High-Resolution Genetic and Physical Mapping of Modifier-of-deafwaddler (mdfw) and Waltzer (Cdh23)

Modifier-of-deafwaddler (mdfw) and waltzer (Cdh23^v) are loci on mouse chromosome 10 encoding factors that are essential for the function of auditory hair cells. The BALB/cByJ-specific mdfw allele encodes a necessary and sufficient modifier that induces progressive early onset hearing loss in CBy-dfw^2J heterozygotes. Recessive mutations in the waltzer locus result in circling behavior and congenital deafness. In this report we present a high-resolution integrated genetic and physical map of mdfw and Cdh23^v. Our genetic analyses localize mdfw between markers D10Mit60 and 148M13T7 within a 1.01-cM region. The Cdh23^v critical interval is fully contained within the mdfw region and localizes between markers 146O23T7 and 148M13T7 within a 0.35-cM interval that is represented in an ≈500-kb BAC contig. Our data suggest that mdfw and Cdh23^v are allelic.     

Physical and Genetic Maps of thedeafwaddlerRegion on Distal Mouse Chr 6

Thedeafwaddler(dfw) mutation, displaying motor ataxia and profound deafness, arose spontaneously in a C3H/HeJ colony and was mapped previously to distal mouse Chr 6. In this study, a high-resolution genetic map was generated by positioning 10 microsatellite markers and 5 known genes on a 968-meioses intersubspecific backcross segregating fordfw[(CAST/Ei–+/+ × C3HeB/FeJ–dfw/dfw) × C3HeB/FeJ–dfw/dfw], giving the following marker order and sex-averaged distances:D6Mit64–(0.10 + 0.10 cM)–Pang–(1.24 + 0.36 cM)–Itpr1–(0.62 + 0.25 cM)–D6Mit108–(0.52 + 0.23 cM)–D6Mit54–(0.21 + 0.15 cM)–D6Mit23, D6Mit107, D6Mit328–(0.72 + 0.27 cM)–D6Mit11–(0.21 + 0.15 cM)–dfw–(0.93 + 0.31 cM)–Gat4, D6Mit55–(0.10 + 0.10 cM)–D6Mit63–(0.31 + 0.18 cM)–Syn2–(0.62 + 0.25 cM)–D6Mit44(Rho). Female and male genetic maps are similar immediately surrounding thedfwlocus, but show marked differences in other areas. A yeast artificial chromosome-based physical map suggests that the closest markers flanking thedfwlocus,D6Mit11(proximal) andGat4, D6Mit55(distal), are contained within 650–950 kb. The human homologues of the flanking lociItpr1(proximal) andSyn2(distal) map to chromosome 3p25–p26, suggesting that the human homologue of thedfwgene is located within this same region.     

Ahl2, a second locus affecting age-related hearing loss in mice

Inbred mouse strains with age-related hearing loss (AHL) provide valuable models for studying the genetic basis of human presbycusis. Here we report the genetic mapping of a second AHL locus in mice (designated Ahl2) that is a major contributor to the 8- to 10-month difference in hearing loss onset times between NOD/LtJ and C57BL/6J mice. A whole-genome linkage scan of 110 progeny from a (C57BL/6JxNOD/LtJ)xNOD/LtJ backcross revealed statistically significant associations of ABR thresholds with markers on chromosome 5, with a peak lod score of 5.5 for D5Mit309. At 6 months of age, backcross progeny that inherited two copies of the recessive NOD/LtJ-derived allele at this locus (genotype ahl2/ahl2) exhibited ABR thresholds that were on average 26 decibels above those of heterozygous mice. Analysis of a (CAST/EixNOD/LtJ)xNOD/LtJ backcross, which segregates strain-specific alleles at both Ahl2 and the Ahl locus on chromosome 10, showed that the hearing loss attributable to Ahl2 is dependent on a predisposing Ahl genotype. The statistically significant effect of Ahl2 observed in crosses with NOD/LtJ was not seen in crosses involving three other strains with early onset AHL: A/J, BUB/BnJ, and SKH2/J.     

