Molecular genetics of growth and development in Populus. I. Triploidy in hybrid poplars

Summary While constructing a genetic linkage map of a hybrid poplar genome (Populus trichocarpa x P. deltoides), we identified several restriction fragment length polymorphismus (RFLPs) for which the parental trees are heterozygous. Although 8 of the 11 F₁ hybrid offspring inherited, as expected, single RFLP alleles from each parent, 3 F₁ trees in the mapping pedigree inherited both maternal alleles along with a single paternal allele at some loci. Aneuploidy or polyploidy in these 3 F₁ trees due to partial or complete nondisj unction during female gametogenesis is the simplest explanation for this finding. Of the 3 f₁ offspring with supernumerary RFLP alleles 2 have triploid nuclear DNA contents as measured by fluorescence flow cytometry; the 3rd F₁ with supernumerary alleles has a sub-triploid nuclear DNA content and is probably aneuploid. Among the tri/aneuploid hybrids, leaf quantitative traits either are skewed toward those values characteristic of the P. trichocarpa female parent (adaxial stomate density, petiole length: blade length ratio; abaxial color) or show transgressive variation (epidermal cell size). Abaxial leaf color was used to screen a large population of P. trichocarpa x P. deltoides hybrids for further evidence of tri/aneuploidy. In each case where a white abaxial leaf surface was observed and the nuclear DNA content measured, the hybrid proved to be tri/aneuploid. All sexually mature female triploids examined were sterile, although the inflorescences completed their development in the absence of embryo formation. The (probably) aneuploid F₁ hybrid is a fertile female. Of 15 female P. trichocarpa parents used in crosses to P. deltoides, 10 produced one or more tri/aneuploid hybrid offspring. In an intraspecific cross using a P. trichocarpa female that had produced triploid hybrids with five different P. deltoides males, no tri/aneuploid offpsring were found.     

Genetic studies of self-fertility in rye (Secale cereale L.). 2. The search for isozyme marker genes linked to self-incompatibility loci

The segregation of several isozyme marker genes has been studied in F₂ inbred families from hybrids between self-sterile and five self-fertile inbred lines (nos. 2, 3, 4, 5, and 8) as well as from interline hybrids. Self-pollination of F₁ hybrids between self-sterile forms and lines 5 and 8 gave an F₂ segregation ratio of 1 heterozygote:1 homozygote for the gene Prx7 (chromosome 1R) against the allele from the line. This is interpreted as a result of tight linkage of the Prx7 gene with the S1 gene in chromosome 1R (recombination at a level of 0–1%). The self-pollination of such hybrids with lines 2,3 and 4 gave normal segregation for the Prx7 gene (1:2:1). This means that these lines carry a self-fertility allele which is not on chromosome 1R. Interline hybrids 5×2, 5×3 and 5×4 had self-fertility alleles for the two S genes and in inbred F₂ progenies gave the expected deviating segregation for the Prx7 gene in a ratio of 2:3:1. The segregation of interline hybrid 5×8 was normal, 1:2:1, as expected. Highly-deviating segregation in an inbred F₂ family of a hybrid with line 5 has also been obtained for another gene from chromosome 1R — Pgi2 (recombination with the S1 locus of 16.7%). By using the same method it has been estimated that line 4 has a self-fertility allele of the S2 locus from chromosome 2R and that the genes -Glu and Est4/11 are linked with it (recombination 16.7% and 17.5–20% respectively). Lines 2 and 3 have a self-fertility allele of the S5 locus from chromosome 5R which is linked with the Est5-7 gene complex (recombination at a level of 28.8–36.0%).     

Inheritance of restriction fragment length polymorphisms and random amplified polymorphic DNAs in coastal Douglas-fir

A total of 225 new genetic loci [151 restriction fragment length polymorphisms (RFLP) and 74 random amplified polymorphic DNAs (RAPD)] in coastal Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco var. menziesii] have been identified using a three-generation outbred pedigree. The Mendelian inheritance of 16 RFLP loci and 29 RAPD loci was demonstrated based on single-locus segregation in a sample of F₂ progeny. One RFLP locus, PtIFG2025, showed segregation distortion. Probe pPtIFG2025 is a loblolly pine cDNA probe encoding for rbcS. The 16 RFLP loci and 23 allozyme loci were also assayed in a sample of 16 Douglas-fir seed-orchard clones. Allelism was determined at 11 of the 16 RFLP loci. RFLPs were able to detect slightly more variation (4.0 alleles per locus) than allozymes (3.1 alleles per locus). The inheritance of an additional 80 RAPD loci was determined based on haploid segregation analysis of megagametophytes from parent tree 013-1. Once 200–300 markers are identified and placed on a genetic map, quantitative trait loci affecting bud phenology will be mapped.     

