Rflps for Somatotropic Genes Identify Quantitative Trait Loci for Growth in Mice

Restriction fragment length polymorphisms for somatotropic genes were tested for associations with body weight and postweaning growth rate in mice. Polymorphisms for growth hormone (GH) and insulin-like growth factor 2 (IGF-2) genes were identified in stock population lines which had been subjected to long-term selection for high 42-day body weight (H lines) or randomly mated (FP and C lines). Two F2 populations of mice (5F2 and MF2) were generated from crosses between a single H line of mice and two unselected control lines and subsequently, two divergently weight selected sublines were generated from each F2 population. The GHh allele which had originally been fixed in three of four H lines and absent from all FP and C lines was found to have a significant (P less than 0.01) effect on 42-day weight and postweaning growth rate in the F2 populations. However, GHh was associated with lower 42-day weight in the F2 populations, suggesting that the positive association between GHh and weight in the stock population was unique to the high weight selected genetic background of those lines. In agreement with this, the frequency of GHh increased in sublines selected for high 42-day weight and decreased in sublines selected for low 42-day weight. The IGF-2H5 allele was associated with higher weights in a sex-dependent manner in 5F2. In the high selected subline derived from 5F2, a significant increase in the frequency of IGF-2H5 was observed. Therefore this allele, in contrast to GHh, appears to be a positive indicator of growth irrespective of genetic background.     

Restriction fragment length polymorphism among Israeli Holstein-Friesian dairy bulls

Israeli Holstein-Friesian dairy bulls were screened for restriction fragment length polymorphisms by hybridizing cloned DNA probes for bovine growth hormone, for chymosin, and for rat muscle beta-actin to restriction endonuclease-digested DNA immobilized on nitrocellulose filters. The population proved to be polymorphic at the growth hormone locus, with evidence consistent with the phenotypes being inherited in allelic fashion. A low level of polymorphism was also observed at one of the beta-actin gene family loci. The chymosin locus was monomorphic with the restriction enzymes utilized. The results illustrate the power of restriction fragment length polymorphism methodology in visualizing genetic variability in dairy cattle populations.     

Construction of a high-density linkage map of Italian ryegrass (Lolium multiflorum Lam) using restriction fragment length polymorphism, amplified fragment length polymorphism, and telomeric repeat associated sequence markers.

To construct a high-density molecular linkage map of Italian ryegrass (Lolium multiflorum Lam), we used a two-way pseudo-testcross F1 population consisting of 82 individuals to analyze three types of markers: restriction fragment length polymorphism markers, which we detected by using genomic probes from Italian ryegrass as well as heterologous anchor probes from other species belonging to the Poaceae family, amplified fragment length polymorphism markers, which we detected by using PstI/MseI primer combinations, and telomeric repeat associated sequence markers. Of the restriction fragment length polymorphism probes that we generated from a PstI genomic library, 74% (239 of 323) of randomly selected probes detected hybridization patterns consistent with single-copy or low-copy genetic locus status in the screening. The 385 (mostly restriction fragment length polymorphism) markers that we selected from the 1226 original markers were grouped into seven linkage groups. The maps cover 1244.4 cM, with an average of 3.7 cM between markers. This information will prove useful for gene targeting, quantitative trait loci mapping, and marker-assisted selection in Italian ryegrass.     

The effects of selection for gain in mice on the direct-maternal genetic correlation

Components of genetic variation for postweaning growth traits were estimated for both control and growth stocks of mice. The effect of phenotypic selection for gain, which genetically combines selection for additive direct and maternal effects, on additive genetic variance components, heritability, and additive genetic correlationsis discussed. Quantitative genetic theory predicts that simultaneous selection for two metric traits in the same direction will cause the genetic correlation between the two traits to become more negative. The results presented in this paper conflict with this theory. The direct-maternal additive genetic correlation was more negative in the control line (with 356 mice) than in the growth-selected line (with 320 mice) for the three traits analyzed (0.310 vs 0.999 for 21-day weight, 0.316 vs 1.000 for 42-day weight, and 0.506 vs 1.000 for gain from 21–42 days). Estimates were obtained by restricted maximum likelihood (REML) computed under a derivative free algorithm (DFREML).     