Neuromuscular degeneration (nmd): a mutation on mouse Chromosome 19 that causes motor neuron degeneration

Neuromuscular degeneration, nmd, is a spontaneous autosomal recessive mutation in the mouse producing progressive hindlimb impairment caused by spinal muscular atrophy. We used an intersubspecific intercross between B6.BKs-nmd ^2J/+ and Mus musculus castaneus (CAST/Ei) to map the nmd mutation to mouse Chromosome (Chr) 19 with the most likely gene order: nmd-(D19Se12, Pygm)-Cntf-Pomc2-D19Mit16-Cyp2c-Got1. nmd maps near muscle deficient, mdf, and has a very similar clinical phenotype, but allele tests and histological differences suggest that nmd is a distinct mutation at a different locus. Although closely linked, nmd recombined with the candidate genes muscle glycogen phosphorylase, Pygm, and ciliary neurotrophic factor, Cntf.     

Y-chromosomal factor is involved in neonatal lethality in (♀ DDD × ♂DH-Dh/+) F1-Dh/+ male mice

The dominant hemimelia(Dh) mutation causes various developmental abnormalities in mice. Most -Dh/+ males, crosses between DDD females and DH-Dh/+ males, have lethal abnormalities during the neonatal period. This is a consequence of synergism among three independent gene loci; that is, theDh allele on chromosome (Chr) 1, the DDD allele on an X Chr-linked locus, and a Y Chr-linked locus in some strains. With regard to the Y Chr derived fromMus musculus musculus (M. m. musculus), the Y Chrs of C57BL/6J and BALB/cA caused lethality, but the Y Chr of C3H/HeJ did not, suggesting that not allM. m. musculus Y Chrs are the same. In the present study, whether Y Chrs derived fromM. m. domesticus andM. m. castaneus could cause lethality was investigated. Among seven inbred strains, including AKR/J, DDD, RF/J, SJL/J, SWR/J, TIRANO/Ei, and CAST/Ei, Y Chrs of AKR/ J, DDD, SJL/J, SWR/J, and TIRANO/Ei caused lethality, but Y Chrs of RF/J and CAST/Ei did not. It was unlikely that the mitochondrial genome of the DDD strain contributed to the lethality. The X Chr-linked locus could not compensate for the role of the Y Chr-linked locus. These results suggest that not allM. m. domesticus Y Chrs are the same.     

The original shaker-with-syndactylism mutation (sy) is a contiguous gene deletion syndrome

Tests for allelism among mice with four different mutant alleles at the shaker-with-syndactylism locus on mouse Chromosome (Chr) 18 provide evidence that the original radiation-induced mutation, sy, is a deletion including at least two genes associated with distinct phenotypes. Mice homozygous for sy have syndactylous feet and other skeletal malformations, are deaf, and exhibit abnormal behavior characteristic of vestibular dysfunction. Two less severe spontaneous mutations, shown to be allelic with sy, cause syndactylism when homozygous (hence named fused phalanges, sy ^fp and sy ^fp-2J ), but do not affect hearing and behavior. Here we describe a third spontaneous mutation allelic with sy that does not affect foot morphology (hence named no syndactylism, sy ^ns ), but that does cause deafness and balance defects when homozygous. Complementation test results indicate that sy ^fp and sy ^fp-2J are alleles of the same gene, but that sy ^ns is an allele of a different gene. The original sy mutation, therefore, includes both of the genes defined by these three spontaneous mutations. Typing of DNA markers in sy/sy mice revealed a deletion of approximately 1 cM in the sy region of Chr 18, including D18Mit52, D18Mit124, D18Mit181, and D18Mit205. The genetic relationships described here will aid in positional cloning efforts to identify the genes responsible for the disparate phenotypes associated with the sy locus. Received: 8 May 1998 / Accepted: 10 July 1998     

