Fine mapping of a murine growth locus to a 1.4-cM region and resolution of linked QTL

Previous work identified a QTL affecting murine size (particularly tail length) in a cross between C57BL/6J and DBA/2J mice and refined its location to an 8-cM region between D1Mit30 and D1Mit57. The present study used recombinant progeny testing to fine map this QTL. Individuals from a partially congenic strain carrying chromosomes recombinant between D1Mit30 and D1Mit57 were mated to DBA/2J, generating 942 progeny. Two QTL affecting 10-week tail length were identified in this population: one at 9.7 cM distal to D1Mit30 (the position estimated in previous work), and another of smaller effect near D1Mit30. A second population (n=787) was generated by mating siblings from the progeny test population that were heterozygous for the same segment of chromosome, including only recombinants between D1Mit265 and D1Mit57. In the latter population, two QTL were also identified: one at 10.2 cM distal to D1Mit30, and another of smaller effect at the distal end of the mapped region (at D1Mit150). When the two populations were analyzed together, the estimated location of the central QTL was 10.2 cM distal to D1Mit30 and there was marginally significant evidence of the distal QTL. The central QTL explained approximately 7% of the phenotypic variance, and the 95% confidence interval for its position (determined by bootstrapping) was a 1.4-cM region, approximately the region from D1Mit451 to D1Mit219. The central QTL also affected tail length and body mass at 3 and 6 weeks of age, but to a lesser degree than 10-week tail length.     

Gene-environment interaction: a significant diet-dependent obesity locus demonstrated in a congenic segment on mouse Chromosome 7

We have previously reported suggestive evidence for a locus on Chromosome (Chr) 7 that affects adiposity in F₂ mice from a CAST/Ei × C57BL/6J intercross fed a high-fat diet. Here we characterize the effect of a high-fat (32.6 Kcal% fat) diet on male and female congenic mice with a C57BL/6J background and a CAST/Ei-derived segment on Chr 7. Adiposity index (AI) and weights of certain fat pads were approximately 50% lower in both male and female congenic mice than in control C57BL/6J mice, and carcass fat content was significantly reduced. The reduction of fat depot weights was not seen, however, in congenic animals fed a low-fat chow diet (12 Kcal% fat). The congenic segment is approximately 25 cM in length, extending from D7Mit213 to D7Mit41, and includes the tub, Ucp2, and Ucp3, genes, all of which are candidate genes for this effect. Some polymorphisms have been found on comparing c-DNA sequences of the Ucp2 gene from C57BL/6J and CAST/Ei mice. These results suggest that one or more genes present in the congenic segment modulate the susceptibility to fat deposition on feeding a high-fat diet. We were unable to show any significant difference between the energy intakes of the congenic and the control C57BL/6J mice on the high-fat diet. Also, measurements of energy expenditure in male mice at 6 weeks of age, during the first 2 weeks of exposure to the high-fat diet, failed to show any differences between control and congenic animals. Received: 30 September 1998 / Accepted: 22 December 1998     

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.     

Alcohol preference in AXB/BXA recombinant inbred mice: gender differences and gender-specific quantitative trait loci

