A novel mouse Chromosome 2 congenic strain with obesity phenotypes

Linkage studies have identified many chromosomal regions containing obesity genes in mice. However, only a few of these quantitative trait loci (QTLs) have been used to guide the production of congenic mouse strains that retain obesity phenotypes. We seek to identify chromosomal regions containing obesity genes in the BSB model of spontaneous obesity because the BSB model is a multigenic obesity model. Previous studies identified QTLs on Chromosomes (Chrs) 2, 6, 7,12, and 15. BSB mice are made by backcross of lean C57BL/6J × Mus spretus. F₁s were backcrossed to C57BL/6J mice to produce BSB progeny. We have constructed a new BSB cross and produced congenic mice with obesity phenotypes by marker-directed selection called B6.S–D2Mit194–D2Mit311. We found a highly significant QTL for percentage body lipid on Chr 2 just proximal to the Agouti locus. Chr 2 congenics were constructed to determine whether the main effects would be detectable. We observed highly significant linkage of the Chr 2 congenic containing Agouti and containing markers distal to D2Mit311 and proximal to D2Mit194. Thus, this congenic contains approximately 14.6 cM or 30 Mb (about 1.1% of the spretus mouse genome) and several hundred genes. The obesity phenotype of the QTL is retained in the congenic. The congenic can now be used to model the genetic and physiological basis for a relatively simple, perhaps monogenic, obesity.     

A High-Resolution Map in the Chromosomal Region Surrounding theLpsLocus

TheLpslocus on mouse chromosome 4 controls host responsiveness to lipopolysaccharide, a major component of the outer membrane of Gram-negative bacteria. The C3H/HeJ inbred mouse strain is characterized by a mutantLpsallele (Lps^d) that renders it hyporesponsive to LPS and naturally tolerant of its lethal effects. To identify theLpsgene by a positional cloning strategy, we have generated a high-resolution linkage map of the chromosomal region surrounding this locus. We have analyzed a total of 1604 backcross mice from a preexisting interspecific backcross panel of 259 (Mus spretus× C57BL/6J)F1 × C57BL/6J and two novel panels of 597 (DBA/2J × C3H/HeJ)F1 × C3H/HeJ and 748 (C57BL/6J × C3H/HeJ)F1 × C3H/HeJ segregating atLps.A total of 50 DNA markers have been mapped in a 11.8-cM span overlapping theLpslocus. This positions theLpslocus within a 1.1-cM interval, flanked proximally by a large cluster of markers, including three known genes (Cd30l, Hxb,andAmbp), and distally by two microsatellite markers (D4Mit7/D4Mit178). The localization of theLpslocus is several centimorgans proximal to that previously assigned.     

Construction of a long-range YAC physical map spanning the 10-cM region between the markers D18Mit109 and D18Mit68 on mouse proximal chromosome 18.

Four yeast artificial chromosome (YAC) contigs, physically approximately 8 Mb, have been constructed spanning a 10-cM region on mouse proximal chromosome 18 and include the sites of 21 known genes, including those near the twirler (Tw) locus and the recently isolated Niemann-Pick type C1 (npc1) gene, formerly designated as the spm locus. This physical map consists of 49 YAC clones that cover roughly 15% of the chromosome. The physical order of 38 microsatellite sequence-tagged sites (STSs) could be assembled and confirmed based on their presence or absence in individual YACs, from proximal D18Mit109 through distal D18Mit68. These YACs provide an important resource for the further characterization and identification of known and unknown genes. The physical map has been integrated with our previously published genetic linkage map and showed an average genetic to physical distance of cM/Mb > 1.1.     

Congenic Strains Confirm the Pleiotropic Effect of Chromosome 4 QTL on Mouse Femoral Geometry and Biomechanical Performance