Growth hormone gene polymorphism associated with selection for milk fat production in lines of cattle

Summary
Fifty-eight calves of both sexes from lines of Red Danish dairy breed selected for high ( n = 36) and low ( n = 22) milk fat production, and 32 heifers from lines of Norwegian Red dairy breed selected for high ( n = 16) and low ( n = 16) milk fat yield were typed for two previously reported restriction fragment length polymorphisms (RFLPs) in the growth hormone gene. The RFLPs are consistent with: (1) an insertion(I)/deletion(D) of approximately 0.9kb in the 3'-region of the growth hormone gene and (2) a polymorphic Msp I(+/−) site in the third intron. A traditional RFLP procedure was used for typing the I/D polymorphism and a polymerase chain reaction (PCR) procedure was developed for typing the Mspl polymorphism. Only the I-MspI(+) and D- Msp I(-) haplotypes were found. In the Red Danish lines the frequency of D- Msp I(-) haplo-type was 0.28 in high line and 0.05 in low line calves, this difference was significant ( P <0.01). The corresponding frequencies in the Norwegian Red lines were 0.09 in the high line and 0.0 in the low line. Attempts to screen for RFLPs in the growth hormone receptor gene and in the insulin-like growth factor-I gene were unsuccessful.     

Intrinsic properties of muscle satellite cells are changed in response to long-term selection of mice for different growth traits

Satellite cell cultures were derived from mice selected long-term over 70 generations for body weight (DU-6, growth), carcass protein amount (DU-6P, protein) and an index combining body weight and endurance treadmill performance (DU-6+LB, growth + fitness) at 42 days of age and from an unselected control line (DU-Ks). They were grown under identical environmental conditions to examine intrinsic cellular differences in proliferation, protein metabolism and responsiveness to growth factors. Growth kinetics (DNA and protein amounts) were determined over a 12-day period. During exponential growth, all growth-selected cultures grew faster than the control culture: (DU-6+LB=DU-6P)>DU-6>DU-Ks. The differences in DNA and protein levels were maintained until day 8. DU-Ks cultures reached similar levels as the growth (DU-6) and protein (DU-6P) cultures in terms of DNA at day 12 of cultivation. Thus, the cultures from the growth and protein lines, but not from the growth + fitness line, exhibited larger protein:DNA ratios (cell size) than the control cultures. Cell cultures from the selected lines were more responsive to serum and epidermal growth factor in terms of [(3)H] thymidine incorporation into DNA, whereas no stimulation by insulin or insulin-like growth factor-I was detectable in cultures from selected lines or controls. During differentiation, protein metabolism in cultures from selected lines was characterised by higher rates of protein synthesis (PS) and degradation (PD), as measured by [(3)H] phenylalanine incorporation or release, respectively, than in control cells. The ratios of the relative differences from the control in PS and PD were only >1.0 in the growth and protein lines. In conclusion, long-term selection for growth therefore modifies the intrinsic capability of satellite cells for proliferation and protein metabolism, with changes being dependent on the selection trait.     

Growth hormone and insulin-like growth factor-I measurements in high growth (hg) mice

