form contains 868 markers (four morphological, 12 isozyme, 26 seed protein, 216 RFLP, 608 RAPD and two specific PCR markers) in eight linkage groups. Of the markers 80 are known genes, including 2 previously cytologically localized genes, the rDNA and the β-tubulin loci. The genetic map covers 754 centimorgans (cM) with an average marker density of 0.8/cM. The correlation between the physical and genetic distances is about 1000–1300 kilobase pairs per centiMorgan. In this map, the linkage relationships of some markers on linkage groups 6, 7, and 8 are different from the previously published one. The cause of this discrepancy was that the genetic linkage of markers displaying distorted segregation (characterized by an overwhelming number of heterozygous individuals) had artificially linked genetic regions that turned out to be unlinked. To overcome the disadvantageous influence of the excess number of heterozygous genotypes on the recombination fractions, we used recently described maximum-likelihood formulas and colormapping, which allowed us to exclude the misleading linkages and to estimate the genetic distances more precisely. Received: 19 October 1998 / Accepted: 15 April 1999     

Inheritance and molecular mapping of a downy mildew resistance gene, Pl 13 in cultivated sunflower (Helianthus annuus L.)

The inheritance of resistance to sunflower downy mildew (SDM) derived from HA-R5 conferring resistance to nine races of the pathogen has been determined and the new source has been designated as Pl ₁₃ . The F₂ individuals and F₃ families of the cross HA-R5 (resistant) × HA 821 (susceptible) were screened against the four predominant SDM races 300, 700, 730, and 770 in separate tests which indicated dominant control by a single locus or a cluster of tightly linked genes. Bulked segregant analysis (BSA) was carried out on 116 F₂ individuals with 500 SSR primer pairs that resulted in the identification of 10 SSR markers of linkage groups 1 (9 markers) and 10 (1 marker) of the genetic map (Tang et al. in Theor Appl Genet 105:1124–1136, 2002) that distinguished the bulks. Of these, the SSR marker ORS 1008 of linkage group 10 was tightly linked (0.9 cM) to the Pl ₁₃ gene. Genotyping the F₂ population and linkage analysis with 20 polymorphic primer pairs located on linkage group 10 failed to show linkage of the markers with downy mildew resistance and the ORS 1008 marker. Nevertheless, validation of polymorphic SSR markers of linkage group 1 along with six RFLP-based STS markers of linkage group 12 of the RFLP map of Jan et al. (Theor Appl Genet 96:15–22, 1998) corresponding to linkage group 1 of the SSR map, mapped seven SSR markers (ORS 965-1, ORS 965-2, ORS 959, ORS 371, ORS 716, and ORS 605) including ORS 1008 and one STS marker (STS10D6) to linkage group 1 covering a genetic distance of 65.0 cM. The Pl ₁₃ gene, as a different source with its location on linkage group 1, was flanked by ORS 1008 on one side at a distance of 0.9 cM and ORS 965-1 on another side at a distance of 5.8 cM. These closely linked markers to the Pl ₁₃ gene provide a valuable basis for marker-assisted selection in sunflower breeding programs.     

A low-density genetic map of onion reveals a role for tandem duplication in the evolution of an extremely large diploid genome

 The bulb onion, Allium cepa L., is a diploid (2n=2x=16) plant with a huge nuclear genome. Previous genetic and cytogenetic analyses have not supported a polyploid origin for onion. We developed a low-density genetic map of morphological markers, randomly amplified polymorphic DNAs (RAPD), and restriction fragment length polymorphisms (RFLP) as a tool for onion improvement and to study the genome organization of onion. A mapping population of 58 F₃ families was produced from a single F₁ plant from the cross of two partially inbred lines (Brigham Yellow Globe 15-23 and Alisa Craig 43). Segregations were established for restoration of male fertility in sterile cytoplasm, complementary light-red bulb color, 14 RAPDs, 110 RFLPs revealed by 90 anonymous cDNA clones, and 2 RFLPs revealed by a cDNA clone of alliinase, the enzyme responsible for the characteristic Allium flavors. Duplicated RFLP loci were detected by 21% of the clones, of which 53% were unlinked (>30 cM), 5% loosely linked (10–30 cM), and 42% tightly linked (<10 cM). This duplication frequency is less than that reported for paleopolyploids but higher than for diploid species. We observed 40% dominant RFLPs, the highest yet reported among plants. Among duplicated RFLP loci, 19% segregated as two loci each with two codominant alleles, 52% segregated as one locus with codominant alleles and one locus with only a dominant fragment, and 29% segregated as two loci with only dominant fragments. We sequenced cDNAs detecting duplicated RFLPs; 63% showed homology to known gene families (e.g., chlorophyll binding proteins, ubiquitin, or RuBISCO), and 37% were unique clones showing significant homology to known genes of low-copy number or no homology to database sequences. Duplicated RFLPs showing linkage could be due to retroviral-like sequences in adjacent coding regions or intrachromosomal, as opposed to whole genome, duplications. Previous cytological analyses and this genetic map support intrachromosomal duplication as a mechanism contributing to the huge onion genome. Received: 3 July 1997 / Accepted: 8 August 1997     