The purpose of the present study was to characterize the C57BL/6J, A/J, and AXB/BXA Recombinant Inbred (RI) strains of mice for voluntary alcohol consumption. Quantitative Trait Locus (QTL) analysis was used to provide provisional location of QTLs for alcohol consumption. The inbred strains were screened for levels of alcohol intake (calculated as alcohol preference and absolute alcohol consumption) by receiving 4 days of forced exposure to a 10% (wt/vol) solution of alcohol, followed by 3 weeks of free choice between water and 10% alcohol. A wide and continuous distribution of values for alcohol consumption and preference was obtained in the AXB/BXA RI strains, confirming polygenic influences on alcohol-related behaviors. Significant gender differences were found for both alcohol preference [F_28,651= 2.12, p < 0.001] and absolute alcohol consumption [F_28,647= 2.57, p < 0.001]. In males, putative QTLs were mapped to chromosomes (Chrs) 2, 5, 7, 10, 11, and 16. Multiple regression analysis indicated that approximately 75% of the genetic variance in alcohol preference in males could be accounted for by three of the QTL regions. Several of the putative QTLs appeared to be male-specific (Tyr on Chr 7; D10Mit126 on Chr 10; D11Mit61 on Chr 11). In females, seven putative QTLs were mapped to Chrs 2, 4, 5, 7, 11, 16, and 19. Approximately 90% of the genetic variance in alcohol preference in females could be accounted for by four QTL regions, as determined by multiple regression. The QTL on Chr 11 near D11Mit35 appeared to be female-specific. This site was close to a female-specific QTL (Alcp2) previously mapped in C57BL/6J × DBA/2J backcrosses by Melo and coworkers (Nat Genet 13, 147, 1996). The QTLs mapped for alcohol preference in the present study must be considered suggestive at the present time, since only D2Mit74 met very strict statistical criteria for significance. However, the concordance across several studies for the loci on Chrs 2, 4, 7, 9, and 11 suggest that some common QTLs influencing alcohol preference have been identified. Confirmation of QTLs mapped in the present study is currently being conducted in a new series of recombinant congenic (RC) strains developed from reciprocal backcrosses between the A/J and C57BL/6J progenitors. The concomitant use of both RI and RC strains developed from the same progenitors should provide a powerful means of detecting, confirming, and mapping QTLs for alcohol-related traits. Received: 25 August 1998 / Accepted: 8 October 1998     

A growth QTL (<Emphasis Type="Italic">Pbwg1</Emphasis>) region of mouse chromosome 2 contains closely linked loci affecting growth and body composition

Previous QTL studies have identified 24 QTLs for body weight and growth from 3 to 10 weeks after birth in an intersubspecific backcross mouse population between C57BL/6J and wild Mus musculus castaneus that has 60% of the body size of C57BL/6J. The castaneus allele at the most potent QTL (Pbwg1) on proximal chromosome 2 retards growth. In this study we have developed a congenic strain with a 44.1-Mb interval containing the castaneus allele at Pbwg1 by recurrent backcrossing to C57BL/6J. The congenic mouse developed was characterized by significantly higher body weight gain between 1 and 3 weeks of age and lower weight of white fat pads at 10 weeks of age than C57BL/6J. However, no clear difference in body weight at 1–10 weeks of age was observed between congenic and C57BL/6J strains. QTL analysis with 269 F₂ mice between the two strains did not identify any QTLs for body weight at 1, 3, 6, and 10 weeks of age, but it discovered eight closely linked QTLs affecting body weight gain from 1 to 3 weeks of age, lean body weight, weight of white fat pads, and body length within the Pbwg1 region. The castaneus alleles at all fat pad QTLs reduced the phenotypes, whereas at the remaining growth and body composition QTLs, they increased the trait values. These results illustrate that Pbwg1, which initially appeared to be a single locus, was resolved into several loci with opposite effects on the composition traits of overall body weight. This gives a reason for the loss of the Pbwg1 effect found in the original backcross population. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Quantitative trait loci for growth traits in C57BL/6J × DBA/2J mice

Quantitative trait loci (QTLs) for body weight and tail length are mapped in an F₂ population of 927 C57BL/6J × DBA/2J mice. We test the concordance between the locations of the mapped QTLs with those detected by changes of marker frequency under artificial selection in a previous experiment with the same base population. The directions of effects of the QTLs are generally in agreement, and in many cases significant QTLs are found in similar map positions, but there are also discrepancies between the two experiments. There are indications of age-specific QTL effects on growth. For body weight traits, the genetic variation in the F₂ appears to result from many loci with relatively small effects. For tail length at 10 weeks, however, a single QTL on Chromosome (Chr) 1 with a peak LOD score of ∼33 contributes most of the genetic variation detected, changes the trait value by about 6%, and explains about 20% of the phenotypic variance of the trait. Received: 4 August 1998 / Accepted: 17 November 1998     