Effects of a recessive gene causing high growth (hg) were studied on two major components of the growth axis in mice. Plasma and pituitary levels of growth hormone and plasma levels of insulin-like growth factor I (IGF-I) were measured in three lines homozygous for hg, each compared with a control line of alike genetic background but wild type for the hg locus (Hg). Line Gh (hghg) and line GH (HgHg) are from a line which had undergone long-term selection for high postweaning weight gain; line Ch (hghg) and line CH (HgHg) were extracted from the second backcross of Gh to C57BL/6J; line L54 (hghg) was from the sixth backcross to C57BL/6J (B6) (HgHg). Pituitary GH levels and plasma IGF-I levels were measured in both sexes at 3, 4.5, 6 and 9 wk of age. Plasma growth hormone was measured in 8- to 12-wk-old males at hourly intervals from 08.00 to 17.00. Body weight in lines homozygous for hg at 6 and 9 wk of age was 10-30% greater than in control lines. The ontogeny of this increased growth depended on genetic background. Pituitary growth hormone content was 52% lower in the two hghg lines measured (lines Ch and Gh) than in control lines at 4.5, 6 and 9 wk. Plasma growth hormone levels were also much lower in hg mice, with values only 20-30% of those in their respective controls. hg lines showed consistently low plasma growth hormone levels throughout the 9 hr sampling period, while control lines expressed the characteristic pulsatile hormone secretion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Growth and body composition of mice selected for high body weight

To elucidate the influence of high body weight selection on body compositional relationships, the accumulation of lipid, protein, and ash was investigated in two lines of mice selected for high 42-day body weight (H lines) and an unselected foundation population (FP). The two H lines differed in population size and were designated as the high-large (HL) and high-small (HS) lines. Logistic body growth curves revealed that HL mice exhibited an accelerated growth rate and reached a higher mature body weight than FP or HS mice. Over the range of body weights examined, HL mice had more lipid, less protein, and less ash than FP or HS mice of the same sex and body weight. However, HL lipid accumulation (relative to body weight increase) was not accelerated in comparison to that of FP mice. This study suggests that the existing model of selection-mediated compositional changes requires expansion to account for the ability of high-growth selection to direct an acceleration of body growth without a correlated enhancement of the relative rate of fat accumulation.     

Milk yield in lines of mice selected for growth or maternal ability.

Two experiments were conducted to measure milk yield as a correlated response to selection for increased 12-day litter weight (maternal lines) for 13 generations and increased and decreased body weight gain from 21 to 42 days of age (postweaning gain lines) for 12 to 14 generations. Milk yield was measured from day 6 to day 21 of lactation by separating litters from their dams for 6 h, then weighing litters before and after a 1.5 h suckling period. Average total milk production was 30.3 +/- 0.9 g and 28.6 +/- 1.1 g in maternal and control lines, respectively. This difference was not significant (P greater than 0.05). Differences among gain lines were significant (P less than 0.01) in the second experiment with average production of 44.1 +/- 1.4 g for increased gain lines, 22.5 +/- 0.9 g for the decreased gain line and 31.2 +/- 1.0 g for the control. These results indicated that response to selection for increased 12-day litter weight was not associated with increases in milk yield, but sizable changes inmilk yield accompanied bi-directional selection for early postweaning gain.     

Growth hormone gene polymorphisms and growth performance traits in Duroc, Landrace and Tao-Yuan pigs

This study investigated the restriction fragment length polymorphism (RFLP) of the porcine growth hormone (pGH) gene in Duroc, Landrace, and Tao-Yuan pigs and its effects on growth performance and levels of plasma growth hormone in peripheral circulation. Genomic DNA extracted from 81 Tao-Yuan, 60 Landrace and 48 Duroc pigs were subjected to Southern blot hybridization with a pGH cDNA probe. Polymorphism was detected with the restriction enzymes TaqI and DraI. A comparison of these three breeds showed significant differences in allelic frequencies. Blood samples for radioimmunoassay (RIA) of GH were collected biweekly during the experimental period from pigs 12 to 40 weeks of age. Tao-Yuan pigs showed a mean plasma GH level (2.51 +/- 1.23 ng/mL) that was much lower than that of the Landrace (3.80 +/- 1.52 ng/mL) and Duroc (4.20 +/- 1.03 ng/mL) pigs (P < 0.05). Moreover, the Tao-Yuan pigs also showed poorer growth performance than the Landrace and the Duroc pigs both in the daily weight gain (0.37 +/- 0.06 vs. 0.67 +/- 0.05 and 0.70 +/- 0.05 kg/day, P < 0.01) and feed efficiency (3.12 +/- 0.28 vs. 2.60 +/- 0.14 and 2.52 +/- 0.12, P < 0.05). These results suggest that the growth performance trait in these pigs is highly correlated with their growth hormone genotype.     