A genetic linkage map of tef [Eragrostis tef (Zucc.) Trotter] based on amplified fragment length polymorphism

A genetic linkage map of tef was constructed with amplified fragment length polymorphism (AFLP) markers using F₅ recombinant inbred lines (RILs) derived by single seed descent from the intraspecific cross of ’Kaye Murri’×’Fesho’. A total of 192 EcoRI/MseI primer combinations were screened for parental polymorphism. Around three polymorphic fragments per primer combination were detected, indicating a low polymorphism level in tef. Fifty primer combinations were selected to assay the mapping population, and 226 loci segregated among 85 F₅ RILs. Most AFLP loci behaved as dominant markers (presence or absence of a band), but about 15% of the loci were codominant. Significant deviations from the expected Mendelian segregation ratio were observed for 26 loci. The genetic linkage map comprised 211 markers assembled into 25 linkage groups and covered 2,149 cM of genome. AFLP is an efficient marker system for mapping plant species with low polymorphism such as tef. This is the first genetic linkage map constructed for tef. It will facilitate the mapping of genes controlling agronomically important traits and cultivar improvement in tef. Received: 27 April 1998 / Accepted: 4 January 1999     

Inheritance of resistance to southern corn rust in tropical-by-corn-belt maize populations

 The inheritance of resistance to southern rust (caused by Puccinia polysora Underw.) was investigated in two F_2:3 populations derived from crossing two temperate-adapted, 100% tropical maize (Zea mays L.) inbred lines (1416-1 and 1497-2) to a susceptible Corn Belt Dent hybrid, B73Ht×Mo17Ht. The inbred lines possess high levels of resistance to southern rust and may be unique sources of resistance genes. Heritability for resistance was estimated as 30% and 50% in the two populations from regression of F_2:3 family mean scores on F₂ parent scores, and as 65% and 75% from variances among F_2:3 families on a single-plot basis. RFLP loci on three chromosomal regions previously known to possess genes for resistance to either southern rust or common rust (P. sorghi Schw.) were used to localize genes affecting resistance to southern rust in selected genotypes of both populations, and to estimate their genetic effects. A single locus on 10S, bnl3.04, was associated with 82–83% of the variation among field resistance scores of selected F_2:3 families in the two populations. Loci on chromosomes 3 (umc26) and 4 (umc31) were significantly associated with resistance in the 1497-2 population, each accounting for 13–15% of the phenotypic variation for F_2:3 field scores. Multiple-marker locus models, including loci from chromosomes 3, 4, and 10 and their epistatic interactions, accounted for 96–99% of the variation in F_2:3 field scores. Similar results were obtained for resistance measured by counting pustules on juvenile plants in the greenhouse. An attempt was made to determine if the major gene for resistance from 1416-1 was allelic to Rpp9, which is also located on 10S. Testcross families from the cross (1416-1×B37Rpp9)×B14AHt were evaluated for resistance to southern rust in Mexico. Neither source of resistance was completely effective in this environment, preventing determination of allelism of the two genes; however, both sources of resistance had better partial resistance to southern rust than did B14AHt. Received: 6 May 1997/Accepted: 19 September 1997     

Genetic analyses of bolting in bulb onion (Allium cepa L.)

Key message

We present the first evidence for a QTL conditioning an adaptive trait in bulb onion, and the first linkage and population genetics analyses of candidate genes involved in photoperiod and vernalization physiology.