A pleiotropic quantitative trait locus (QTL) for bone geometry and mechanical performance in mice was mapped to distal chromosome 4 via an intercross of recombinant congenic mice HcB-8 and HcB-23. To study the QTL in isolation, we have generated C3H.B10-(rs6355453-rs13478087) (C.B.4.3) and C3H.B10-(rs6369860-D4Mit170) (C.B.4.2) congenic strains that harbor ~20 Mb and ~3 Mb, respectively, of chromosome 4 overlapping segments from C57BL/10ScSnA (B10) within the locus on a C3H/DiSnA (C3H) background. Using 3-point bend testing and standard beam equations, we phenotyped these mice for femoral mid-diaphyseal geometry and biomechanical performance. We analyzed the results via 2-way ANOVA, using sex and genotype as factors. In the C.B.4.3 strain, we found that homozygous B10/B10 male mice had smaller cross sectional area (CSA) and reduced total displacement than homozygous C3H/C3H mice. Sex by genotype interaction was also observed for maximum load and stiffness for C3H/C3H and B10/B10 mice, respectively. In C.B.4.2 strain, we found that homozygous B10/B10 mice had lower total displacement, post-yield displacement (PYD), stiffness, yield load and maximum load than mice harboring C3H allele. Sex by genotype interaction was observed in B10/B10 mice for perimeter, outer minor axis (OMA) and CSA. There were no significant differences in tissue level mechanical performance, which suggest that the QTL acts primarily on circumferential bone size. These data confirm the prior QTL mapping data and support other work demonstrating the importance of chromosome 4 QTL on bone modeling and bone responses to mechanical loading.     

Tss (Tail-short Shionogi), a new short tail mutation found in the BALB/cMs strain, maps quite closely to the Tail-short (Ts) locus on mouse chromosome 11.

A spontaneous morphological mutation characterized by a short and kinky tail (Tail-short Shionogi: Tss) was observed in a BALB/cMs mouse breeding colony. The inheritance mode of the Tss mutation is semi-dominant, and homozygotes (Tss/Tss) are probably embryonic lethal. The viability of the Tss/+ heterozygotes appear to be influenced by the mating partner: 47.1% of the (BALB/cMs-Tss/+ x C57BL/6J)F1 embryos were the mutant phenotype, whereas there were no (BALB/cMs-Tss/+ x A/J)F1 embryos with the mutant phenotype. The Tss locus was mapped by linkage analysis between microsatellite markers D11Mit128 and D11Mit256 on mouse Chromosome 11. These results suggest that the Tss mutation is a new allele on the Tail-short (Ts) locus.     

A novel mouse Chr5 locus <Emphasis Type="Italic">Diht</Emphasis> controls dopamine-induced hypothermia

Mice of the strain C3H.PRI-Flv^r, carrying genetically determined resistance to flaviviruses, have been shown to be more sensitive to the hypothermic effect of dopamine than congenic flavivirus-susceptible C3H/HeJARC mice. In the current study, the greater sensitivity to dopamine-induced hypothermia observed in flavivirus-resistant mice was shown to be dose-dependent, with strain differences being the most prominent at a moderate dose of apomorphine (1 mg/kg). In addition, hypothermic responses to apomorphine were shown to be under developmental regulation; aging increased the potency of apomorphine-induced hypothermia and abrogated strain and sex differences observed in young mice. Linkage analysis of mouse strain-dependent co-inheritance between flavivirus resistance and greater sensitivity to the hypothermic effect of dopamine was performed using two genetically unrelated flavivirus-susceptible and two highly congenic flavivirus-resistant mouse strains in parallel with C3H.PRI-Flv^r-and C3H/HeJARC reference strains. This study has revealed a clear segregation between flavivirus resistance conferred by the Flv locus and sensitivity to dopamine-controlled hypothermia conferred by a novel locus, Diht. Parallel studies in F1 and F2 heterozygote mice showed that the high sensitivity to hypothermic effect of dopamine (Diht^high) is inherited as the Chr5-linked dominant trait. The novel locus, Diht, has been mapped proximal to the Flv locus on a distal part of mouse Chr5 between microsatellite markers D5Mit41 and D5Mit158.     

Interspecific recombinant congenic strains between C57BL/6 and mice of the Mus spretus species: a powerful tool to dissect genetic control of complex traits

Complex traits are under the genetic control of multiple genes, often with weak effects and strong epistatic interactions. We developed two new collections of mouse strains to improve genetic dissection of complex traits. They are derived from several backcrosses of the Mus spretus SEG/Pas or STF/Pas strains on the C57BL/6J background. Each of the 55 interspecific recombinant congenic strains (IRCSs) carries up to eight SEG/Pas chromosomal segments with an average size of 11.7 Mb, totalizing 1.37% of the genome. The complete series covers 39.7% of the SEG/Pas genome. As a complementary resource, six partial or complete interspecific consomic strains were developed and increased genome coverage to 45.6%. To evaluate the usefulness of these strains for QTL mapping, 16 IRCSs were compared with C57BL/6J for seven hematological parameters. Strain 66H, which carries three SEG/Pas chromosomal segments, had lower red blood cell volume and higher platelet count than C57BL/6J. Each chromosomal segment was isolated in a congenic strain to evaluate individual effects. Congenic strains were combined to assess epistasis. Our data show that both traits were controlled by several genes with complex epistatic interactions. IRCSs are therefore useful to unravel QTL with small effects and gene-by-gene interactions.     