Direct and correlated responses to selection for increased postweaning gain in mice

Summary Mass selection for increased body weight gain from 21 to 42 days of age was practiced for 12 generations in four replicate lines of ICR-albino mice. Response to selection averaged 0.56±0.03 g. per generation. This response represented an increase of 7.0 genetic standard deviation units and 3.4 phenotypic standard deviation units in 12 generations. The realized heritability pooled over the four replicates was 0.24±0.02. Sizable positive correlated responses were found for 42 and 56-day weight and gain from 42 to 56 days. Much smaller positive correlated responses were noted for 12-day litter weight and 12-day individual weight. Neither litter size nor weaning weight were significantly altered by selection for increased postweaning gain. Reproductive efficiency measured as percent fertile matings declined significantly in the selected lines.Supported in part by a grant from the Virginia Agricultural Foundation.Published with the approval of the Director of the Research Division, Virginia Polytechnic Institute and State University.     

Leptin levels in lines of mice developed by long-term divergent selection on fat content

The mouse lines were developed by long-term selection for fatness, after which the fat line (F) had about a 5-fold (23% vs 4%) higher fat percentage than the lean (L) line at 14 weeks; but the lines differed little in fat-free body weight. To assess the contribution of genetic changes in leptin hormone level to the selection response, plasma leptin levels were assayed in these lines in generation 60 and in an unselected control (C) from the same base population. With access to food prior to assay, the F, C and L lines had 16.5, 0.91 and 0.26 ng/ml leptin, respectively. In fasted animals these levels were much lower: 2.98, 0.171 and 0.0087 ng/ml, respectively. Thus the leptin levels differ greatly between the lines, with the fattest mice showing the highest level: almost 20 times higher than the control and 60-300 times higher than the L line. These correlated selection effects are an order of magnitude greater than the direct selection response, and believed to be much larger than seen for any hormonal or other trait. Correlations between leptin level and fat amount were high (over 0.86) in fed or fasted animals of the F line, indicative of leptin resistance.     

Identification of quantitative trait loci influencing traits related to energy balance in selection and inbred lines of mice.

Energy balance is a complex trait with relevance to the study of human obesity and maintenance energy requirements of livestock. The objective of this study was to identify, using unique mouse models, quantitative trait loci (QTL) influencing traits that contribute to variation in energy balance. Two F2 resource populations were created from lines of mice differing in heat loss measured by direct calorimetry as an indicator of energy expenditure. The HB F2 resource population originated from a cross between a noninbred line selected for high heat loss and an inbred line with low heat loss. Evidence for significant QTL influencing heat loss was found on chromosomes 1, 2, 3, and 7. Significant QTL influencing body weight and percentage gonadal fat, brown fat, liver, and heart were also identified. The LH F2 resource population originated from noninbred lines of mice that had undergone divergent selection for heat loss. Chromosomes 1 and 3 were evaluated. The QTL for heat loss identified on chromosome 1 in the HB population was confirmed in the LH population, although the effect was smaller. The presence of a QTL influencing 6-wk weight was also confirmed. Suggestive evidence for additional QTL influencing heat loss, percentage subcutaneous fat, and percentage heart was found for chromosome 1.     