Molecular characterization of four intra-t mouse recombinants

Recombination in the proximal region of mouse chromosome 17 is greatly reduced in heterozygotes carrying the wild-type and thet complex-type chromosomes. The reason for this is the presence of two non-overlapping inversions in thet complex. Rare crossing-over does, however, occur within thet complex of thet/+ heterozygotes. Here we characterize four such exceptional intra-t recombinants,t ^Tu1 throught ^Tu4 . To map the positions of the genetic exchange in these four recombinants, we analyzed them with DNA probes specific for 16 loci distributed over thet complex. The analysis revealed that in three of the four recombinants, an equal crossing-over occurred in the short region between the two inversions, producing chromosomes carrying either the proximal inversion only (t ^Tu1 andt ^Tu4 ) or the distal inversion only (t ^Tu2 ). In the fourth recombinant (t ^Tu3 ), unequal crossing-over occurred within the proximal inversion between lociD17Leh119 andD17Leh66, producing a chromosome in which the region containing lociTcp-1, T, andD17Tu5 has been duplicated. The duplication of theBrachyury locus leads to the suppression of the tail-shortening effect normally produced by the interaction of the dominant (T) and recessive (tct) alleles at this locus so that theT/t ^Tu3 mice have normal tails.     

Identification of a New Chemically Induced Allele (Lp) at the Loop-Tail Locus: Morphology, Histology, and Genetic Mapping

Loop-tail (Lp) is a semidominant mutation that affects neurulation in mice. Heterozygous animals are characterized by a looped-tail appearance (pig tail) and wobbly head movements while homozygous embryos exhibit a neural tube closure defect that extends from the caudal midbrain to the tip of the tail. The Lp gene has been finely mapped to the distal part of chromosome 1, and a positional cloning strategy has been initiated to isolate the defective gene. This study represents the characterization of a new Lp allele (Lp^m1Jus) induced by N-ethyl-N-nitrosurea mutagenesis. Lp^m1Jus/+ mice have a looped-tail appearance, and both Lp^m1Jus/Lp^m1Jus homozygotes and Lp/Lp^m1Jus compound heterozygotes fail to initiate neural tube closure along most of the embryonic axis. These data indicate that the Lp^m1Jus allele causes a neural tube defect and overall phenotype similar to that of the original Lp allele. Segregation analysis of 90 (Lp^m1Jus/+ × C57BL/6J)F₁ × C57BL/6J looped-tail mice with seven markers that define the Lp genetic map (D1Mit455/D1Mit146/D1Mit148/D1Mit270–1 cM–D1Mit113–0.4 cM–Lp–0.2 cM–D1Mit149–0.8 cM–D1Mit115) showed significant linkage between Lp^m1Jus and all loci analyzed (P < 0.0001). Eight crossovers were detected with the proximal cluster of D1Mit455, D1Mit146, D1Mit148, and D1Mit270, indicating a recombination rate higher than expected in this region, and a single recombinant was encountered with the distal markers D1Mit149 and D1Mit115. Based on these phenotypic and genetic data, Lp^m1Jus is most likely allelic to Lp, thereby representing a valuable additional tool for the positional cloning of the Lp gene and its subsequent molecular characterization.     

Characteristics of the Cfu-S Population In Mice Carrying the SlJ Allele

Abstract Abstract. A tentative characterization of haemopoietic stem cells with respect to their organ distribution, seeding fraction and colony formation in the spleen, radiosen-sitivity and humoral regulation was attempted in mice heterozygous for the mutant allele Sl^J and in their normal littermates. Sl^J/+ mice were characterized by a deficient CFU-s content of the blood and spleen and had slightly lower femoral CFU-s numbers. This CFU-s distribution could not be explained by differences in seeding efficiency ‘f’ between CFU-s of Sl^J/+ and +/+ origin in lethally irradiated recipients used in the CFU-s assay. the seeding fraction of CFU-s of +/+ origin did not differ in +/+ and Sl^J/+ recipients. However, in irradiated SI^J/+ recipient mice a 30% decrease was observed in the number of the colonies derived from splenic and femoral CFU-s of both +/+ and Sl^J/+ origin. the serum level of SHSF (splenic haemopoiesis stimulating factor) was decreased in Sl^J/+ mice, but significantly increased in Sl/Sl^d mice, as compared to their respective normal +/+ littermates. Endogenous colony formation in Sl^J/+ spleens was deficient in comparison to that observed in +/+ spleens, and distinct sex differences were observed. However, mutant and normal CFU-s from spleen and bone marrow had a similar survival following in-vitro y irradiation. Femurs and spleens of both Sl^J/+ and +/+ origin were implanted into both Sl^J/+ and +/+ hosts. Six weeks later the Sl^J/+ grafts contained less CFU-s than the +/+ grafts. These data show that the splenic stroma of Sl^J/+ mice is not defective in its capacity to lodge injected CFU-s but is deficient in its ability to maintain CFU-s under ‘steady-state’ conditions and stimulate their colony formation in a ‘perturbed state’. Some of the characteristics of Sl^J/+ mice segregate them from Sl/Sl^d mice, i.e. a deficient splenic CFU-s content, normal seeding fractions ‘f’ of CFU-s from spleen and bone marrow in the presence of an almost compensated anemia, and decreased serum levels of SHSF. the study of the Sl^J trait may be a useful extension of the current Sl/Sl^d model for exploration of hereditary defects in haematopoietic stroma.     