Mapping murine loci for seizure response to kainic acid

Mature DBA/2J (D2) mice are very sensitive to seizures induced by various chemical and physical stimuli, whereas C57BL/6J (B6) mice are relatively seizure resistant. We have conducted a genome-wide search for quantitative trait loci (QTLs) influencing the differential sensitivity of these strains to kainic acid (KA)-induced seizures by studying an F₂ intercross population. Parental, F₁, and F₂ mice (8–10 weeks of age) were injected subcutaneously with 25 mg/kg of KA and observed for 3 h. Latencies to focal and generalized seizures and status epilepticus were recorded and used to calculate an overall seizure score. Results of seizure testing indicated that the difference in susceptibility to KA-induced seizures between D2 and B6 mice is a polygenic phenomenon with at least 65% of the variance due to genetic factors. First-pass genome screening (10-cM marker intervals) in F₂ progeny (n = 257) documented a QTL of moderate effect on Chromosome (Chr) 1 with a peak LOD score of 5.5 (17% of genetic variance explained) localized between D1Mit30 and D1Mit16. Provisional QTLs of small effect were detected on Chr 11 (D11Mit224–D11Mit14), 15 (D15Mit6–D15Mit46) and 18 (D18Mit9–D18Mit144). Multiple locus models generally confirmed the Mapmaker/QTL results and also provided evidence for another QTL on Chr 4 (D4Mit9). Multilocus analysis of seizure severity suggested that additional loci on Chrs 5 (D5Mit11), 7 (D7Mit66), and 15 (D15Nds2) might also contribute to KA-induced seizure response. Overall, our results document a complex genetic determinism for KA-induced seizures in these mouse strains with contributions from as many as eight QTLs. Received: 16 April 1996 / Accepted: 21 October 1996     

The replicability of QTLs for murine alcohol preference drinking behavior across eight independent studies

On the basis of eight independent quantitative trait loci (QTL) studies of ethanol (alcohol) preference drinking in mice, a meta-analysis was carried out to examine the replicability of QTLs across studies and to enhance the power of QTL detection and parameter estimation. To avoid genetic heterogeneity, we analyzed only studies of mapping populations derived from the C57BL/6 (B6) and DBA/2 (D2) inbred progenitor strains. Because these studies were carried out in five different laboratories, there were substantial differences in testing procedure, data analysis, and especially in the choice of mapping population (BXD recombinant inbred strains, F₂, backcross, selected lines, or congenic strains). Despite this, we found several QTLs that were sufficiently robust as to appear consistently across studies given the strengths and weaknesses of the mapping populations employed. These were on Chromosomes (Chrs) 2 (proximal to mid), 3 (mid to distal), 4 (distal), and 9 (proximal to mid). The P value for each of these QTLs, combined across all applicable studies, ranged from 10⁻⁷ to 10⁻¹⁵, with the additive effect of each QTL accounting for 3–5% of the trait variance extrapolated to an F₂ population. Two other QTLs on Chrs 1 (distal) and 11 (mid) were less consistent, but still reached overall significance (P < .0001). Received: 18 April 2001 / Accepted: 25 July 2001     

An interstitial telomere array proximal to the distal telomere of mouse chromosome 13

Lambda clones of mouse DNA from BALB/c and C57BL/10, each containing an array of telomere hexamers, were localized by FISH to a region close to the telomere of Chr 13. Amplification of mouse genomic DNA with primers flanking SSRs within the cloned DNA showed several alleles, which were used to type eight sets of RI strains. The two lambda clones contained allelic versions of the interstitial telomere array, Tel-rs4, which is 495 bp in C57BL/10 and which includes a variety of sequence changes from the consensus telomere hexamer. Comparison of the segregation of the amplification products of the SSRs with the segregation of other loci in an interspecies backcross (C57BL/6JEi × SPRET/Ei) F₁× SPRET/Ei shows recombination suppression, possibly associated with ribosomal DNA sequences present on distal Chr 13 in Mus spretus, when compared with recombination in an interstrain backcross, (C57BL/6J × DBA/J) F₁× C57BL/6J, and with the MIT F₂ intercross. Analysis of recombination in females using a second interstrain backcross, (ICR/Ha × C57BL/6Ha) F₁× C57BL/6Ha, also indicates recombination suppression when compared with recombination in males of the same strains, using backcross C57BL/6Ha × (ICR/Ha × C57BL/6Ha) F₁. Thus, more than one cause may contribute to recombination suppression in this region. The combined order of the loci typed was D13Mit37–D13Mit30–D13Mit148–(D13Rp1, 2, 3, 4, Tel-rs4)–D13Mit53–D13Mit196–D13Mit77–(D13Mit78, 35). Data from crosses where apparently normal frequencies of recombination occur suggest that the telomere array is about 6 map units proximal to the most distal loci on Chr 13. This distance is consistent with evidence from markers identified in two YAC clones obtained from the region. Received: 24 September 1996/Accepted: 20 January 1997     

