Strain distribution patterns for genetic markers in the LSXSS recombinant-inbred series

We present the strain distribution patterns (SDPs) of 118 SSLP markers and three pigmentation genes that have been characterized in 27 strains from the LSXSS RI series. This coarse map provides a resource for linkage studies of phenotypes that are heritable in the LSXSS RI series. The LSXSS recombinant inbred (RI) strains were derived from the Long-Sleep (LS) and Short-Sleep (SS) selected lines of mice that were selected for differential sensitivity to ethanol but are also differentially sensitive to a variety of other alcohols, barbiturates, sedative hypnotics, and general anesthetics. Since the parents were not inbred, two atypical factors are present in these SDPs. First, more than two alleles are frequently found in these RIs, and second, some alleles can be uniquely associated with one or the other parent while other alleles may be found in both parental lines. To validate the markers found in the parental line, we genotyped all parental mice from one generation of both the LS and SS lines, thus leading to a set of marker SDPs that are useful for further phenotypic association and identification of provisional QTLs. Received: 15 November 1995 / Accepted: 6 February 1996     

Mapping quantitative trait loci mediating sensitivity to etomidate

Long- and Short-Sleep (LS and SS) mice were selectively bred for differences in ethanol-induced loss of the righting reflex (LORR) and have been found to differ in LORR induced by various anesthetic agents. We used a two-stage mapping strategy to identify quantitative trait loci (QTLs) affecting duration of LORR caused by the general anesthetic etomidate and brain levels of etomidate (BEL) following regain of the righting reflex. Analysis of recombinant-inbred strains derived from a cross between LS and SS mice (LSXSS) yielded a heritability estimate of 0.23 for etomidate-induced LORR and identified one marker that showed suggestive linkage for a QTL, on mouse Chromosome (chr) 12. Mapping in an F(2) population derived from a cross between inbred LS and SS (ILS and ISS) revealed a significant QTL for etomidate-induced LORR on Chr 12, and two significant QTLs mediating BEL on Chrs 6 and 12. Several QTLs showing suggestive linkage for etomidate-induced LORR and BEL were also identified in the F(2) population. Brain levels of etomidate in the RI and F(2) mice suggested that differences in LORR were due to differential central nervous system sensitivity, rather than differential etomidate metabolism. Interestingly, the region on Chr 7 has also been identified as a region influencing ethanol-induced LORR, suggesting the possibility of a common genetic mechanism mediating etomidate and ethanol sensitivity. These QTL regions need to be further narrowed before the testing of candidate genes is feasible.     

Parental genetic contributions in the AXB and BXA recombinant inbred mouse strains

Recombinant inbred (RI) strains are a valuable tool in mouse genetics to rapidly map the location of a new locus. Because RI strains have been typed for hundreds of genetic markers, the genotypes of individual strains within an RI set can be examined to identify specific strain(s) containing the desired region(s) of interest (e.g., one or more quantitative trait loci, QTLs) for subsequent phenotype testing. Specific RI strains might also be identified for use as progenitors in the construction of consomic (chromosome substitution strains or CSSs) or congenic lines or for use in the RI strain test (RIST). To quickly identify the genetic contributions of the parental A/J (A) and C57BL/6J (B) strains, we have generated chromosome maps for each commercially available AXB and BXA RI strain, in which the genetic loci are colorcoded to signify the parent of origin. To further assist in strain selection for further breeding schemes, the percentages of A and B parental contributions were calculated, based on the total number of typed markers in the database for each strain. With these data, one can rapidly select the RI strain(s) carrying the desired donor and recipient strain region(s). Because points of recombination are known, starting with RI mice to generate CSSs or congenic lines immediately reduces genomewide screening to those donor-strain regions not already homozygous in the recipient strain. Two examples are presented to demonstrate potential uses of the generated chromosome maps: to select RI strains to construct congenic lines and to perform an RIST forAliq1, a QTL linked to ozone-induced acute lung injury survival.     

Evaluation of LEXF/FXLE rat recombinant inbred strains for genetic dissection of complex traits.