The genetics of self-incompatibility in Brassica campestris L. ssp. Oleifera Metzg. I. Characteristics of S-locus. Control of self-incompatibility

Self-incompatibility in Brassica campestris c.v. Arlo is controlled by a single locus sporophytic system. The identity and expression of the S alleles were determined in eight inbred and two hybrid families. It was found that co-dominance of alleles is more frequent in the stigma, whereas dominance relations between pairs of alleles predominate in the pollen. A linear order of dominance was established between six S alleles and alleles high, intermediate and low in the dominance series were recognized.In considering the variation in the expression of compatibility and the segregation ratios in inbred, F₁, F₂ and backcross progenies, the presence of a specific S allele conditioning self-fertility, or a single dominant self-compatibility factor independent of the S locus could not be established. Instead, self-compatibility in this cultivar was ascribed to the segregation of a polygenic complex which is capable of modifying the incompatibility reaction to the point of self-fertility, or to a reduction in the strength of the reaction due to the presence of S alleles low in the dominance series.     

Estimation of the position and effect of a lethal factor locus on a molecular marker linkage map

In the mapping of DNA markers the distortion of segregation of marker genotypes is often observed, which may be caused by a lethal factor acting in filial generations derived from distant crosses. A method is presented for estimating the recombination values between a lethal factor locus and neighboring molecular markers, and the relative viability or fertilization ability of gametes or zygotes affected by the lethal factor in an F₂ population using the maximum likelihood method and the expectation conditional maximization (ECM) algorithm. Three selection models of gamete or zygote were considered, and the most likely one was determined by goodness of fit of the observed frequency of the phenotypes to the expected ones under the models. The method was applied to segregation data of molecular markers of an F₂ population consisting of 144 individuals derived from a cross between an Indica and a Japonica rice variety. The presence of a lethal factor locus (L) located on chromosome III that caused partial gametic selection in both the male and female sides was suggested. The locus L was tightly linked to RFLP marker number 23 of the RFLP linkage map of Saito et al. (1991a), and the fertilization chance of a male or female gamete possessing the lethal factor was, on average, 41.5% of that of the normal gamete.     

An allele responsible for seedling death in Pinus radiata D. Don

 When inbred, most outcrossing species show high mortality, manifested at several life stages. The occurrence of homozygotes for deleterious or lethal alleles is believed to be responsible. Here, we report the identification of an allele responsible for the death of selfed Pinus radiata D. Don seedlings in their first month after germination. Among 291 S₁ seedlings of plus-tree 850.55, 76 died within 1 month of emergence. Their death appears to be caused by a single recessive lethal allele, SDPr (seedling death in Pinus radiata). SDPr is located in a linkage group with 28 RAPD markers, the closest of which is ai05800a. Of the 76 seedlings that died, megagametophytes of 73 could be genotyped. Of these, 71 had the null (no band) allele of ai05800a; only two had the band allele. Of the 190 surviving S₁ diploids that were genotyped, only two individuals were homozygous for the null allele of ai05800a. By two different methods, the map distance between SDPr and ai05800a was estimated to be between 1.0 and 2.7 cM respectively. The frequency of band and null alleles in the combined population of dead and surviving seedlings and in un-sown seeds shows no evidence of selection at this locus prior to germination. Received: 30 September 1997 / Accepted: 29 October 1997     

Construction of an RFLP linkage map for cultivated sunflower

 An RFLP linkage map was constructed for cultivated sunflower Helianthus annuus L., based on 271 loci detected by 232 cDNA probes. Ninety-three F₂ plants of a cross between inbred lines RHA 271 and HA 234 were used as the mapping population. These genetic markers plus a fertility restoration gene, Rf ₁, defined 20 linkage groups, covering 1164 cM of the sunflower genome. Of the 71 loci 202 had codominant genotypic segregation, with the rest showing dominant segregation. Thirty-two of the 232 probes gave multiple locus segregation. There were 39 clusters of tightly linked markers with 0 cM distance among loci. This map has an average marker-to-marker distance of 4.6 cM, with 11 markerless regions exceeding 20 cM. Received: 17 June 1997 / Accepted: 19 June 1997     