Decreased frequency of the rat growth hormone transgene in mouse populations with or without selection for increased adult body weight

Summary Frequencies of mice with the rat growth hormone (rGH) transgene were examined in lines derived from two genetic bases (P/W and P/C). The genetic bases were developed from males (P) with the rGH transgene, mated with non-transgenic females of different origin: a line previously selected for large body size (W) and a corresponding unselected control line (C). They were maintained for six generations under random mating with or without selection for increased 42-day body weight. The frequencies of P/W and P/C males with the rGH transgene wer 0.075 and 0.300, respectively at generation 0 of the genetic bases. They were significantly (P<0.05) lower than the expected frequency (about 0.5). At generation 6, the frequencies had decreased further both in selected and unselected lines (ranging from 0.025 to 0.125). Decreased frequencies of mice with the transgene were confirmed in a separate experiment testing segregation of the transgene. The reasons for these decreases are not clear. The results suggest that transgenes need to be monitored when transgenic animals are mated with animals of different origin.Animal Research Centre Contribution No. 1697     

Characterization of a major X-linked quantitative trait locus influencing body weight of mice

Growth rate in mice is an archetypal quantitative trait that has long been studied genetically, physiologically, and metabolically, but its genetic basis is still poorly understood due to its complex inheritance and the influence of environment. We measured differences in 17 growth-related traits between a pair of partially congenic lines that differ for a segment of the X chromosome containing a quantitative trait locus (QTL) that we identified in a genomewide QTL scan. The QTL has a large effect on mean body weight of approximately 20% at all ages, and affects early growth rate to a greater extent than late growth rate. Feed is converted to body mass more efficiently in the high chromosome segment-bearing line than the low line. The weights of various internal organs are affected to a somewhat greater extent by the QTL than body weight. The proportional change in body length is smaller than body weight, but this may be an effect of scale. Body weight at late ages appears to allow the most efficient detection of allelic differences at the QTL, although assignment of genotypic state based on phenotype is never unambiguous.     

A centromeric region on chromosome 6(6H) affects dormancy in an induced mutant in barley

Genetic control of seed dormancy in barley (Hordeum vulgare L.) has mostly been described in terms of quantitative variation. Although some molecular markers for dormancy QTL have been identified, the corresponding genes involved in the regulation of the process have not been cloned. Induced barley mutants may constitute useful material to study the physiology and genetics of seed dormancy. The objective of this study was to identify the genetic control of this trait in a mutant (TL43) produced in the barley cv. Triumph. This mutant was selected for reduced dormancy and reduced sensitivity to abscisic acid (ABA). Two sets of F6 barley lines were selected for high and low levels of dormancy from a cross between the original dormant parent and the sodium azide-induced non-dormant TL43 mutant. Unexpectedly, given the near-isogenic nature of these two genotypes, polymorphism was detected for an SSR located in the centromeric region of chromosome 6(6H) out of a total of 92 molecular markers evenly distributed along the genome. Fortunately, upon three cycles of intensive divergent selection, every dormant and non-dormant F5 line consistently showed the genotype for this region identical to Triumph and TL43, respectively. Based on the mutagenic effect presumably attributed to sodium azide, mostly single point mutations, it cannot be clearly established if such extensive genomic variation on chromosome 6(6H) is due to the mutagenic treatment or may be an introgression from an unknown source. The means that could originate such heterogeneity are discussed; however, regardless of its origin, this genomic region shows a strong association with the expression of seed dormancy and provides an additional genetic locus for further studies of the mechanistic basis of this complex trait. In addition, since TL43 shows reduced sensitivity to ABA, the response to this hormone was determined on the F6 seed from the two sets of selected F5 lines. The results confirmed that the initial level of dormancy in the seed lot is the most important factor in determining ABA sensitivity.     

Growth and body composition changes in mice selected for high post-weaning weight gain on two levels of feeding