Abstract

Economic production of bulb onion (Allium cepa L.) requires adaptation to photoperiod and temperature such that a bulb is formed in the first year and a flowering umbel in the second. ‘Bolting’, or premature flowering before bulb maturation, is an undesirable trait strongly selected against by breeders during adaptation of germplasm. To identify genome regions associated with adaptive traits we conducted linkage mapping and population genetic analyses of candidate genes, and QTL analysis of bolting using a low-density linkage map. We performed tagged amplicon sequencing of ten candidate genes, including the FT-like gene family, in eight diverse populations to identify polymorphisms and seek evidence of differentiation. Low nucleotide diversity and negative estimates of Tajima’s D were observed for most genes, consistent with purifying selection. Significant population differentiation was observed only in AcFT2 and AcSOC1. Selective genotyping in a large ‘Nasik Red × CUDH2150’ F₂ family revealed genome regions on chromosomes 1, 3 and 6 associated (LOD > 3) with bolting. Validation genotyping of two F₂ families grown in two environments confirmed that a QTL on chromosome 1, which we designate AcBlt1, consistently conditions bolting susceptibility in this cross. The chromosome 3 region, which coincides with a functionally characterised acid invertase, was not associated with bolting in other environments, but showed significant association with bulb sucrose content in this and other mapping pedigrees. These putative QTL and candidate genes were placed on the onion map, enabling future comparative studies of adaptive traits.     

Quantitative trait locus analysis of a recombinant inbred line population derived from a Lycopersicon esculentum x Lycopersicon cheesmanii cross

Quantitative trait loci influencing fruit traits were identified by restriction fragment length polymorphism (RFLP) analysis in a population of recombinant inbred lines (RIL) derived from a cross of the cultivated tomato, Lycopersicon esculentum with a related wild species Lycopersicon cheesmanii. One hundred thirty-two polymorphic RFLP loci spaced throughout the tomato genome were scored for 97 F₈ RIL families. Fruit weight and soluble solids were measured in replicated trials during 1991 and 1992. Seed weight was measured in 1992. Significant (P<0.01 level) quantitative trait locus (QTL) associations of marker loci were identified for each trait. A total of 73 significant marker locus-trait associations were detected for the three traits measured. Fifty-three of these associations were for fruit weight and soluble solids, many of which involved marker loci signficantly associated with both traits. QTL with large effects on all three traits were detected on chromosome 6. Greater homozygosity at many loci in the RIL population as compared to F₂ populations and greater genomic coverage resulted in increased precision in the estimation of QTL effects, and large proportions of the total phenotypic variance were explained by marker class variation at significant marker loci for many traits. The RIL population was effective in detecting and discriminating among QTL for these traits previously identified in other investigations despite skewed segregation ratios at many marker loci. Large additive effects were measured at significant marker loci. Lower fruit weight, higher soluble solids, and lower seed weight were generally associated with RFLP alleles from theL. cheesmanii parent.     

Detection and integration of quantitative trait loci for grain yield components and oil content in two connected recombinant inbred line populations of high-oil maize

Improvement in grain yield is an important objective in high-oil maize breeding. In this study, one high-oil maize inbred was crossed with two normal maize inbreds to produce two connected recombinant inbred line (RIL) populations with 282 and 263 F_7:8 families, respectively. The field experiments were conducted under four environments, and eight grain yield components and grain oil content were evaluated. Two genetic linkage maps were constructed using 216 and 208 polymorphic SSR markers. Quantitative trait loci (QTL) were detected for all traits under each environment and in combined analysis. Meta-analysis was used to integrate genetic maps and detected QTL in both populations. A total of 199 QTL were detected, 122 in population 1 and 87 in population 2. Seven, 11 and 19 QTL showed consistency across five environments, across two RIL populations and with respective F_2:3 generations, respectively. 183 QTL were integrated in 28 meta-QTL (mQTL). QTL with contributions over 15% were consistently detected in 3–4 cases and integrated in mQTL. Each mQTL included 3–19 QTL related to 1–4 traits, reflecting remarkable QTL co-location for grain yield components and oil content. Further research and marker-assisted selection (MAS) should be concentrated on 37 consistent QTL and four genetic regions of mQTL with more than 10 QTL at bins 3.04–3.05, 7.02, 8.04–8.05 and 9.04–9.05. Near-isogenic lines for 100-grain-weight QTL at bin 7.02–7.03, for ear-length QTL at bin 7.02–7.03 and for rows-per-ear QTL at bin 3.08 are now in construction using MAS. Co-located candidate genes could facilitate the identification of candidate genes for grain yield in maize.     