Recombinant inbred (RI) strains are formed from an outcross between two well-characterized inbred stains followed by at least 20 generations of inbreeding. RI strains can be utilized for the analysis of many complex phenotypic traits. The LEXF/FXLE RI strain set consists of 34 RI strains derived by reciprocal crossing of LE/Stm and F344/Stm. Here we report on genetic dissections of complex traits using this RI set and their parental strains. We have developed strain distribution patterns for 232 informative simple sequence length polymorphism markers. The framework map covers the rat genome except for chromosome Y. Seventy-six phenotype parameters, which included physiological and behavioral traits, were examined for these RI lines. Quantitative trait locus (QTL) analysis of these parameters revealed 27 significant and 91 suggestive QTLs, illustrating the potential of this RI resource for the detection of underlying gene functions for various phenotypes. Although this RI set was originally developed to study susceptibility to chemical-induced tumors, it has been shown to be equally powerful for a wide spectrum of traits. The LEXF/FXLE RI strains have been deposited at the National Bio Resource Project for the Rat in Japan and are maintained under specific pathogen-free conditions. They are available at http://www.anim.med.kyoto-u.ac.jp/nbr.     

The genetic contribution to heart rate and heart rate variability in quiescent mice

Recent studies have suggested a genetic component to heart rate (HR) and HR variability (HRV). However, a systematic examination of the genetic contribution to the variation in HR and HRV has not been performed. This study investigated the genetic contribution to HR and HRV using a wide range of inbred and recombinant inbred (RI) mouse strains. Electrocardiogram data were recorded from 30 strains of inbred mice and 29 RI strains. Significant differences in mean HR and total power (TP) HRV were identified between inbred strains and RI strains. Multiple significant differences within the strain sets in mean low-frequency (LF) and high-frequency (HF) power were also found. No statistically significant concordance was found between strain distribution patterns for HR and HRV phenotypes. Genomewide interval mapping identified a significant quantitative trait locus (QTL) for HR [LOD (likelihood of the odds) score = 3.763] on chromosome 6 [peak at 53.69 megabases (Mb); designated HR 1 (Hr1)]. Suggestive QTLs for TP were found on chromosomes 2, 4, 5, 6, and 14. A suggestive QTL for LF was found on chromosome 16; for HF, we found one significant QTL on chromosome 5 (LOD score = 3.107) [peak at 53.56 Mb; designated HRV-high-frequency 1 (Hrvhf1)] and three suggestive QTLs on chromosomes 2, 11 and 15. In conclusion, the results demonstrate a strong genetic component in the regulation of resting HR and HRV evidenced by the significant differences between strains. A lack of correlation between HR and HRV phenotypes in some inbred strains suggests that different sets of genes control the phenotypes. Furthermore, QTLs were found that will provide important insight to the genetic regulation of HR and HRV at rest.     

The genetic structure of recombinant inbred mice: High-resolution consensus maps for complex trait analysis

Background

Recombinant inbred (RI) strains of mice are an important resource used to map and analyze complex traits. They have proved particularly effective in multidisciplinary genetic studies. Widespread use of RI strains has been hampered by their modest numbers and by the difficulty of combining results derived from different RI sets.

Results

We have increased the density of typed microsatellite markers 2- to 5-fold in each of several major RI sets that share C57BL/6 as a parental strain (AXB, BXA, BXD, BXH, and CXB). A common set of 490 markers was genotyped in just over 100 RI strains. Genotypes of another ~1100 microsatellites were generated, collected, and error checked in one or more RI sets. Consensus RI maps that integrate genotypes of ~1600 microsatellite loci were assembled. The genomes of individual strains typically incorporate 45-55 recombination breakpoints. The collected RI set - termed the BXN set - contains approximately 5000 breakpoints. The distribution of recombinations approximates a Poisson distribution and distances between breakpoints average about 0.5 cM. Locations of most breakpoints have been defined with a precision of < 2 cM. Genotypes deviate from Hardy-Weinberg equilibrium in only a small number of intervals.

Conclusions

Consensus maps derived from RI strains conform almost precisely with theoretical expectation and are close to the length predicted by the Haldane-Waddington equation (X3.6 for a 2-3 cM interval between markers). Non-syntenic associations among different chromosomes introduce predictable distortions in QTL data sets that can be partly corrected using two-locus correlation matrices.     