Analysis of reciprocal congenic lines reveals the C3H/HeJ genome to be highly resistant to ApcMin intestinal tumorigenesis

Genetic background affects polyp development in the Multiple intestinal neoplasia (Apc(Min)) mouse model. The Modifier of Min 1 (Mom1) locus accounts for approximately 50% of the variation in polyp multiplicity. We generated reciprocal congenic lines, such that the recipient C57BL/6J (B6) strain carries a donor C3H/HeJ (C3H) Mom1 allele, and the recipient C3H strain carries a donor B6 Mom1 allele. Hybrid progeny from congenic females mated to B6 Apc(Min/+) males were analyzed. A single C3H Mom1 locus on the B6 background reduced small intestinal polyp numbers by 50% and colon polyp incidence by 66% compared to their susceptible B6 Mom1(S/S)Apc(Min/+) siblings. These findings show that the C3H genome contains a resistant Mom1(R) locus. The reciprocal congenic line, which carries the susceptible B6 Mom1(S) locus on the C3H background, reduced small intestinal polyp numbers by 80% and colon polyp incidence by 95% compared to B6 Mom1(S/S)Apc(Min/+) mice. These data demonstrate that unidentified modifiers in the C3H strain can suppress intestinal polyp multiplicity in Apc(Min/+) mice, and act in the absence of a resistant Mom1(R) locus.     

Low resolution mapping around the flavivirus resistance locus (Flv) on mouse Chromosome 5

Although the phenomenon of innate resistance to flaviviruses in mice was recognized many years ago, it was only recently that the genetic locus (Flv) controlling this resistance was mapped to mouse Chromosome (Chr) 5. Here we report the fine mapping of the Flv locus, using 12 microsatellite markers which have recently been developed for mouse Chr 5. The new markers were genotyped in 325 backcross mice of both (C3H/HeJxC3H/ RV)F₁xC3H/HeJ and (BALB/cxC3H/RV)F₁xBALB/c backgrounds, relative to Flv. The composite genetic map that has been constructed identifies three novel microsatellite loci, D5Mit68, D5Mit159, and D5Mit242, tightly linked to the Flv locus. One of those loci, D5Mit159, showed no recombinations with Flv in any of the backcross mice analyzed, indicating tight linkage (<0.3 cM). The other two, D5Mit68 and D5Mit242, exhibited two and one recombinations with Flv (0.6 and 0.3 cM) respectively, defining the proximal and distal boundaries of a 0.9-cM segment around this locus. The proximal flanking marker, D5Mit68, maps to a segment on mouse Chr 5 homologous to human Chr 4. This, together with the previous data produced by our group, locates Flv to a region on mouse Chr 5 carrying segments that are conserved on either human Chr 4, 12, or 7, but present knowledge does not allow precise identification of the syntenic element.     

High-throughput genotyping of advanced congenic lines by high resolution melting analysis for identification of Bbaa2, a QTL controlling Lyme arthritis

Congenic mapping is a powerful strategy to identify genomic loci regulating quantitative traits. Generating congenic lines is an iterative process of refinement that is both time and resource intensive. Here we report an alternative to traditional microsatellite marker analysis or costly high-density oligonucleotide single nucleotide polymorphism (SNP) arrays for congenic genotyping. The identification of inherited SNP variability in congenic lines using high resolution melting analysis (HRMA) represents a novel application of the method. The blocked probe HRMA approach offers a scalable, low cost, closed-tube system that benefits from rapid turnaround times, and unequivocal interpretation. The markedly higher prevalence of SNPs relative to microsatellites in the genome allows much greater flexibility for the identification of new genotyping landmarks as congenic intervals are refined. We have adopted this approach in our development of B6.C3-Bbaa2 congenic lines for the identification of loci regulating murine Lyme arthritis severity. As a result, we have been able to fully genotype individuals prior to weaning age, and expand our number of breeding cages without increasing our colony budget. Thus far, 26 SNP markers have been successfully mapped to the Bbaa2 locus. This has facilitated the identification of 20 novel B6.C3-Bbaa2 congenic lines spanning the original interval.     