Compound Heterozygosity of the Functionally Null Cdh23v-ngt and Hypomorphic Cdh23ahl Alleles Leads to Early-onset Progressive Hearing Loss in Mice

The waltzer (v) mouse mutant harbors a mutation in Cadherin 23 (Cdh23) and is a model for Usher syndrome type 1D, which is characterized by congenital deafness, vestibular dysfunction, and prepubertal onset of progressive retinitis pigmentosa. In mice, functionally null Cdh23 mutations affect stereociliary morphogenesis and the polarity of both cochlear and vestibular hair cells. In contrast, the murine Cdh23^ahl allele, which harbors a hypomorphic mutation, causes an increase in susceptibility to age-related hearing loss in many inbred strains. We produced congenic mice by crossing mice carrying the v niigata (Cdh23^v-ngt) null allele with mice carrying the hypomorphic Cdh23^ahl allele on the C57BL/6J background, and we then analyzed the animals’ balance and hearing phenotypes. Although the Cdh23^v-ngt/ahl compound heterozygous mice exhibited normal vestibular function, their hearing ability was abnormal: the mice exhibited higher thresholds of auditory brainstem response (ABR) and rapid age-dependent elevation of ABR thresholds compared with Cdh23^ahl/ahl homozygous mice. We found that the stereocilia developed normally but were progressively disrupted in Cdh23^v-ngt/ahl mice. In hair cells, CDH23 localizes to the tip links of stereocilia, which are thought to gate the mechanoelectrical transduction channels in hair cells. We hypothesize that the reduction of Cdh23 gene dosage in Cdh23^v-ngt/ahl mice leads to the degeneration of stereocilia, which consequently reduces tip link tension. These findings indicate that CDH23 plays an important role in the maintenance of tip links during the aging process.     

Selection of human cells having two different types of mutations in individual cells (genetic/artificial mutants)

Summary We have previously reported the establishment and characterization of B cell lines from patients and family members with various types of adenine phosphoribosyltransferase (APRT) deficiencies. These cell lines contain, at the APRT locus, three different alleles (APRT ^* 1, APRT ^* Q0, and APRT ^* J) that are clearly distinguishable from each other. From five genetically heterozygous cell lines with two different genotypes (APRT ^* 1/APRTQ0 and APRT ^* 1/APRT ^* J), we have selected 48 clones resistant to 2,6-diaminopurine. Resistance to this adenine analogue is a characteristic of cells having defects in both of the APRT alleles in individual cells. The mutant clones from a cell line from a complete-type heterozygote had APRT activities close to zero (mean=0.04 nmol/min per milligram protein) in the cell extracts, while 15 clones from four cell lines from the four Japanese-type heterozygotes had significant enzyme activities (mean=3.88 nmol/min per milligram protein). Kinetic studies on two of the mutants from two Japancse-type heterozygous cell lines have shown that affinity to substrate 5-phosphoribosyl-1-pyrophosphate was reduced, indicating that APRT in those clones reflected the characteristics of the Japanese-type enzyme. The data presented here indicate that clones we obtained are genetic/artificial mutants, each having a genetic mutation in a single allele (APRT ^* J or APRT ^* Q0) and an artificially produced mutation in the other previously functional allele (APRT ^*1). The present procedure provided the only diagnostic method for Japanese-type APRT heterozygotes (APRT ^* 1/APRT ^* J).     