Genetic linkage maps of two interspecific grape crosses (Vitis spp.) used to localize quantitative trait loci for downy mildew resistance

Two populations (Pop) segregating quantitatively for resistance to downy mildew (DM), caused by Plasmopara viticola, were used to construct genetic maps and to carry out quantitative trait locus (QTL) analysis. Pop1 comprised of 174 F₁ individuals from a cross of ‘Moscato Bianco’, a susceptible Vitis vinifera cultivar, and a resistant individual of Vitis riparia. Pop2 consisted of 94 progeny from a cross of two interspecific hybrids, ‘VRH3082 1-42’ and ‘SK77 5/3’, with resistance traits inherited from Vitis rotundifolia and Vitis amurensis, respectively. Resistance of progeny was measured in field and greenhouse conditions by visual evaluation of disease symptoms on leaves. Linkage maps of 1037.2 and 651 cM were built essentially with simple sequence repeat markers and were enriched with gene-derived single-strand conformational polymorphism and single-nucleotide polymorphism markers. Simple interval mapping and Kruskall–Wallis analysis detected a stable QTL involved in field resistance to DM on linkage group (LG) 7 of the Pop1 integrated map co-localized with a putative Caffeoyl-CoA O-methyltransferase-derived marker. Additional QTLs were detected on LGs 8, 12 and 17. We were able to identify genetic factors correlated with resistance to P. viticola with lower statistical significance on LGs 1, 6 and 7 of the Pop2 map. Finally, no common QTLs were found between the two crosses analyzed. A search of the grapevine genome sequence revealed either homologues to non-host-, host- or defense-signalling genes within the QTL intervals. These positional candidate genes may provide new information about chromosomal regions hosting phenotypic loci.     

QTL analysis for capsaicinoid content in Capsicum

Pungency or “heat” found in Capsicum fruit results from the biosynthesis and accumulation of alkaloid compounds known as capsaicinoids in the dissepiment, placental tissue adjacent to the seeds. Pepper cultivars differ with respect to their level of pungency because of quantitative and qualitative variation in capsaicinoid content. We analyzed the segregation of three capsaicinoids: capsaicin, dihydrocapsaicin and nordihydrocapsaicin in an inter-specific cross between a mildly pungent Capsicum annuum ‘NuMex RNaky’ and the wild, highly pungent C. frutescens accession BG2814-6. F₃ families were analyzed in three trials in California and in Israel and a dense molecular map was constructed comprised mostly of loci defined by simple sequence repeat (SSR) markers. Six QTL controlling capsaicinoid content were detected on three chromosomes. One gene from the capsaicinoid biosynthetic pathway, BCAT, and one random fruit EST, 3A2, co-localized with QTL detected in this study on chromosomes 3 and 4. Because one confounding factor in quantitative determination of capsaicinoid is fruit size, fruit weight measurements were taken in two trials. Two QTL controlling fruit weight were detected, however, they did not co-localize with QTL detected for capsaicinoid content. The major contribution to the phenotypic variation of capsaicinoid content (24–42% of the total variation) was attributed to a digenic interaction between a main-effect QTL, cap7.1, and a marker located on chromosome 2 that did not have a main effect on the trait. A second QTL, cap7.2 is likely to correspond to the QTL, cap, identified in a previous study as having pronounced influence on capsaicinoid content.     

Molecular linkage mapping in rye (Secale cereale L.)

A rye linkage map containing clones from rye, wheat, barley, oat and rice genomic and cDNA libraries, known-function genes and microsatellite markers, was created using an F₂ population consisting of 110 F₂-derived F₃ families. Both co-dominant and dominant markers were added to the map. Of all probes screened, 30.8% were polymorphic, and of those polymorphic 79.3% were mapped. The current map contains 184 markers present in all seven linkage groups covering only 727.3 cM. This places a marker about every 3.96 cM on average throughout the map; however, large gaps are still present. The map contains 60 markers that have been integrated from previous rye maps. Surprisingly, no markers were placed between the centromere and C1–1RS in the short arm of 1R. The short arm of chromosome 4 also lacked an adequate number of polymorphic markers. The population showed a remarkable degree of segregation distortion (72.8%). In addition, the genetic distance observed in rye was found to be very different among the maps created by different mapping populations. Received: 10 January 2000 / Accepted: 26 May 2000     

A genetic linkage map of cowpea (Vigna unguiculata) developed from a cross between two inbred, domesticated lines