Further characterization of loss of heterozygosity enhanced by p53 abrogation in human lymphoblastoid TK6 cells: disappearance of endpoint hotspots

Loss of heterozygosity (LOH) is the predominant mechanism of spontaneous mutagenesis at the heterozygous thymindine kinase locus (tk) in TK6 cells. LOH events detected in spontaneous TK(-) mutants (110 clones from p53 wild-type cells TK6-20C and 117 clones from p53-abrogated cells TK6-E6) were analyzed using 13 microsatellite markers spanning the whole of chromosome 17. Our analysis indicated an approximately 60-fold higher frequency of terminal deletions in p53-abrogated cells TK6-E6 compared to p53 wild-type cells TK6-20C whereas frequencies of point mutations (non-LOH events), interstitial deletions, and crossing over events were found to increase only less than twofold by such p53 abrogation. We then made use of an additional 17 microsatellite markers which provided an average map-interval of 1.6Mb to map various LOH endpoints on the 45Mb portion of chromosome 17q corresponding to the maximum length of LOH tracts (i.e. from the distal marker D17S932 to the terminal end). There appeared to be four prominent peaks (I-IV) in the distribution of LOH endpoints/Mb of Tk6-20C cells that were not evident in p53-abrogated cells TK6-E6, where they appeared to be rather broadly distributed along the 15-20Mb length (D17S1807 to D17S1607) surrounding two of the peaks that we detected in TK6-20C cells (peaks II and III). We suggest that the chromosomal instability that is so evident in TK6-E6 cells may be due to DNA double-strand break repair occurring through non homologous end-joining rather than allelic recombination.     

Further characterization of loss of heterozygosity enhanced by p53 abrogation in human lymphoblastoid TK6 cells: disappearance of endpoint hotspots

Loss of heterozygosity (LOH) is the predominant mechanism of spontaneous mutagenesis at the heterozygous thymindine kinase locus (tk) in TK6 cells. LOH events detected in spontaneous TK⁻ mutants (110 clones from p53 wild-type cells TK6-20C and 117 clones from p53-abrogated cells TK6-E6) were analyzed using 13 microsatellite markers spanning the whole of chromosome 17. Our analysis indicated an approximately 60-fold higher frequency of terminal deletions in p53-abrogated cells TK6-E6 compared to p53 wild-type cells TK6-20C whereas frequencies of point mutations (non-LOH events), interstitial deletions, and crossing over events were found to increase only less than twofold by such p53 abrogation. We then made use of an additional 17 microsatellite markers which provided an average map-interval of 1.6 Mb to map various LOH endpoints on the 45 Mb portion of chromosome 17q corresponding to the maximum length of LOH tracts (i.e. from the distal marker D17S932 to the terminal end). There appeared to be four prominent peaks (I–IV) in the distribution of LOH endpoints/Mb of Tk6-20C cells that were not evident in p53-abrogated cells TK6-E6, where they appeared to be rather broadly distributed along the 15–20 Mb length (D17S1807 to D17S1607) surrounding two of the peaks that we detected in TK6-20C cells (peaks II and III). We suggest that the chromosomal instability that is so evident in TK6-E6 cells may be due to DNA double-strand break repair occurring through non homologous end-joining rather than allelic recombination.     

Overlapping mouse subcongenic strains successfully separate two linked body fat QTL on distal MMU 2

Background

Mouse chromosome 2 is linked to growth and body fat phenotypes in many mouse crosses. With the goal to identify the underlying genes regulating growth and body fat on mouse chromosome 2, we developed five overlapping subcongenic strains that contained CAST/EiJ donor regions in a C57BL/6J^hg/hg background (hg is a spontaneous deletion of 500 Kb on mouse chromosome 10). To fine map QTL on distal mouse chromosome 2 a total of 1,712 F2 mice from the five subcongenic strains, plus 278 F2 mice from the HG2D founder congenic strain were phenotyped and analyzed. Interval mapping (IM) and composite IM (CIM) were performed on body weight and body fat traits on a combination of SNP and microsatellite markers, which generated a high-density genotyping panel.

Results

Phenotypic analysis and interval mapping of total fat mass identified two QTL on distal mouse chromosome 2. One QTL between 150 and 161 Mb, Fatq2a, and the second between 173.3 and 175.6 Mb, Fatq2b. The two QTL reside in different congenic strains with significant total fat differences between homozygous cast/cast and b6/b6 littermates. Both of these QTL were previously identified only as a single QTL affecting body fat, Fatq2. Furthermore, through a novel approach referred here as replicated CIM, Fatq2b was mapped to the Gnas imprinted locus.