Quantitative trait loci on chromosome 5 for susceptibility to frequency-specific effects on hearing in DBA/2J mice

The DBA/2J strain is a model for early-onset, progressive hearing loss in humans, as confirmed in the present study. DBA/2J mice showed progression of hearing loss to low-frequency sounds from ultrasonic-frequency sounds and profound hearing loss at all frequencies before 7 months of age. It is known that the early-onset hearing loss of DBA/2J mice is caused by affects in the ahl (Cdh23^ahl) and ahl8 (Fscn2^ahl8) alleles of the cadherin 23 and fascin 2 genes, respectively. Although the strong contributions of the Fscn2^ahl8 allele were detected in hearing loss at 8- and 16-kHz stimuli with LOD scores of 5.02 at 8 kHz and 8.84 at 16 kHz, hearing loss effects were also demonstrated for three new quantitative trait loci (QTLs) for the intervals of 50.3–54.5, 64.6–119.9, and 119.9–137.0 Mb, respectively, on chromosome 5, with significant LOD scores of 2.80–3.91 for specific high-frequency hearing loss at 16 kHz by quantitative trait loci linkage mapping using a (DBA/2J × C57BL/6J) F₁ × DBA/2J backcross mice. Moreover, we showed that the contribution of Fscn2^ahl8 to early-onset hearing loss with 32-kHz stimuli is extremely low and raised the possibility of effects from the Cdh23^ahl allele and another dominant quantitative trait locus (loci) for hearing loss at this ultrasonic frequency. Therefore, our results suggested that frequency-specific QTLs control early-onset hearing loss in DBA/2J mice.     

Increased physical activity cosegregates with higher intake of carbohydrate and total calories in a subcongenic mouse strain

C57BL/6 J (B6) and CAST/EiJ (CAST), the inbred strain derived from M. musculus castaneus, differ in nutrient intake behaviors, including dietary fat and carbohydrate consumption in a two-diet-choice paradigm. Significant quantitative trait loci (QTLs) for carbohydrate (Mnic1) and total energy intake (Kcal2) are present between these strains on chromosome (Chr) 17. Here we report the refinement of the Chr 17 QTL in a subcongenic strain of the B6.CAST- ^{D17Mit19-D17Mit91} congenic mice described previously. This new subcongenic strain possesses CAST Chr 17 donor alleles from 4.8 to 45.4 Mb on a B6 background. Similar to CAST, the subcongenic mice exhibit increased carbohydrate and total calorie intake per body weight, while fat intake remains equivalent. Unexpectedly, this CAST genomic segment also confers two new physical activity phenotypes: 22% higher spontaneous physical activity levels and significantly increased voluntary wheel-running activity compared with the parental B6 strain. Overall, these data suggest that gene(s) involved in carbohydrate preference and increased physical activity are contained within the proximal region of Chr 17. Interval-specific microarray analysis in hypothalamus and skeletal muscle revealed differentially expressed genes within the subcongenic region, including neuropeptide W (Npw); glyoxalase I (Glo1); cytochrome P450, family 4, subfamily f, polypeptide 1 (Cyp4f15); phospholipase A2, group VII (Pla2g7); and phosphodiesterase 9a (Pde9a). This subcongenic strain offers a unique model for dissecting the contributions and possible interactions among genes controlling food intake and physical activity, key components of energy balance.     

Identification of a new CML target antigen controlled by a gene associated with theQa-2 locus

BALB/cBy anti-BALB/cJ spleen cells were tested in a secondary cellmediated lympholysis assay. The effector cells generated displayed a positive cytotoxic effect against Con A lymphoblasts from only those strains that were typed serologically as having theQa-2 ^a allele. Confirmation that the target antigen is controlled by a locus closely associated with or identical toQa-2 was obtained by the findings that target cells from B6.K2 (Qa-2 ^a,Qa-3 ^a) mice were lysed by the effector cells, while those from theQa-2, 3 congenic strain B6.K1 (Qa-2 ^b,Qa-3 ^b) were not. The fact that target cells from aQa-2-positive/Qa-3-negative strain (DBA/1,Qa-2 ^ai,Qa-3 ^b) were killed indicates that the target antigen is controlled, at least in part, by theQa-2 locus, not the Qa-3.There is no observedH-2 genetic restriction for this cytotoxic effect, since target cells which have theQa-2 ^a allele but differ from the stimulator cells at theH-2K, D, andI regions were lysed efficiently.     