 We have constructed a genetic linkage map within the cultivated gene pool of cowpea (2n=2x=22) from an F₈ recombinant inbred population (94 individuals) derived from a cross between the inbreds IT84S-2049 and 524B. These breeding lines, developed in Nigeria and California, show contrasting reactions against several pests and diseases and differ in several morphological traits. Parental lines were screened with 332 random RAPD decamers, 74 RFLP probes (bean, cowpea and mung bean genomic DNA clones), and 17 AFLP primer combinations. RAPD primers were twice as efficient as AFLP primers and RFLP probes in detecting polymorphisms in this cross. The map consists of 181 loci, comprising 133 RAPDs, 19 RFLPs, 25 AFLPs, three morphological/classical markers, and a biochemical marker (dehydrin). These markers identified 12 linkage groups spanning 972 cM with an average distance of 6.4 cM between markers. Linkage groups ranged from 3 to 257 cM in length and included from 2 to 41 markers, respectively. A gene for earliness was mapped on linkage group 2. Seed weight showed a significant association with a RAPD marker on linkage group 5. This map should facilitate the identification of markers that “tag” genes for pest and disease resistance and other traits in the cultivated gene pool of cowpea. Received: 16 September 1996 / Accepted: 25 April 1997     

Quantitative trait loci and candidate gene mapping of aluminum tolerance in diploid alfalfa

Aluminum (Al) toxicity in acid soils is a major limitation to the production of alfalfa (Medicago sativa subsp. sativa L.) in the USA. Developing Al-tolerant alfalfa cultivars is one approach to overcome this constraint. Accessions of wild diploid alfalfa (M. sativa subsp. coerulea) have been found to be a source of useful genes for Al tolerance. Previously, two genomic regions associated with Al tolerance were identified in this diploid species using restriction fragment length polymorphism (RFLP) markers and single marker analysis. This study was conducted to identify additional Al-tolerance quantitative trait loci (QTLs); to identify simple sequence repeat (SSR) markers that flank the previously identified QTLs; to map candidate genes associated with Al tolerance from other plant species; and to test for co-localization with mapped QTLs. A genetic linkage map was constructed using EST-SSR markers in a population of 130 BC₁F₁ plants derived from the cross between Al-sensitive and Al-tolerant genotypes. Three putative QTLs on linkage groups LG I, LG II and LG III, explaining 38, 16 and 27% of the phenotypic variation, respectively, were identified. Six candidate gene markers designed from Medicago truncatula ESTs that showed homology to known Al-tolerance genes identified in other plant species were placed on the QTL map. A marker designed from a candidate gene involved in malic acid release mapped near a marginally significant QTL (LOD 2.83) on LG I. The SSR markers flanking these QTLs will be useful for transferring them to cultivated alfalfa via marker-assisted selection and for pyramiding Al tolerance QTLs.     

A molecular genetic map of cassava (Manihot esculenta Crantz)

 A genetic linkage map of cassava has been constructed with 132 RFLPs, 30 RAPDs, 3 microsatellites, and 3 isoenzyme markers segregating from the heterozygous female parent of an intraspecific cross. The F₁ cross was made between ‘TMS 30572’ and ‘CM 2177-2’, elite cassava cultivars from Nigeria and Colombia, respectively. The map consists of 20 linkage groups spanning 931.6 cM or an estimated 60% of the cassava genome. Average marker density is 1 per 7.9 cM. Since the mapping population is an F₁ cross between heterozygous parents, with unique alleles segregating from either parent, a second map was constructed from the segregation of 107 RFLPs, 50 RAPDs, 1 microsatellite, and 1 isoenzyme marker from the male parent. Comparison of intervals in the male-and female-derived maps, bounded by markers heterozygous in both parents, revealed significantly less meiotic recombination in the gametes of the female than in the male parent. Six pairs of duplicated loci were detected by low-copy genomic and cDNA sequences used as probes. Efforts are underway to saturate the cassava map with additional markers, to join the male- and female-derived maps, and to elucidate genome organization in cassava. Received: 5 July 1996/Accepted: 22 November 1996     

The inheritance of resistance to Verticillium wilt caused by race 1 isolates of <Emphasis Type="Italic">Verticillium dahliae</Emphasis> in the lettuce cultivar La Brillante