Conclusions

The integration of subcongenic strains, high-density genotyping, and CIM succesfully partitioned two previously linked QTL 20 Mb apart, and the strongest QTL, Fatq2b, was fine mapped to a ~2.3 Mb region interval encompassing the Gnas imprinted locus.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-014-1191-8) contains supplementary material, which is available to authorized users.     

Mouse male sterility and histoincompatibility (mshi) maps between the D10Mit51/168/212 cluster and D10Mit213

The recessive male sterility and histoincompatibility (mshi) mutation in the mouse generates pleiotropic effects on graft transplantation and male reproduction. Previous analysis of backcross mice typed for mshi either by testicular morphology or by allograft rejection has located each trait to a 20-cM region on proximal mouse Chr 10. Here we present the microsatellite polymorphism analysis of a new 276-member intraspecific backcross panel—including a set of 135 males typed for sterility and histoincompatibility—that places both features controlled by mshi within a 1.7-cM interval between markers D10Mit51/168/212 and D10Mit213. In addition, this analysis has allowed an explicit test of a two-gene model for the mshi locus and has provided a measurement of the penetrance of the mshi-generated histogenic phenotype in both male (88.4 ± 3.9%) and female (91.0 ± 3.5%) mutants. The fine-structure map presented should facilitate a chromosome walk across this region and, ultimately, the molecular identification of the gene or genes affected by this interesting mutation. Received: 28 August 1998 / Accepted 18 December 1998     

High resolution genetic and physical mapping of the mouse asebia locus: A key gene locus for sebaceous gland differentiation

The asebia (ab) mutation in the mouse is an autosomal recessive trait with hypoplastic sebaceous glands. As a first step toward cloning the ab gene, we report here the genetic mapping of the ab locus with respect to Chromosome 19 microsatellite markers. 644 backcross progeny were generated by mating (CAST/EiJ × DBA/1LacJ-ab^2J/ab^2J) F₁ heterozygous females and parental ab^2J/ab^2J mutant males. Our results located the ab gene to an interval of 1.6 cM on mouse Chromosome 19 defined by flanking markers D19Mit11 and D19Mit53/D19Mit27, and identified a tightly linked polymorphic marker, D19Mit67, that co-segregates with the mutation in the backcross progeny examined. This places ab locus 4 cM distal to its present position as indicated in Mouse Genome Database at The Jackson Laboratory. We have also mapped a yeast artificial chromosome (YAC) contig in this locus interval which suggests the ab interval to be less than one megabase of DNA.     

Three loci on mouse chromosome 6 influence onset and final incidence of type I diabetes in NOD.C3H congenic strains

The development of insulin-dependent diabetes mellitus in both human and mouse is dependent on the interaction between genetic and environmental factors. The analysis of newly created NOD.C3H congenic strains for spontaneous and cyclophosphamide-induced diabetes has allowed the definition of three controlling genetic loci on mouse chromosome 6. A NOD-derived susceptibility allele at the Idd6 locus strongly influences the onset of diabetes in spontaneous diabetes. A NOD-derived resistance allele at the Idd19 locus affects the final diabetes incidence observed in both models, while a novel locus, provisionally termed Idd20, appears to control Idd19 in an epistatic manner. Decreased diabetes incidence is observed in CY-induced diabetes when Idd20 is homozygous for the C3H allele, while heterozygosity is associated with an increase in diabetes incidence. The Idd20, Idd19, and Idd6 candidate regions fall respectively within genetically defined intervals of 4, 7, and 4.5 cM on mouse chromosome 6. From our YAC contig, Idd6 would appear to localize within a ca. 1.5-Mb region on distal chromosome 6.     

Genome-wide search for genes that modulate inflammatory arthritis caused by Ali18 mutation in mice