QTL analysis of Al tolerance in recombinant inbred lines of Arabidopsis thaliana

Quantitative trait loci (QTLs) and epistasis for Arabidopsis thaliana aluminum (Al) tolerance were analyzed using a recombinant inbred (RI) population of 100 lines derived from a cross between Landsberg erecta and Columbia (Col). Root growth of the RI population was determined in hydroponics using solutions containing 0 or 4 micro M of AlCl(3 )and a series of nutrients, except P(i), at pH 5.0. Al tolerance was defined as relative root length [RRL: plus Al/minus Al (%)], and the RI lines ranged from 22.6 to 97.4% with a broad sense heritability of 0.99. Using the composite interval mapping method, two significant single factor QTLs (P<0.05) were detected by RRL on chromosomes 1 and 4, where the Col allele showed positive and negative effects on the Al tolerance. These QTLs could explain about 43% of the total variation of Al tolerance among the RI population. On the other hand, five epistatic loci pairs were identified by the complete pair-wise search method (P<0.0005). No single factor QTL and epistatic loci pairs were shared by the root length in the control and the RRL, suggesting that the loci identified by the RRL would be specific for Al treatment and controlling Al tolerance among the RI population.     

Mapping of QTLs conferring resistance to bacterial leaf streak in rice

A large F₂ and a RI population were separately derived from a cross between two indica rice varieties, one of which was highly resistant to bacterial leaf streak (BLS) and the other highly susceptible. Following artificial inoculation of the RI population and over 2 years of testing, 11 QTLs were mapped by composite interval mapping (CIM) on six chromosomes. Six of the QTLs were detected in both seasons. Eight of the QTLs were significant following stepwise regression analysis, and of these, 5 with the largest effects were significant in both seasons. The detected QTLs explained 84.6% of the genetic variation in 1997. Bulked segregant analysis (BSA) of the extremes of the F₂ population identified 3 QTLs of large effect. The 3 QTLs were dentical to 3 of the 5 largest QTLs detected by CIM. The independent detection of the same QTLs using two methods of analysis in separate mapping populations verifies the existence of the QTLs for BLS and provides markers to ease their introduction into elite varieties. Received: 13 October 1999 / Accepted: 29 October 1999     

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     

RFLP Mapping of Genes Conferring Complete and Partial Resistance to Blast in a Durably Resistant Rice Cultivar

Moroberekan, a japonica rice cultivar with durable resistance to blast disease in Asia, was crossed to the highly susceptible indica cultivar, CO39, and 281 F(7) recombinant inbred (RI) lines were produced by single seed descent. The population was evaluated for blast resistance in the greenhouse and the field, and was analyzed with 127 restriction fragment length polymorphism (RFLP) markers. Two dominant loci associated with qualitative resistance to five isolates of the fungus were tentatively named Pi-5(t) and Pi-7(t). They were mapped on chromosomes 4 and 11, respectively. To identify quantitative trait loci (QTLs) affecting partial resistance, RI lines were inoculated with isolate PO6-6 of Pyricularia oryzae in polycyclic tests. Ten chromosomal segments were found to be associated with effects on lesion number (P < 0.0001 and LOD > 6.0). Three of the markers associated with QTLs for partial resistance had been reported to be linked to complete blast resistance in previous studies. QTLs identified in greenhouse tests were good predictors of blast resistance at two field sites. This study illustrates the usefulness of RI lines for mapping a complex trait such as blast resistance and suggests that durable resistance in the traditional variety, Moroberekan, involves a complex of genes associated with both partial and complete resistance.     

Fine Mapping QTL for Drought Resistance Traits in Rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) Using Bulk Segregant Analysis

Drought stress is a major limitation to rice (Oryza sativa L.) yields and its stability, especially in rainfed conditions. Developing rice cultivars with inherent capacity to withstand drought stress would improve rainfed rice production. Mapping quantitative trait loci (QTLs) linked to drought resistance traits will help to develop rice cultivars suitable for water-limited environments through molecular marker-assisted selection (MAS) strategy. However, QTL mapping is usually carried out by genotyping large number of progenies, which is labour-intensive, time-consuming and cost-ineffective. Bulk segregant analysis (BSA) serves as an affordable strategy for mapping large effect QTLs by genotyping only the extreme phenotypes instead of the entire mapping population. We have previously mapped a QTL linked to leaf rolling and leaf drying in recombinant inbred (RI) lines derived from two locally adapted indica rice ecotypes viz., IR20/Nootripathu using BSA. Fine mapping the QTL will facilitate its application in MAS. BSA was done by bulking DNA of 10 drought-resistant and 12 drought-sensitive RI lines. Out of 343 rice microsatellites markers genotyped, RM8085 co-segregated among the RI lines constituting the respective bulks. RM8085 was mapped in the middle of the QTL region on chromosome 1 previously identified in these RI lines thus reducing the QTL interval from 7.9 to 3.8 cM. Further, the study showed that the region, RM212–RM302–RM8085–RM3825 on chromosome 1, harbours large effect QTLs for drought-resistance traits across several genetic backgrounds in rice. Thus, the QTL may be useful for drought resistance improvement in rice through MAS and map-based cloning.     