High-resolution genetic mapping of bacterial blight resistance gene <Emphasis Type="Italic">Xa10</Emphasis>

Bacterial blight of rice, caused by Xanthomonas oryzae pv. oryzae (Xoo), is the most devastating disease of rice (Oryza sativa L). Rice lines that carry resistance (R) gene Xa10 confer race-specific resistance to Xoo strains harboring avirulence (Avr) gene avrXa10. Here we report on genetic study, disease evaluation and fine genetic mapping of the Xa10 gene. The inheritance of Xa10-mediated resistance to PXO99^A(pHM1avrXa10) did not follow typical Mendelian inheritance for single dominant gene in F₂ population derived from IR24 × IRBB10. A locus might be present in IRBB10 that caused distorted segregation in F₂ population. To eliminate this locus, an F₃ population (F₃-65) was identified, which showed normal Mendelian segregation ratio of 3:1 for resistance and susceptibility. A new near-isogenic line (F₃-65-1743) of Xa10 in IR24 genetic background was developed and designated as IRBB10A. IRBB10A retained similar resistance specificity as that of IRBB10 and provided complete resistance to PXO99^A(pHM1avrXa10) from seedling to adult stages. Linkage analysis using existing RFLP markers and F₂ mapping population mapped the Xa10 locus to the proximal side of E1981S with genetic distance at 0.93 cM. With five new RFLP markers developed from the genomic sequence of Nipponbare, Xa10 was finely mapped at genetic distance of 0.28 cM between proximal marker M491 and distal marker M419 and co-segregated with markers S723 and M604. The physical distance between M491 and M419 on Nipponbare genome is 74 kb. Seven genes have been annotated from this 74-kb region and six of them are possible Xa10 candidates. The results of this study will be useful in Xa10 cloning and marker-assisted breeding.     

The R1 gene conferring race-specific resistance to Phytophthora infestans in potato is located on potato chromosome V.

Summary Late blight in potato is caused by the fungusPhytophthora infestans and can inflict severe damage on the potato crop. Resistance toP. infestans is either based on major dominantR genes conferring vertical, race-specific resistance or on minor genes inducing horizontal, unspecific resistance. A dihaploid potato line was identified which carried theR1 gene, conferring vertical resistance to allP. infestans races, with the exception of those homozygous for the recessive virulence allele of the locusV1. The F₁ progeny of a cross between this resistant parent P(R1) and P(r), a line susceptible to all races, was analysed for segregation ofR1 and of restriction fragment length polymorphism (RFLP) markers distributed on the potato RFLP map comprising more than 300 loci. TheR1 locus was mapped to chromosome V in the interval between RFLP markers GP21 and GP179. The map position ofR1 was found to be very similar to the one ofRx2, a dominant locus inducing extreme resistance to potato virus X.     

Segregation analysis and RFLP mapping of the R1 and R3 alleles conferring race-specific resistance to Phytophthora infestans in progeny of dihaploid potato parents

Phytophthora infestans (Mont.) de Bary is the most important fungal pathogen of the potato (Solanum tuberosum). The introduction of major genes for resistance from the wild species S. demissum into potato cultivars is the earliest example of breeding for resistance using wild germplasm in this crop. Eleven resistance alleles (R genes) are known, differing in the recognition of corresponding avirulence alleles of the fungus. The number of R loci, their positions on the genetic map and the allelic relationships between different R variants are not known, except that the R1 locus has been mapped to potato chromosome V The objective of this work was the further genetic analysis of different R alleles in potato. Tetraploid potato cultivars carrying R alleles were reduced to the diploid level by inducing haploid parthenogenetic development of 2n female gametes. Of the 157 isolated primary dihaploids, 7 set seeds and carried the resistance alleles R1, R3 and R10 either individually or in combinations. Independent segregation of the dominant R1 and R3 alleles was demonstrated in two F₁ populations of crosses among a dihaploid clone carrying R1 plus R3 and susceptible pollinators. Distorted segregation in favour of susceptibility was found for the R3 allele in 15 of 18 F₁ populations analysed, whereas the RI allele segregated with a 1:1 ratio as expected in five F₁ populations. The mode of inheritance of the R10 allele could not be deduced as only very few F₁ hybrids bearing R10 were obtained. Linkage analysis in two F₁ populations between R1, R3 and RFLP markers of known position on the potato RFLP maps confirmed the position of the R1 locus on chromosome V and localized the second locus, R3, to a distal position on chromdsome XI.     