QTL influencing baseline hematocrit in the C57BL/6J and DBA/2J lineage: age-related effects

Baseline serum hematocrit varies substantially in the population. While additive genetic factors account for a large part of this variability, little is known about the genetic architecture underlying the trait. Because hematocrit levels vary with age, it is plausible that quantitative trait loci (QTL) that influence the phenotype also show an age-specific profile. To investigate this possibility, hematocrit was measured in three different age cohorts of mice (150, 450, and 750 days) of the C57BL/6J (B6) and the DBA2/J (D2) lineage. QTL were searched in the B6D2F₂ intercross and the BXD recombinant inbred (RI) strains. The effects of these QTL were explored across the different age groups. On the phenotypic level, baseline serum hematocrit declines with age in a sex-specific manner. In the B6D2F₂ intercross, suggestive QTL that influence the phenotype were located on Chromosomes (Chr) 1, 2, 7, 11, 13, and 16. With the exception of the QTL on Chr 2, all of these QTL exerted their largest effect at 750 days. The QTL on Chr 1, 2, 7, 11 and 16 were confirmed in the BXD RIs in a sex- and age-specific manner. Linkage analysis in the BXD RIs revealed an additional significant QTL on Chr 19. Baseline serum hematocrit is influenced by several QTL that appear to vary with the age and sex of the animal. These QTL primarily overlap with QTL that have been shown to regulate hematopoietic stem cell phenotypes.     

Quantitative trait loci affecting growth in high growth (hg) mice

A genome-wide scan was performed in order to identify Quantitative Trait Loci (QTL) associated with growth in a population segregating high growth (hg), a partially recessive mutation that enhances growth rate and body size in the mouse. A sample of 262 hg/hg mice was selected from a C57BL/6J-hg/hg× CAST/EiJ F₂ cross and typed with 79 SSLP markers distributed across the genome. Eight significant loci were identified through interval mapping. Loci on Chromosomes (Chrs) 2 and 8 affected the growth rate of F₂ mice. Loci on Chr 2 and 11 affected growth rate and carcass lean mass (protein and ash). A locus on Chr 9 modified femur length and another one in Chr 17 affected both carcass lean mass and femur length, but none of these had significant effects on growth rate. Loci on Chrs 5 and 9 modified carcass fat content. Additive effects were positive for C57BL/6J alleles, except for the two loci affecting carcass fatness. Typing of selected markers in 274 +/+ F₂ mice revealed significant interactions between hg and other growth QTL, which were detected as changes in gene action (additive or dominant) and in allele substitution effects. Knowledge about interactions between loci, especially when major genes are involved, will help in the identification of positional candidate genes and in the understanding of the complex genetic regulation of growth rate and body size in mammals. Received: 29 June 2000 / Accepted: 22 November 2000     

Genetic determinants of weight of fast- and slow-twitch skeletal muscles in old mice

The main goal of the study was to explore the genetic architecture underlying muscle weight in old mice. Weight of soleus, tibialis anterior (TA), extensor digitorum longus (EDL), and gastrocnemius muscles was measured in the C57BL/6J (B6) and DBA/2J (D2) strains and derivative generations: a panel of the BXD recombinant inbred (RI) strains and a B6D2 F₂ intercross at the age of 800 days. The between-strain difference in muscle weight (B6 > D2) ranged between 16% and 38%. Linkage analysis identified suggestive quantitative trait loci (QTL) on Chromosomes (Chr) 2, 6, 7, 8, 19, and X that influenced muscle weight in the 800-day-old group. Comparison of weights at 200, 500, and 800 days revealed a variable effect of age among the four muscles. Linkage analysis in the B6D2 F₂ population combined across the three different age groups identified muscle-, sex-, and age-specific QTL on Chr 1, 2, 3, 5, 6, 8, 9, 11, 13, 17, X, and Y. Genetic factors that influence the rate of weight change (within-strain weight difference at two ages) over the lifespan of BXD RIs were mapped to the markers D2Mit369 and D3Mit130 at the genome-wide p < 0.05 for TA muscle in males (between 200 and 800 days) and females (between 500 and 800 days), respectively. Analysis of all age groups supported previous findings that the genetic effects may be muscle-, age-, and sex-specific.     