Verticillium wilt of lettuce caused by Verticillium dahliae can cause severe economic damage to lettuce producers. Complete resistance to race 1 isolates is available in Lactuca sativa cultivar (cv.) La Brillante and understanding the genetic basis of this resistance will aid development of new resistant cultivars. F₁ and F₂ families from crosses between La Brillante and three iceberg cultivars as well as a recombinant inbred line population derived from L. sativa cv. Salinas 88 × La Brillante were evaluated for disease incidence and disease severity in replicated greenhouse and field experiments. One hundred and six molecular markers were used to generate a genetic map from Salinas 88 × La Brillante and for detection of quantitative trait loci. Segregation was consistent with a single dominant gene of major effect which we are naming Verticillium resistance 1 (Vr1). The gene described large portions of the phenotypic variance (R ² = 0.49–0.68) and was mapped to linkage group 9 coincident with an expressed sequence tag marker (QGD8I16.yg.ab1) that has sequence similarity with the Ve gene that confers resistance to V. dahliae race 1 in tomato. The simple inheritance of resistance indicates that breeding procedures designed for single genes will be applicable for developing resistant cultivars. QGD8I16.yg.ab1 is a good candidate for functional analysis and development of markers suitable for marker-assisted selection.     

Identification of molecular markers linked to quantitative trait loci for soybean resistance to corn earworm

 One hundred and thirty nine restriction fragment length polymorphisms (RFLPs) were used to construct a soybean (Glycine max L. Merr.) genetic linkage map and to identify quantitative trait loci (QTLs) associated with resistance to corn earworm (Helicoverpa zea Boddie) in a population of 103 F₂-derived lines from a cross of ‘Cobb’ (susceptible) and PI229358 (resistant). The genetic linkage map consisted of 128 markers which converged onto 30 linkage groups covering approximately 1325 cM. There were 11 unlinked markers. The F₂-derived lines and the two parents were grown in the field under a plastic mesh cage near Athens, Ga., in 1995. The plants were artificially infested with corn earworm and evaluated for the amount of defoliation. Using interval-mapping analysis for linked markers and single-factor analysis of variance (ANOVA), markers were tested for an association with resistance. One major and two minor QTLs for resistance were identified in this population. The PI229358 allele contributed insect resistance at all three QTLs. The major QTL is linked to the RFLP marker A584 on linkage group (LG) ‘M’ of the USDA/Iowa State University public soybean genetic map. It accounts for 37% of the total variation for resistance in this cross. The minor QTLs are linked to the RFLP markers R249 (LG ‘H’) and Bng047 (LG ‘D1’). These markers explain 16% and 10% of variation, respectively. The heritability (h²) for resistance was estimated as 64% in this population. Received: 15 October 1997 / Accepted: 4 November 1997     

Molecular mapping of a rice gene conditioning thermosensitive genic male sterility using AFLP, RFLP and SSR techniques

The discovery and application of the thermosensitive genic male sterility (TGMS) system has great potential for revolutionizing hybrid seed production technology in rice. Use of the TGMS system in two-line breeding is simple, inexpensive, efficient, and eliminates the limitations associated with the cytoplasmic-genetic male sterility (CMS) system. An F₂ population developed from a cross between a TGMS indica mutant, TGMS–VN1, and a fertile indica line, CH1, was used to identify molecular markers linked to the TGMS gene and to subsequently determine its chromosomal location on the linkage map of rice. Bulk segregant analysis was performed using the AFLP technique. From the survey of 200 AFLP primer combinations, four AFLP markers (E2/M5–600, E3/M16–400, E5/M12–600, and E5/M12–200) linked to the TGMS gene were identified. All the markers were linked to the gene in the coupling phase. All except E2/M5–200 were found to be low-copy sequences. However, the marker E5/M12–600 showed polymorphism in RFLP analysis and was closely linked to the TGMS gene at a distance of 3.3 cM. This marker was subsequently mapped on chromosome 2 using doubled-haploid mapping populations derived from the crosses IR64×Azucena and CT9993×IR62666, available at IRRI, Philippines, and Texas Tech University, respectively. Linkage of microsatellite marker RM27 with the TGMS gene further confirmed its location on chromosome 2. The closest marker, E5/M12–600, was sequenced so that a PCR marker can be developed for the marker-assisted transfer of this gene to different genetic backgrounds. The new TGMS gene is tentatively designated as tms4(t). Received: 13 July 1999 / Accepted: 27 July 1999