High-resolution genetic map and YAC contig around the mouse neurological locus reeler

Mutations at the recessive reeler locus (rl) on mouse Chromosome (Chr) 5 result in abnormal development of multiple central nervous system components, including the cerebral and cerebellar cortices. These abnormalities are characterized by highly disorganized laminar structures thought to have arisen from a post-migration failure of neuronal organization events that are probably mediated through cell-cell interactions. As a result of a mutagenesis scheme designed to generate visible recessive mutations induced by the drug chlorambucil, we had previously recovered a new allele of the reeler locus (rl ^Alb ) that is likely to involve a deletion based on the known mechanisms of chlorambucil action. We have constructed a high-resolution genetic map from two intercrosses segregating this allele. Our first cross, in which the mutation was outcrossed to the 101 strain prior to intercrossing, consisted of 196 meioses and resulted in the positioning of four loci proximal to rl, with D5Mit1 being the closest (2.6±1.1 cM). The second cross consisted of intercrossing rl heterozygotes derived from an outcross to the C57BL/6 strain. A total of 318 mice (636 meioses) gave rise to a panel of 41 recombinants, which were used to map a total of 14 loci within a 6.4-cM interval bounded by D5Mit1 and the En-2 gene. A yeast artificial chromosome contig consisting of clones containing two of these loci, D5Mit72 (located 0.31 cM distal to rl), and D5Mit61 (no recombinants with rl), has been assembled and is being used to locate the rl gene.     

Effects ofPax3Modifier Genes on Craniofacial Morphology, Pigmentation, and Viability: A Murine Model of Waardenburg Syndrome Variation

Waardenburg syndrome type 1 is caused by mutations inPAX3.Over 50 humanPAX3mutations that lead to hearing, craniofacial, limb, and pigmentation anomalies have been identified. APAX3mutant allele, segregating in a family, can show reduced penetrance and variable expressivity that cannot be explained by the nature of the mutation alone. TheMus musculus Pax3mutationSp^d(Splotch-delayed, Pax3[formula]), coisogenic on the C57BL/6J (B₆) genetic background, produces in heterozygotes a white belly spot with 100% penetrance and very few other anomalies. By contrast, manySp^d/+ BC₁progeny [F₁♀Sp^d/+ (♀Sp^d/+ B₆× ♂ +/+Mus spretus) × ♂ +/+ B₆] exhibit highly variable craniofacial and pigmentary anomalies. Of the BC₁Sp^d/+ progeny, 23.9% are estimated to be nonviable, and 32.1% are nonpenetrant for the white belly spot. The penetrance and expressivity of theSp^d/+ genotype are controlled in part by the genetic background and the sex of the individual. A minimum of two genes interact withSp^dto influence the craniofacial features of these mice. One of these genes may be either X-linked or sex-influenced, while the other is autosomal. TheA-locus (Agouti) or a gene closely linked toAalso plays a role in determining craniofacial features. At least one additional gene, possibly theA-locus or a gene linked toA,interacts withSp^dand determines the presence and size of the white belly spot. The viability of BC₁mice is influenced by at least three factors:Sp^d,A-locus alleles or a gene closely linked to theA-locus, and the sex of the mouse. These BC₁mice provide an opportunity to identify genes that interact with and modify the expression ofPax3and serve as a model to identify the genes that modify the expression of humanPAX3mutations.     

Fine Genetic Map of Mouse Chromosome 10 around the Polycystic Kidney Disease Gene,jcpk,and Ankyrin 3

A chlorambucil (CHL)-induced mutation of thejcpk(juvenile congenital polycystic kidney disease) gene causes a severe early onset polycystic kidney disease. In an intercross involvingMus musculus castaneus, jcpkwas precisely mapped 0.2 cM distal toD10Mit115and 0.8 cM proximal toD10Mit173.In addition, five genes,Cdc2a, Col6a1, Col6a2, Bcr,andAnk3were mapped in both thisjcpkintercross and a (BALB/c × CAST/Ei)F₁× BALB/c backcross. All five genes were eliminated as possible candidates forjcpkbased on the mapping data. Thejcpkintercross allowed the orientation of theAnk3gene relative to the centromere to be determined.D10Mit115, D10Mit173, D10Mit199,andD10Mit200were separated genetically in this cross. The order and genetic distances of all markers and gene loci mapped in thejcpkintercross were consistent with those derived from the BALB/c backcross, indicating that the CHL-induced lesion has not generated any gross chromosomal abnormalities detectable in these studies.     