Intersubspecific subcongenic mouse strain analysis reveals closely linked QTLs with opposite effects on body weight

A previous genome-wide QTL study revealed many QTLs affecting postnatal body weight and growth in an intersubspecific backcross mouse population between the C57BL/6J (B6) strain and wild Mus musculus castaneus mice captured in the Philippines. Subsequently, several closely linked QTLs for body composition traits were revealed in an F₂ intercross population between B6 and B6.Cg-Pbwg1, a congenic strain on the B6 genetic background carrying the growth QTL Pbwg1 on proximal chromosome 2. However, no QTL affecting body weight has been duplicated in the F₂ population, except for mapping an overdominant QTL that causes heterosis of body weight. In this study, we developed 17 intersubspecific subcongenic strains with overlapping and nonoverlapping castaneus regions from the B6.Cg-Pbwg1 congenic strain in order to search for and genetically dissect QTLs affecting body weight into distinct closely linked loci. Phenotypic comparisons of several developed subcongenic strains with the B6 strain revealed that two closely linked but distinct QTLs that regulate body weight, named Pbwg1.11 and Pbwg1.12, are located on an 8.9-Mb region between D2Mit270 and D2Mit472 and on the next 3.6-Mb region between D2Mit205 and D2Mit182, respectively. Further analyses using F₂ segregating populations obtained from intercrosses between B6 and each of the two selected subcongenic strains confirmed the presence of these two body weight QTLs. Pbwg1.11 had an additive effect on body weight at 6, 10, and 13?weeks of age, and its castaneus allele decreased it. In contrast, the castaneus allele at Pbwg1.12 acted in a dominant fashion and surprisingly increased body weight at 6, 10, and 13?weeks of age despite the body weight of wild castaneus mice being 60% of that of B6 mice. These findings illustrate the complex genetic nature of body weight regulation and support the importance of subcongenic mouse analysis to dissect closely linked loci.     

A Locus for Circadian Period of Locomotor Activity on Mouse Proximal Chromosome 3

Lengthened circadian period of locomotor activity is a characteristic of a congenic strain of mice carrying a nonsense mutation in exon 5 of the carbonic anhydrase II gene, car2. The null mutation in car2 is located on a DBA/2J inbred strain insert on proximal chromosome 3, on an otherwise C57BL/6J genomic background. Since reducing the size of the congenic region would narrow the possible candidate genes for period, two recombinant congenic strains (R1 and R2) were developed from the original congenic strain. These new congenic strains were assessed for period, genetic composition, and the presence of immunoreactive carbonic anhydrase II. R1 mice were homozygous DBA/2J for the distal portion of the original DBA/2J insert, while R2 mice were homozygous DBA/2J for the proximal portion. R1 mice had a significantly lengthened period compared to R2 mice and wild-type C57BL/6J mice, indicating that the gene(s) affecting period is likely found within the reduced DBA/2J insert (?1 cM) in the R1 mice. The R1 mice also possessed the null mutation in car2. This study confirmed the presence of a gene(s) affecting period on proximal chromosome 3 and significantly reduced the size of the congenic region and the number of candidate genes. Future studies will focus on identifying the gene influencing period.     

Genetic Differences in Androgen Receptors and in Autoregulation of Testicular Human Chorionic Gonadotropin Binding Sites in the Mouse

Abstract

The autoregulation of testicular human chorionic gonadotropin (hCG) binding sites was studied in two strains of mice known to differ in their endocrine and reproductive characteristics (C57BL/10J and DBA/2J), and in their F₁ progeny (B10D2F₁). Basal hCG binding levels were higher in C57BL/10J than in DBA/2J mice, while B10D2F₁ mice had intermediate levels. Twenty-four h after injection, hCG produced dose-related changes in hCG binding in C57BL/10J and B10D2F₁ mice not observed in DBA/2J mice. However, 72 h after treatment with hCG there was a decrease in hCG binding in all the strains studied. These results suggest the participation of genetic factors in determining basal levels, dose-related changes and temporal response of testicular hCG binding sites to hCG administration. Androgen receptor levels were measured in the same strains of mice. DBA/2J mice had higher receptor levels in the kidney and coagulating gland, and lower levels in the hypothalamus and seminal vesicle when compred to C57BL/10J mice. B10D2F₁ mice had androgen receptor levels similar to those measured in C57BL/10J mice in all tissues studied, with the exception of the coagulating gland, where levels were similar to those observed in DBA/ZJ mice. These observations may indicate the existence of several loci coding for androgen receptors, with only one being expressed per tissue     