Genetic loci affecting body weight and fatness in a C57BL/6J × PWK/PhJ mouse intercross

To determine the genetic variation that contributes to body composition in the mouse, we interbred a wild-derived strain (PWK/PhJ; PWK) with a common laboratory strain (C57BL/6J; B6). The parental, F₁, and F₂ mice were phenotyped at 18 weeks old for body weight and composition using dual-energy X-ray absorptiometry (DEXA). A total of 479 (244 male and 235 female) F₂ mice were genotyped for 117 polymorphic markers spanning the autosomes. Twenty-eight suggestive or significant linkages for four traits (body weight, adjusted lean and fat weight, and percent fat) were detected. Of these, three QTLs were novel: one on the proximal portion of Chr 5 for body weight (Bwq8; LOD = 4.7), one on Chr 3 for lean weight (Bwtq13; LOD = 3.6), and one on Chr 11 for percent fat (Adip19; LOD = 5.8). The remaining QTLs overlapped previously identified linkages, e.g., Adip5 on Chr 9. One QTL was sex-specific (present in males only) and seven were sex-biased (more prominent in one sex than the other). Most alleles that increased body weight were contributed by the B6 strain, and most alleles that increased percent fat were contributed by the PWK strain. Eight pairs of interacting loci were identified, none of which exactly overlapped the main-effect QTLs. Many of the QTLs found in the B6 × PWK cross map to the location of previously reported linkages, suggesting that some QTLs are common to many strains (consensus QTLs), but three new QTLs appear to be particular to the PWK strain. The location and type of QTLs detected in this new cross will assist in future efforts to identify the genetic variation that determines the ratio of lean to fat weight as well as body size in mice.     

Insect resistance in recombinant inbred soybean lines derived from non-resistant parents

Larval performance of Helicoverpa zea (Boddie) (corn earworm) (Lepidoptera: Noctuidae) was examined on 240 recombinant inbred (RI) soybean, Glycine max (L.) Merrill, lines. These homozygous RI were derived from an intraspecific cross of genetically distant, non-resistant, parents, Minsoy from China and Noir 1 from Hungary. Based upon a genetic map of more than 500 molecular markers, each RI line presented a unique genotype composed of a mixture of different parental alleles. The RI lines exhibited transgressive segregation with respect to their defensive effects on H. zea, such that the range of RI phenotypes far exceeded that of the parents. Similar effects were observed on the soybean looper, Pseudoplusia includens (Walker) (Lepidoptera: Noctuidae). We identified several independent quantitative trait loci (QTLs) linked to molecular markers that were associated with H. zea larval development parameters. Two QTLs affected several different traits including larval weight and developmental rate; other QTLs affected only a single trait each, i.e., larval weight, pupal weight, developmental rate, nutritional efficiency or survival. The results demonstrate that the increased range of defensive effects among the segregant RI lines is due to recombination among several parental genes that together quantitatively control plant defensive traits.Several alternative responses by herbivores have been proposed relative to plant hybrid swarms, hybrid avoidance due to higher hybrid resistance than either parent, hybrid preference due to lower resistance than either parent, hybrid equivalency to one or the other parent, or hybrid intermediacy. Within this RI population, we observed all of the proposed responses by H. zea, as might be expected when defensive traits are controlled by several genes.     

Distribution of body weight, blood insulin and lipid levels in the SMXA recombinant inbred strains and the QTL analysis.