Many of inflammatory diseases, including inflammatory arthritis, are multifactorial bases. The Ali18 semidominant mutation induced by N-ethyl-N-nitrosourea in the C3HeB/FeJ (C3H) genome causes spontaneous inflammation of peripheral limbs and elevated immunoglobulin E (IgE) levels in mice. Although the Ali18 locus was mapped to a single locus on chromosome 4, the arthritic phenotype of Ali18/+ mice was completely suppressed in F1 hybrid genetic backgrounds. To determine the chromosomal locations of the modifier loci affecting the severity of arthritis, an autosomal genome scan of 22 affected Ali18/+ F2 mice was conducted using C57BL/6J as a partner strain. Interestingly, regions on chromosomes 1 and 3 in C3H showed significant genetic interactions. Moreover, 174 N2 (backcross to Ali18/Ali18) and 267 F2 animals were used for measurement of arthritis scores and plasma IgE levels, and also for genotyping with 153 genome-wide single nucleotide polymorphism (SNP) markers. In N2 populations, two significant trait loci for arthritis scores on chromosomes 1 and 15 were detected. Although no significant scores were detected in F2 mice besides chromosome 4, a suggestive score was detected on chromosome 3. In addition, a two-dimensional genome scan using F2 identified five suggestive scores of chromosomal combinations, chromosomes 1 × 10, 2 × 6, 3 × 4, 4 × 9, and 6 × 15. No significant trait loci affecting IgE levels were detected in both N2 and F2 populations. Identification of the Ali18 modifier genes by further detailed analyses such as congenic strains and expression profiling may dissect molecular complexity in inflammatory diseases.     

Positional cloning of "Lisch-Like", a candidate modifier of susceptibility to type 2 diabetes in mice

In 404 Lep(ob/ob) F2 progeny of a C57BL/6J (B6) x DBA/2J (DBA) intercross, we mapped a DBA-related quantitative trait locus (QTL) to distal Chr1 at 169.6 Mb, centered about D1Mit110, for diabetes-related phenotypes that included blood glucose, HbA1c, and pancreatic islet histology. The interval was refined to 1.8 Mb in a series of B6.DBA congenic/subcongenic lines also segregating for Lep(ob). The phenotypes of B6.DBA congenic mice include reduced beta-cell replication rates accompanied by reduced beta-cell mass, reduced insulin/glucose ratio in blood, reduced glucose tolerance, and persistent mild hypoinsulinemic hyperglycemia. Nucleotide sequence and expression analysis of 14 genes in this interval identified a predicted gene that we have designated "Lisch-like" (Ll) as the most likely candidate. The gene spans 62.7 kb on Chr1qH2.3, encoding a 10-exon, 646-amino acid polypeptide, homologous to Lsr on Chr7qB1 and to Ildr1 on Chr16qB3. The largest isoform of Ll is predicted to be a transmembrane molecule with an immunoglobulin-like extracellular domain and a serine/threonine-rich intracellular domain that contains a 14-3-3 binding domain. Morpholino knockdown of the zebrafish paralog of Ll resulted in a generalized delay in endodermal development in the gut region and dispersion of insulin-positive cells. Mice segregating for an ENU-induced null allele of Ll have phenotypes comparable to the B.D congenic lines. The human ortholog, C1orf32, is in the middle of a 30-Mb region of Chr1q23-25 that has been repeatedly associated with type 2 diabetes.     

Recombinant congenic strains derived from A/J and C57BL/6J: a tool for genetic dissection of complex traits

Complex genetic traits can be dissected in mice, using well-defined sets of recombinant inbred strains, congenic strains, and recombinant congenic strains (RCS). We report the creation of a series of 37 independent RCS derived from the commonly used inbred strains of laboratory mouse A/J (A) and C57BL/6J (B6). These RCS were derived by systematic inbreeding of independent pairs of animals from a (F1 x A) x A and a (F1 x B) x B double backcross (N3), to create AcB and BcA strains, respectively. Fifteen AcB strains and 22 BcA strains at between 18 and 30 generations of inbreeding have been generated, are healthy, and show stable breeding performance. These strains have been genotyped for a total of 625 informative microsatellite DNA markers covering the entire genome, with an average spacing of 2.6 cM. Haplotype analyses indicate that on average, AcB and BcA strains contain 13.25% of the donor genome, a value close to the 12.5% expected from the breeding scheme used in their creation. In the AcB set, approximately 79% of the B6 genome has been transferred in independent strains, while in the BcA set approximately 84% of the A genome is represented on the B6 background. This represents an excellent coverage of congenic segments from both parental genomes in the two sets of strains, which can now be used to map simple and complex traits in a genome-wide fashion. As an example of the power of AcB/BcA strains as a mapping tool, the 37 strains were typed for susceptibility to infection with Legionella pneumophila, a monogenic trait controlled by the Lgn1 locus on Chromosome 13. Analysis of the strain distribution pattern of L. pneumophila susceptibility allowed direct mapping of Lgn1 to a 3-cM interval. The AcB/BcA set should prove a useful tool with which to investigate the complex genetic basis of known interstrain differences between A and B6 for many important diseases.