The genetic locus controlling supernodulation in soybean (Glycine max L.) co-segregates tightly with a cloned molecular marker.

Summary The genetic locus (nts) controlling nitrate-tolerant nodulation, supernodulation, and diminished autoregulation of nodulation of soybean (Glycine max (L.) Merill) was mapped tightly to the pA-132 molecular marker using a restriction fragment length polymorphism (RFLP) detected by subclone pUTG-132a. The nts (nitrate-tolerant symbiotic) locus of soybean was previously detected after its inactivation by chemical mutagenesis. Mutant plant lines were characterized by abundant nodulation (supernodulation) and tolerance to the inhibitory effects of nitrate on nodule cell proliferation and nitrogen fixation. The large number of RFLPs between G. max line nts382 (homozygous for the recessive nts allele) and the more primitive soybean G. soja (P1468.397) allowed the detection of co-segregation of several diagnostic markers with the supernodulation locus in F₂ families. We located the nts locus on the tentative RFLP linkage group E about 10 cM distal to pA-36 and directly next to marker pA-132. This very close linkage of the molecular marker and the nts locus may allow the application of this clone as a diagnostic probe in breeding programs as well as an entry point for the isolation of the nts gene.     

Molecular genetics of growth and development in Populus. III. A genetic linkage map of a hybrid poplar composed of RFLP,STS, and RAPD markers

We have evaluated three DNA-based marker types for linkage map construction in Populus: RFLPs detected by Southern blot hybridization, STSs detected by a combination of PCR and RFLP analysis, and RAPDs. The mapping pedigree consists of three generations, with the F₁ produced by interspecific hybridization between a P. trichocarpa female and a P. deltoides male. The F₂ generation was made by inbreeding to the maximum degree permitted by the dioecious mating system of Populus. The applicability of STSs and RAPDs outside the mapping pedigree has been investigated, showing that these PCR-based marker systems are well-suited to breeding designs involving interspecific hybridization. A Populus genome map (343 markers) has been constructed from a combination of all three types. The length of the Populus genome is estimated to be 2400–2800 cM.Abbreviations RFLP restriction fragment length polymorphism - STS sequence-tagged site - PCR polymerase chain reaction - RAPD random amplified polymorphic DNA     

Genetic characterization of apospory in tetraploid Paspalum notatum based on the identification of linked molecular markers

Tetraploid Paspalum notatum (bahiagrass) is a valuable forage grass with aposporous apomictic reproduction. In a previous study, we showed that apospory in bahiagrass is under the control of a single dominant gene with a distorted segregation ratio. The objective of this work was to identify molecular markers linked to apospory in tetraploid P. notatum and establish a preliminary syntenic relationship with the genomic region associated with apospory in P. simplex. A F₁ population of 290 individuals, segregating for apospory, was generated after crossing a completely sexual plant (Q4188) with a natural aposporous apomictic plant (Q4117). The whole progeny was classified as sexual or aposporous by embryo sacs analysis. A bulked segregant analysis was carried out to identify molecular markers co-segregating with apospory. Four hundred RAPD primers, 30 AFLP primers combinations and 85 RFLP clones were screened using DNA from both parental genotypes and aposporous and sexual bulks. Linkage analysis was performed with cytological and genetic information from the complete progeny. Cytoembryological analysis showed 219 sexual and 71 aposporous F₁ individuals. Seven different molecular markers (2 RAPD, 4 AFLP and 1 RFLP) were found to be completely linked to apospory. The RFLP probe C1069, mapping to the telomeric region of the long arm of rice chromosome 12, was one of the molecular markers completely linked to apospory in P. notatum. This marker had been previously associated with apospory in P. simplex. A preliminary map of the chromosome region carrying the apospory locus was constructed.     