High-Resolution Linkage Map of Mouse Chromosome 13 in the Vicinity of the Host Resistance LocusLgn1

Natural resistance of inbred mouse strains to infection withLegionella pneumophilais controlled by the expression of a single dominant gene on chromosome 13, designatedLgn1.The genetic difference atLgn1is phenotypically expressed as the presence or absence of intracellular replication ofL. pneumophilain host macrophages. In our effort to identify theLgn1gene by positional cloning, we have generated a high-resolution linkage map of theLgn1chromosomal region. For this, we have carried out extensive segregation analysis in a total of 1270 (A/J × C57BL/6J) × A/J informative backcross mice segregating the resistance allele of C57BL/6J and the susceptibility allele of A/J. Additional segregation analyses were carried out in three preexisting panels of C57BL/6J ×Mus spretusinterspecific backcross mice. A total of 39 DNA markers were mapped within an interval of approximately 30 cM overlapping theLgn1region. Combined pedigree analyses for the 5.4-cM segment overlappingLgn1indicated the locus order and the interlocus distances (in cM):D13Mit128–(1.4)–D13Mit194–(0.1)–D13Mit147–(0.9)–D13Mit36–(0.9)–D13Mit146–(0.2)–Lgn1/D13Mit37–(1.0)–D13Mit70.Additional genetic linkage studies of markers not informative in the A/J × C57BL/6J cross positionedD13Mit30, -72, -195,and-203, D13Gor4, D13Hun35,andMtap5in the immediate vicinity of theLgn1locus. The marker density and resolution of this genetic linkage map should allow the construction of a physical map of the region and the isolation of YAC clones overlapping the gene.     

The fate of several migrant genes in isolated populations of Drosophila melanogaster

Sepia-eyed flies carrying the slow electrophoretic variant of either Est-6 or Adh were introduced in low numbers and at infrequent intervals into populations of wildtype flies (+ ^se /+ ^se ) that were also homozygous for the fast moving variant of either Est-6 (50 populations) or Adh (50 populations). After 24 generations, the frequency of the sepia alleles was approximately 25%, although there was considerable variation from population to population. The fate of the Est-6 slow allele corresponded closely to that of sepia (which is located ten map units distant), although one population retained the slow allozyme variant but rejected sepia. The Adh slow allele was also retained by many populations. A number of them retained Adh-S but not sepia, and vice versa; these loci are on different chromosomes. The advantage of sepia heterozygotes was estimated to be about twice that of wildtype homozygotes. The data suggest that the selective advantage resides not with the sepia locus itself, but with a nearby chromosomal region.Financial support for work reported here was supplied under grant number GM24850, National Institutes of Health.     

Cerebellar deficient folia (cdf): A new mutation on mouse Chromosome 6

Cerebellar deficient folia, cdf, is a spontaneous autosomal recessive mutation in the mouse with unique pathology; the cerebellar cortex of the cdf/cdf mouse has only 7 folia instead of 10, which is the normal count for the C3H/HeJ strain in which this mutation arose. The cerebellum of the cdf/cdf mouse is hypoplastic and contains mineral deposits in the ventral vermis that are not present in controls. We used an intersubspecific intercross between C3H/HeSnJ-cdf/+ and Mus musculus castaneus (CAST/Ei) to map the cdf mutation to Chromosome (Chr) 6. The most likely gene order is D6Mit16–(cdf, D6Mit3)–D6Mit70–D6Mit29–D6Mit32, which positions cdf distal to lurcher (Lc) and proximal to motor neuron degeneration 2 (mnd2). The definitive visible phenotypes and histopathologies of cdf, Lc, and mnd2 support our mapping evidence that cdf is a distinct gene. The novel pathology of cdf should help elucidate the complicated process of cerebellar folia patterning and development. cdf recombined with mouse atonal homolog 1, Math1, the mouse homolog of the Drosophila atonal gene. Received: 2 August 1996 / Accepted: 2 October 1996