Summary Two lines of mice were selected for high post-weaning weight gain (3 to 6 weeks) adjusted for 3 week weight. One line (F) was grown on freely available food and the other (S) on a feeding scale set at the same level for all mice. Food intake of the S line averaged 80% of the F line. The realised heritabilities after 6 generations of selection were 0.38±0.06 and 0.33±0.07 for the F and S lines, respectively. In generation 7, mice from the F and S lines and from an unselected control line (C) were compared on both free and set levels of feeding from 3 weeks to 9 weeks of age. Measurements taken were growth rate, appetite, food conversion efficiency (weight gain/food intake) and body composition (fat, protein, ash, water). The F and S lines grew more rapidly and efficiently than the C line on both levels of feeding, each line performing best on the level of feeding on which it was selected. The average genetic correlation between growth rates of the same line on the two feeding levels was 0.54±0.10. The F line grew 19% faster and was 9% more efficient than the S line on free feeding but the S line grew 15% faster and was 15% more efficient than the F line on set feeding. Relative to the C line, food intake per day on free feeding was 4% higher in the F line and 6% lower in the S line. There was no difference between the lines in food intake/g body weight. The rate of deposition of all body components increased in both selection lines. In the F, S and C lines respectively, efficiencies of gains in body components (10²x gain/food) were 1.79, 1.31 and 1.06 for fat, 1.53, 1.63 and 1.22 for protein and 5.88, 6.45 and 4.98 for protein + water. Apparently energy lost as heat was reduced in both the F and S lines. The partitioning of energy retained was altered in favour of more fat in the F line and more protein in the S line.     

Diallel Analysis of Growth Traits in Mice

Two replications of a complete diallel cross experiment were performed among four partially inbred lines of mice. These inbred lines originated from a random-bred ICR strain and were produced by 12 generations of full sibbing (F congruent to 92%). Individual body weight was recorded for each animal at 12, 21, 42 and 56 days of age. Body weight gain traits were examined for intervals 12-21, 21-42 and 42-56 days. Simultaneous least squares analyses of inbred and linecrossed groups were used. Sex differences were highly significant for all traits. Replicate differences were significant but made a small contribution to the total variation. Inbred lines differed greatly. Crosses showed growth trends similar to their contemporary maternal and paternal inbreds. Heterosis was highly significant for all traits except 21-day weight. Inbreds were heavier at 12 days of age, but linecrossed progeny were superior to inbreds for all postweaning weights. General combining ability was highly significant for 12- and 56-day weights and 21-42-day gain. Specific combining ability was highly significant for 21-day weight, 12-21- and 42-56-day gain. Significant maternal effects were found for all individual weights but not for 12-21- and 21-42-day gain. Residual reciprocal effects were significant for all traits. Estimated variances among linecrossed groups contained a large maternal component, a fluctuating additive genetic component and consistent non-additive genetic influence on all growth parameters measured.     

Replicated selection for insulin-like growth factor-1 and body weight in mice

Summary Five generations of divergent selection for plasma concentration of insulin-like growth factor-1 (IGF-1) and for 12-week body weight were carried out in mice, including randomly selected control lines for each trait. All lines were replicated once (12 lines in total). Each replicate line consisted of eight male and eight female parents per generation. Litter size was standardized to eight pups at birth. Mass selection was applied in the selected lines and within-family random selection in the control lines. Blood was taken from the orbital sinus of individual mice at 12 weeks of age for IGF-1 assay. Realized heritabilities were 0.10±0.01 for IGF-1 and 0.41 ± 0.02 for 12-week weight. The realized genetic correlation between IGF-1 and 12-week weight was 0.58 ± 0.01, with a phenotypic correlation of 0.38. Although the genetic correlation between IGF-1 and body weight in mice is moderately positive, 12-week weight responded 3.5 times as fast to weight selection as to selection for IGF-1.     

Discrimination between selected lines of pigs using AFLP markers

Amplified fragment length polymorphic (AFLP) markers were used to discriminate between lines of pigs, divergently selected over seven generations for components of efficient lean growth rate. A total of 270 animals with 30 animals per line were genotyped for 239 polymorphic AFLP markers. Canonical variate analysis identified linear combinations of the AFLP marker scores that grouped animals by selection line with no overlap between selection lines. Cluster analysis of AFLP marker scores identified 10 groups of animals with 226 of the 270 animals clustered into nine groups, each consisting of animals from only one selection line. AFLP marker genotyping, using the EcoRI and TaqI restriction enzymes, provided an effective means of discriminating between animals of different selection lines that have arisen from one base population.