Mapping of resistance to the potato cyst nematode Globodera rostochiensis from the wild potato species Solanum vernei

A population of diploid potato (Solanum tuberosum) was used for the genetic analysis and mapping of a locus for resistance to the potato cyst nematode Globodera rostochiensis, introgressed from the wild potato species Solanum vernei. Resistance tests of 108 genotypes of a F₁ population revealed the presence of a single locus with a dominant allele for resistance to G. rostochiensis pathotype Ro1. This locus, designated GroV1, was located on chromosome 5 with RFLP markers. Fine-mapping was performed with RAPD and SCAR markers. The GroV1 locus was found in the same region of the potato genome as the S. tuberosum ssp. andigena H1 nematode resistance locus. Both resistance loci could not excluded to be allelic. The identification of markers flanking the GroV1 locus offers a valuable strategy for marker-assisted selection for introgression of this nematode resistance.Abbreviations BSA bulked segregant analysis - RAPD random-amplified polymorphic DNA - RFLP restriction fragment length polymorphism - SCAR sequence-characterized amplified region     

Molecular mapping of a major gene conferring resistance to maize mosaic virus

 The objective of this study was to determine the genetic basis of resistance to maize mosaic virus (MMV). Molecular markers were used to map resistance loci to MMV in a set of 91 maize (Zea mays L.) recombinant inbred lines (RILs), derived from the cross between Hi31 (a B68 conversion resistant to MMV) and Kil4 (a Thai inbred susceptible to MMV). The RILs were evaluated for MMV resistance in disease nurseries in Hawaii in the winter of 1993 and the summer of 1994. Twenty-eight highly susceptible RILs were chosen for gene mapping using the pooled-sampling approach. Initial evidence from the pooled DNA indicated that restriction fragment length polymorphism (RFLP) probes on chromosome 3 near the centromere were biased to the susceptible parent allele. Analysis of 91 RILs at 103 RFLP loci confirmed the presence of a major MMV resistance gene on chromosome 3. The resistant allele at this locus, previously named Mv1, is present in the resistant parent Hi31 and traces back to the Argentine parent used in conferring common rust resistance to B68. We conclude that resistance to MMV in B68 and Caribbean flints involves a major gene mv1 on chromosome 3 located between RFLP markers umc102 and php20508. Received: 12 June 1996 / Accepted: 5 July 1996     

Segregation at the SCN resistance locus rhg1 in soybean is distorted by an association between the resistance allele and reduced field emergence

Segregation distortion has been reported repeatedly in soybean (Glycine max [L.] Merr.) inbred line populations segregating for the soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) resistance gene rhg1. In each reported case, the frequency of the SCN resistance allele at the rhg1 locus was lower than expected. Segregation distortion was studied in 51 F₄ populations by counting the number of plants predicted to be homozygous resistant, susceptible, and heterozygous for rhg1 based on the genetic markers Satt309, CTA, or TMA5. Significant (P<0.05) segregation distortion was observed in 44 out of the 51 F₄ populations. When the heterozygotes were ignored, there were significantly fewer homozygous-resistant plants than expected in 33 populations. To study whether differential field emergence was a cause of the segregation distortion, three near isogenic line (NIL) populations segregating at the rhg1 locus for SCN resistance from plant introduction 88788 were tested. Population sizes ranged from 32 to 44 NILs and emergence was determined in field experiments in three environments. In each population, SCN-resistant NILs had significantly (P<0.05) less field emergence than susceptible NILs. In the population with the greatest effect, field emergence of resistant NILs was 6% less than susceptible NILs, with the entire population having an average emergence rate of 46%. Equations were derived to describe the effect of selection on segregation ratios over generations of population development and the observed emergence rates were transformed into fitness factors. Depending on assumptions of gene action, it was predicted from these fitness factors that segregation distortions were in the range of those reported previously for the rhg1 locus and were similar to what was observed on average across the 51 F₄ populations. While other factors might also be involved, the results suggest that reduced field emergence associated with the SCN resistance allele contributes to previously reported segregation distortion at the rhg1 locus.     

An RFLP linkage map for loblolly pine based on a three-generation outbred pedigree

A genetic linkage map for loblolly pine (Pinus taeda L.) was constructed using segregation data from a three-generation outbred pedigree consisting of four grandparents, two parents, and 95 F₂ progeny. The map was based predominantly on restriction fragment length polymorphism (RFLP) loci detected by cDNA probes. Sixty-five cDNA and three genomic DNA probes revealed 90 RFLP loci. Six polymorphic isozyme loci were also scored. One-fourth (24%) of the cDNA probes detected more than 1 segregating locus, an indication that multigene families are common in pines. As many as six alleles were observed at a single segregating locus among grandparents and it was not unusual for the progeny to segregate for three or four alleles per locus. Multipoint linkage analysis placed 73 RFLP and 2 isozyme loci into 20 linkage groups; the remaining 17 RFLP and 4 isozyme loci were unlinked. The mapped RFLP probes provide a new set of codominant markers for genetic analyses in loblolly pine.     