In the SMXA recombinant inbred (RI) strains, we measured body weight, blood insulin and lipid (triglyceride, total cholesterol and phospholipid) levels in each strain. In the five traits, mean values of substrains varied remarkably and showed a continuous spectrum of distribution, suggesting control by multiple genes at distinct loci for each trait. We also screened for quantitative trait loci (QTLs) involved in the five traits. Suggestive QTLs for body weight (Chromosomes 1 and 6), insulin (Chromosomes 1, 3, 10 and 17), triglyceride (Chromosomes 4 and 11) and phospholipid (Chromosome 18) levels were detected. The SMXA RI strains are unique tools for analyzing genetic factors that influence body weight, blood insulin and lipids levels.     

Analysis of the Mesotelencephalic Dopamine System by Quantitative-Trait Locus Introgression

One of the significant factors that affect brain dopamine function is the activity of tyrosine hydroxylase (TH), the first and rate-limiting enzyme in catecholamine biosynthesis. For the analysis of the genetically determined role of dopamine function and TH in behavior and in the regulatory mechanisms of the mesotelencephalic dopamine system we devised a novel genetic strategy (Vadasz; Mouse Genome 88:16–18; 1990). We hypothesized that phenotypic introgression and recombinant fixation could ensure the transfer of Quantitative Trait Loci (QTL) from one strain onto the genetic background of another strain, and new, genetically very similar quasi-congenic strains could be created that would carry individual QTLs, or QTLs in various combinations. Here we summarize the construction of the first set of QTL Introgression strains, and present evidence that QTLs that are responsible for the continuous variation of mesencephalic tyrosine hydroxylase activity (TH/MES), have been transferred onto the C57BL/6By (B6) strain background from BALB/cJ (C) and CXBI (I) donor strains with high and low TH/MES, respectively. The QTL transfer was carried out in two directions by repeated backcross-intercross cycles with concomitant selection for the extreme high and low expressions of TH/MES in replicates, resulting in four QTL Introgression lines. Analysis of regional brain TH activities in the course of the QTL introgression indicated that (a) TH activity in B6.I lines showed quite limited heritability, (b) TH/MES was not highly correlated with striatal TH, and (c) the control of hypothalamic and olfactory tubercle TH activities was largely independent from that of TH/MES. Examination of the open-field (OF) behavior data demonstrated that TH activity did not correlate significantly with OF behavior. After 5 backcross-intercross cycles, TH/MES in each replicate line was still significantly different from that of the B6 background strain. A genomewide scanning of microsatellite markers in the QTL introgression lines demonstrated that about 96% of the markers were of background (B6) type. These results indicate the successful transfer of TH/MES QTLs. After the QTL transfer phase of the experiment altogether more than 100 new RQI strains were initiated in the QTL Introgression lines by strict brother × sister mating. After fixing the introgressed QTLs, ten of the inbred RQI strains were tested for TH/MES. The results showed that in one of the new RQI strains TH/MES was restored to a level that is characteristic to the C donor strain, while TH/MES values in some other strains were between those of the background and donor strains, confirming our hypothesis that phenotypic introgression and recombinant fixation can ensure a virtually complete transfer of QTLs. We conclude from this study that complex, continuously distributed neural traits can successfully be subjected to QTL introgression, and the results raise the possibility that the RQI method can be efficiently applied for gene mapping of complex neural and behavioral traits even if their phenotypic expression is sensitive to confounding developmental and environmental variations, genetic interactions, and genotype-environment interactions.     

Differential effects of pentobarbital and ethanol in mice

The duration of the loss of righting reflex (RR) after ethanol, 4 g/kg, intraperitoneally (i.p.), was significantly longer in “long-sleep” (LS) than in “short-sleep” (SS) mice. This effect was shown to be correlated with differences in brain sensitivities to ethanol. In contrast, pentobarbital sodium (PB), 50 mg/kg, i.p., produced a significantly longer loss of RR in SS than in LS mice. The PB concentrations in the brain were the same in both mouse strains at the time of RR recovery suggesting equal sensitivities of the central nervous systems to PB. The rates of disappearance of PB from the blood were the same in both strains, but the apparent volume of distribution of PB in the LS strain was greater than in SS mice.In addition, C57BL/6J mice were found to be more sensitive than DBA/2J mice to PB, 50 mg/kg. In contrast, C57BL mice are known to be less sensitive than the DBA strain to ethanol. The PB concentration in the brain of DBA mice at the recovery of the RR was significantly greater than in C57BL mice. The apparent volumes of distribution of PB were not different in the two strains, but the rate of disappearance of PB from the blood of C57BL mice was significantly greater than for the DBA strain. In conclusion, factors which govern the brain sensitivities of selected mouse strains to ethanol and pentobarbital may not be equivalent.     