The genetics of aryl hydrocarbon hydroxylase induction in mice: A single gene difference between C57BL/6J and DBA/2J

Twenty-one inbred strains of mice were surveyed for inducibility of hepatic aryl hydrocarbon hydroxylase (AHH) activity by the carcinogen 3-methylcholanthrene (MC). In 11 strains given MC, AHH activity increased 1.3- to 5-fold (inducible), whereas ten strains responded with a less than 0.5-fold increase (noninducible). Neither the inducible nor the noninducible class was homogeneous, and in each considerable variation was found in both the basal activity of AHH and the response to MC. Strains DBA/2J and C57BL/6J were chosen to represent the noninducible and inducible classes, respectively. In the crosses (C57BL/6 × DBA/2)F₁ × DBA/2 and (C57BL/6 × DBA/2)F₂, inducibility segregated as a single autosomal dominant gene. The gene symbols Ahh ⁱ and Ahh ⁿ are proposed for the alleles present in C57BL/6J and DBA/2J, respectively. No genetic linkage was found between the Ahh locus and the following loci: b, d, Es-1, Es-3, Gpd-1, Hbb, Id-1, Pgm-1, and sex. Some implications of this work in the study of mammalian enzyme induction and chemically induced carcinogenesis are discussed. There is a positive correlation between AHH inducibility and the development of an inflammatory response to the topical application of the carcinogen 7,12-dimethylbenzanthracene.     

Influences of inbreeding and genetics on telomere length in mice

We measured telomere lengths of blood leukocytes in several inbred and outbred mammalian species, using a telomere-specific fluorescent probe and flow cytometry. Humans, non-human primates, and three outbred populations of Peromyscus mice (Peromyscus leucopus, Peromyscus maniculatus, and Peromyscus polionotus) have short telomeres. Two common strains of laboratory mice, C57BL/6J and DBA/2J, have telomeres several times longer than most other mammals surveyed. Moreover, the two inbred laboratory mouse strains display significantly different telomere lengths, suggesting the existence of strain-specific genetic determinants. To further examine the effects of inbreeding, we studied three Peromyscus leucopus inbred lines (GS109, GS16A1, and GS16B), all derived from the outbred P. leucopus stock. Telomeres of all three inbred lines are significantly lengthened relative to outbred P. leucopus, and the three lines display strain-specific significantly different telomere lengths, much like the C57BL/6J and DBA/2J strains of M. musculus. To further characterize the genetic inheritance of telomere length, we carried out several crosses to obtain hybrid F₁ mice between parental strains displaying the phenotype of long and short telomeres. In all F₁ mice assayed, peripheral blood leukocyte telomere length was intermediate to that of the parents. Additionally, we generated F₂ mice from a cross of the (P. leucopus outbred × GS16B)F₁. Based on the distribution of telomere length in the F₂ population, we determined that more than five loci contribute to telomere length regulation in Peromyscus. We concluded that inbreeding, through unknown mechanisms, results in the elongation of telomeres, and that telomere length for a given species and/or sub-strain is genetically determined by multiple segregating loci.     