Inheritance and genetic linkage of fusarium wilt {Fusarium oxysporum f.sp. cucumerinum race 1) and scab {Cladosporium cucumerinum) resistance genes in cucumber (Cucumis sativus)

The inheritance of resistance of the cucumber cv. SMR 18 to the race 1 of Fusarium oxysporum f.sp. cucumerinum, the linkage relationship between resistance to race 1 of F. oxysporum f.sp. cucumerinum, resistance to Cladosporium cucumerinum and fruit spine colour, and the reactions of several cucumber cultivars to inoculations with race 1 of F. oxysporum f.sp. cucumerinum and C. cucumerinum were examined. The inbred line Straight 8 (P,), which has white fruit spines and is susceptible to both fusarium wilt and scab was crossed with the inbred line SMR 18 (P₂), which has black fruit spines and resistance to both diseases. When F, F₂, F₃, BC₁P₁ BC₁P₂ and BC₁P₁ selfed progenies were inoculated at the cotyledon stage with a suspension of spores of race 1 of F. oxysporum f.sp. cucumerinum, the ratios of resistant to susceptible plants indicated that resistance was conferred by a single dominant gene, designated Fcu-1. When 171 BC^! plants were selfed and from each resulting F₂ family different groups of 15–25 seedlings each were tested for resistance to either disease, segregation data indicated that the Fcu-1 locus and the Ccu locus for C. cucumerinum resistance were completely linked. No evidence for linkage was found between the Fcu-1 (Ccu) locus and the B locus for fruit spine colour. Among the 59 cultivars tested at the seedling stage, 15 were susceptible, while the remainder were highly resistant to inoculations with both pathogens.     

Mouse serum amyloid P-component (SAP) levels controlled by a locus on chromosome 1

Recombinant inbred strains were used to demonstrate the existence of a major locus on chromosome 1, designated Sap, which controls the endogenous concentration of the mouse acute phase reactant, serum amyloid P-component (SAP). Levels of SAP were associated with alleles at the Ly-9 locus in two sets of RI strains: BXD (C57BL/6J × DBA/2) and BXH (C57BL/6J × C3H/HeJ). Low endogenous levels of SAP were present in the C57BL/6J progenitor strain and in most of the RI strains which inherited the Ly-9 ^ballele. High levels of SAP were present in the DBA/2J and C3H/HeJ progenitors and in most of the RI strains which inherited the Ly-9 ^aallele. In the BXD strains 91% of the genetic variation of SAP levels was accounted for by segregation at the Ly-9 locus while an additional 9% was attributed to genetic factors unlinked to Ly-9. In the BXH strains the percentage of genetic variation accounted for by Ly-9 segregation was reduced to 46%, while 54% was accounted for by other genetic factors. Because of background genetic variation it was not possible to detect any crossovers between Sap and Ly-9. However, in the BXD strains the linkage between Sap and Ly-9 appears to be quite close. The B6.C-H-25 ^ccongenic strain, which carries a segment of BALB/c chromosome 1 including the minor histocompatibility locus H-25 on a C57BL/6By background, had the same endogenous SAP level as the BALB/c donor strain.     

Mapping genes essential for embryo development in Arabidopsis thaliana.

Summary We have previously isolated and characterized over 90 recessive mutants of Arabidopsis thaliana defective in embryo development. These emb mutants have been shown to differ in lethal phase, extent of abnormal development, and response in culture. We demonstrate in this report the value and efficiency of mapping emb genes relative to visible and molecular markers. Sixteen genes essential for embryo development were mapped relative to visible markers by analyzing progeny of selfed F₁ plants. Embryonic lethals are now the most common type of visible marker included on the linkage map of Arabidopsis. Backcrosses were used in several cases to orient genes relative to adjacent markers. Three genes were located to chromosome arms with telotrisomics by screening for a reduction in the percentage of aborted seeds produced by F₁ plants. A restriction fragment length polymorphism (RFLP) mapping strategy that utilizes pooled EMB/EMB F₂ plants was devised to increase the efficiency of mapping embryonic lethals relative to molecular markers. This strategy was tested by demonstrating that the biol locus of Arabidopsis is within 0.5 cM of an existing RFLP marker. Mapping embryonic lethals with both visible and molecular markers may therefore help to identify large numbers of genes with essential functions in Arabidopsis.