Biochemical genetic variants in mice selectively bred for sensitivity or resistance to ethanol-induced sedation

The distribution of biochemical genetic variants was examined among eight inbred strains of mice, which served as contributors to a heterogeneous stock of mice (HS), and in short-sleep (SS) and long-sleep (LS) mice, selectively bred from the HS stock for differential ethanol sensitivity. Fifteen loci for enzymes of alcohol and aldehyde metabolism, as well as 12 other biochemical loci, were investigated. Thirteen of these loci exhibited allelic variation between strains, of which six were separately fixed in the SS and LS mice. Comparisons of genetic similarity coefficients, based upon the distributions of allelic variants for the loci examined, with behavioural sensitivities (sleep-time) to an acute dose of ethanol for the inbred and selected strains of mice, indicated no correlations between these data. This suggests that this collective group of loci are not useful indicators of the genes selectively bred in the SS and LS strains, which are responsible for the differential sensitivities to acute doses of ethanol.     

The AXB and BXA set of recombinant inbred mouse strains

The recombinant inbred (RI) set of strains, AXB and BXA, derived from C57BL/6J and A/J, originally constructed and maintained at the University of California/San Diego, have been imported into The Jackson Laboratory and are now in the 29th to 59th generation of brother-sister matings. Genetic quality control testing with 45 proviral and 11 biochemical markers previously typed in this RI set indicated that five strains had been genetically contaminated sometime in the past, so these strains have been discarded. The correct and complete strain distribution patterns for 56 genetic markers are reported for the remaining RI strain set, which consists of 31 living strains and 8 extinct strains for which DNA is available. Two additional strains, AXB 12 and BXA 17, are living and may be added to the set pending further tests of genetic purity. The progenitors of this RI set differ in susceptibility to 27 infectious diseases as well as atherosclerosis, obesity, diabetes, cancer, cleft palate, and hydrocephalus. Thus, the AXB and BXA set of RI strains will be useful in the genetic analysis of several complex diseases.     

Genetic Analysis of Audiogenic Seizure Susceptibility in C57bl/6j x Dba/2j Recombinant Inbred Strains of Mice

The inheritance of susceptibility to audiogenic seizures (ASs) was studied in the C57BL/6J (B6) and DBA/2J (D2) progenitor strains, their reciprocal F₁ hybrids, backcross generations and in 21 B6 x D2 recombinant inbred (RI) strains of mice at 21 days of age. All of the D2 mice tested experienced ASs, whereas none of the B6 mice responded to the sound. Although 23% of the F₁ mice experienced wild running, they were generally as resistant to ASs as their B6 parents. Mice of the F₁ x B6 backcross generation were also resistant to ASs. In the F₁ x D2 backcross generation, however, a significant preponderance (72%) of AS-susceptible mice was found. No significant association was observed between any of the four coat-color phenotypes that were segregating in this generation and susceptibility to ASs. A continuous distribution of mean seizure severity scores and several new audiogenic response phenotypes, distinctly different from the phenotypes of either progenitor strain, were found among the 21 RI strains. These and the results from the F₁ x D2 backcross generation suggest that the difference in AS susceptibility between 21-day-old B6 and D2 mice cannot be under the control of a single locus. In addition, no association was found between AS susceptibility and the chromosome 4 markers Lyb-2, Mup-1 and b among the 21 RI strains. An association was observed, however, between AS susceptibility and the Ah locus. Several of the RI strains that were AS resistant at 21 days of age became AS susceptible as adults. One RI strain was susceptible to ASs at both young and adult ages. The B6, D2 and F₁ mice were completely resistant to ASs at adult ages. Genetic differences were found among the RI strains for the incidence, onset, duration, and type of severity of ASs. A remarkable amount of phenotypic variability in the audiogenic response, which can be attributed only to the influence of environmental factors, occurred within several of the RI strains. A multiple-factor mode of inheritance involving a physiological threshold can account for our observations.