Immune response to the H-X antigen on P815-X2

In a preceding report, the detection of an H-2-linked immune response to the H-X ^d antigen on the P815-X2 mastocytoma was demonstrated by the significantly increased survival of (C57BL/6 × DBA/2)F₁ (B6D2F1) male hybrids (H-X ^b ) compared with female siblings (H-X ^{b/H-X d} ) after injection with the histocompatible tumor (H-X ^d ). This interpretation was supported by the absence of this sex effect in reciprocal D2B6F1 hybrids (H-X ^d and H-X ^{d/H-X b} ). Additional findings presented in this paper support the conclusion that this sex effect is due to a true immunological response to H-X ^d : (a) Reciprocal (DBA/2 × C57BL/6 H-2 mutant)F₁ hybrids, as well as D2B6F1, failed to exhibit the sex effect: (b) the demonstration of the sex effect in (BALB/c × DBA/2)F₁ and (BALB/c-H-2 ^dm2 × DBA/2)F₁ hybrids and in (C57BL/10 × DBA/2)F₁ hybrids was consistent with the known H-X incompatibilities between the strains BALB/c and DBA/2 and C57BL/10 and DBA/2, respectively, previously demonstrated by skin grafting; and (c) the sex effect was not abrogated by castration of male B6D2F1 hybrids. Variability in the presence or absence of the sex effect was observed in various [recombinant inbred (RI) × DBA/2]F₁ hybrids and may be attributed to the influence of a regulatory non-H-2 gene which is closely linked to the gene coding for mouse kidney-androgen-regulated protein (KAP) but androgen-independent, or to variability in inheritance of the H-X ^b allele among the RI lines. It is proposed that the P815-X2 model may be utilized to type RI lines derived from a cross between C57BL/6 and DBA/2 for their H-X genotypes.Abbreviations B C57BL/6 origin allele - B6 C57BL/6 - B10 C57BL/10 - B6D2F1 (C57BL/6 × DBA/2)F₁ - B6 ^m D2F1 (C57BL/6 H-2 mutant × DBA/2)F₁ - bm10 B6.C-H-2 ^bm10 - C BALB/c - D DBA/2 origin allele - D2 DBA/2 - dm2 BALB/c-H-2 ^dm2 - H-X X chromosome-determined histocompatibility antigen of the mouse - Ir gene, immune response gene - KAP kidney androgenregulated protein - MST median survival time - RI recombinant inbred - SDP strain distribution pattern     

Confirmation and dissection of QTL controlling resistanceto malaria in mice

We developed an F₁₁ AIL population from an F₁ cross of A/J (susceptible) and C57BL/6J (resistant) mouse strains. One thousand F₁₁ mice were challenged with P.c. chabaudi 54X, and 340 mice selected from the phenotypic extremes for susceptibility and resistance were genotyped for microsatellite markers on Chromosomes (Chrs) 5, 8, and 17. QTL originally detected in backcross and F₂ populations were confirmed on the three chromosomes within narrower genomic regions, by maximum likelihood and regression analyses. Each of the previously mapped QTL on Chrs 5 and 17 resolved into two linked QTLs. The distal and proximal QTLs on Chrs 5 and 17, respectively, map to the previously reported QTL.     

A large QTL for fear and anxiety mapped using an F2 cross can be dissected into multiple smaller QTLs

Using chromosome substitution strains (CSS), we previously identified a large quantitative trait locus (QTL) for conditioned fear (CF) on mouse chromosome 10. Here, we used an F₂ cross between CSS‐10 and C57BL/6J (B6) to localize that QTL to distal chromosome 10. That QTL accounted for all the difference between CSS‐10 and B6. We then produced congenic strains to fine‐map that interval. We identified two congenic strains that captured some or all the QTL. The larger congenic strain (Line 1: 122.387121–129.068 Mb; build 37) appeared to account for all the difference between CSS‐10 and B6. The smaller congenic strain (Line 2: 127.277–129.068 Mb) was intermediate between CSS‐10 and B6. We used haplotype mapping followed by quantitative polymerase chain reaction to identify one gene that was differentially expressed in both lines relative to B6 (Rnf41) and one that was differentially expressed between only Line 1 and B6 (Shmt2). These cis‐eQTLs may cause the behavioral QTLs; however, further studies are required to validate these candidate genes. More generally, our observation that a large QTL mapped using CSS and F₂ crosses can be dissected into multiple smaller QTLs shows a weaknesses of two‐stage approaches that seek to use coarse mapping to identify large regions followed by fine‐mapping. Indeed, additional dissection of these congenic strains might result in further subdivision of these QTL regions. Despite these limitations, we have successfully fine‐mapped two QTLs to small regions and identified putative candidate genes, showing that the congenic approach can be effective for fine‐mapping QTLs .