Genetic analysis of seed size, yield and days to flowering in a chickpea recombinant inbred line population derived from a Kabuli × Desi cross

Quantitative traits, seed size, yield and days to flowering were studied in a chickpea intraspecific recombinant inbred line (RIL) population (F_6:7) derived from a Kabuli × Desi cross. The population was evaluated in two locations over 2 years. Days to flowering was also evaluated in the greenhouse under short-day conditions. Seed size was the most heritable trait (0.90), followed by days to flowering (0.36) and yield (0.14). Negative and significant correlation was found between yield and seed size in the second year where environmental homogeneity was tested by analysing the controls included in each assay. During the first year, the environment was not considered homogeneous for yield in either location. Quantitative trait loci (QTLs) for the three characters were detected in linkage group (LG) 4. In relation to seed size, two QTLs were located in LG4 (QTL_SW1) and LG8 (QTL_SW2). QTL_SW1 accounted 20.3% of the total phenotypic variation and QTL_SW2 explained 10.1%. A QTL for yield (QTL_YD) was located in LG4 explaining around 13% of variation. QTL_YD might be pleiotropic with QTL_SW1. For days to flowering, a QTL (QTL_DF1) was located in LG4 for all environments analysed explaining around 20% of variation. QTL_DF1 was closely linked to QTL_SW1 and QTL_YD in LG4.     

Selection for higher yields and heat resistance in Pima cotton has caused genetically determined changes in stomatal conductances

Gas exchange analysis was used to characterize photosynthetic and stomatal responses to key environmental stimuli in five commercial lines of Pima cotton ( Gossypium barbadense L.) which represent a gradient of selection for higher yields and heat resistance, and a primitive, uncultivated G. barbadense. At constant light and vapor pressure deficit, stomatal conductance increased linearly with air temperature in the 23 to 36$C range in all five commercial lines, and conductance at each temperature increased as a function of selection. In contrast, photosynthetic rates had a low sensitivity to temperature in the 23 to 36$C range, particularly in the advanced lines. In a segregating F₂ population from a cross between the advanced line, Pima S-6, and the primitive cotton, B368, the slope of the stomatal response to temperature in each F₂ plant was positively correlated with the stomatal conductance measured at 40$C. An analysis of the frequency distribution of stomatal conductance in F₁ and F₂ progeny of the cross showed that the differences in stomatal conductance between lines were genetically determined. These data indicate that selection for higher yield and heat resistance in Pima cotton has caused genetically determined changes in stomatal properties. Characterization of the relationship between the altered stomatal properties and the attained increases in heat resistance and yields could make it possible to use these physiological traits as selection criteria in future breeding programs.     

Two consensus quantitative trait loci clusters controlling anthesis–silking interval, ear setting and grain yield might be related with drought tolerance in maize

Unravelling the molecular basis of drought tolerance will provide novel opportunities for improving crop yield under water-limited conditions. The present study was conducted to identify quantitative trait loci (QTLs) controlling anthesis–silking interval (ASI), ear setting percentage (ESP) and grain yield (GY). The mapping population included 234 F₂ plants derived from the cross X178 (drought tolerant) × B73 (drought susceptible). The corresponding F_2:3 progenies, along with their parents, were evaluated for the above-mentioned traits under both well-watered and water-stressed field conditions in three different trials carried out in central and southern China. Interval mapping and composite interval mapping identified 45 and 65 QTLs for the investigated traits, respectively. Two QTL clusters influencing ASI and ESP on chromosomes 1 (bin 1.03) and 9 (bins 9.03–9.05) were identified in more than two environments, showing sizeable additive effects and contribution to phenotypic variance; these two QTL clusters influenced GY only in one environment. No significant interaction was detected between the two genomic regions. A comparative analysis of these two QTL clusters with the QTLs controlling maize drought tolerance previously described in three mapping populations confirmed and extended their relevance for marker-assisted breeding to improve maize production under water-limited conditions.     

Predation's role in repeated phenotypic and genetic divergence of armor in threespine stickleback

Predator-driven divergent selection may cause differentiation in defensive armor in threespine stickleback: (1) predatory fish and birds favor robust armor, whereas (2) predaceous aquatic insects favor armor reduction. Although (1) is well established, no direct experimental evidence exists for (2). I examined the phenotypic and genetic consequences of insect predation using F₂ families from crosses between freshwater and marine stickleback populations. I measured selection on body size, and size-adjusted spine (dorsal and pelvic) and pelvic girdle length, by splitting juvenile F₂ families between control and insect predation treatments, set in pond enclosures. I also examined the effect of insect predation on Ectodysplasin ( Eda ), a gene physically linked to quantitative trait loci for lateral plate number, spine length, and body shape. Insect predation resulted in: (1) significant selection for larger juvenile size, and shorter dorsal spine and pelvic girdle length, (2) higher mortality of individuals missing the pelvic girdle, and (3) selection in favor of the low armor Eda allele. Predatory insects favor less stickleback armor, likely contributing to the widespread reduction of armor in freshwater populations. Because size strongly influences mate choice, predator-driven divergent selection on size may play a substantial role in byproduct reproductive isolation and speciation in threespine stickleback.     

Early generation selection for high yielding cowpea genotypes in additive series intercropping systems with sorghum

Defining appropriate selection strategies for developing cowpea varieties adapted to additive series intercropping systems is an important requirement for cowpea breeders and producers in sub‐Saharan Africa. One hundred and forty‐three F_2:3 cowpea families and their subsequent 99 F_3:4 families derived from a cross between a sole bred cultivar, Apagbaala, and a traditional variety, SARC‐L02, were evaluated under additive series intercropping with sorghum. Intercropping imposed a strong selection pressure for days to flowering such that 31% of F_2:3 families that flowered after 50 days produced too few grains to permit their subsequent evaluation in the F_3:4 generation. Narrow‐sense heritabilities estimated by parent–offspring regression were high for 100 seed weight and days to flowering, moderate for biomass, low for grain yield and insignificant for branches/plant and pods/plant. Retrospective selection at 40% intensity based on F₃ grain yield recovered 5 of the 10 top yielding families in the F₄. No significant difference was observed between mean grain yield of selected and rejected families (at the 40% selection intensity) as estimated by a t‐test. Sole and intercrop yields produced by six advanced breeding lines included as controls showed poor correlation, and suggests selecting cultivars under the target cropping system will produce better selection response.     

Population differentiation in an annual legume: genetic architecture

Abstract. The presence or absence of epistasis, or gene interaction, is explicitly assumed in many evolutionary models. Although many empirical studies have documented a role of epistasis in population divergence under laboratory conditions, there have been very few attempts at quantifying epistasis in the native environment where natural selection is expected to act. In addition, we have little understanding of the frequency with which epistasis contributes to the evolution of natural populations. In this study we used a quantitative genetic design to quantify the contribution of epistasis to population divergence for fitness components of a native annual legume, Chamaecrista fasciculata . The design incorporated the contrast of performance of F₂ and F₃ segregating progeny of 18 interpopulation crosses with the F₁ and their parents. Crosses were conducted between populations from 100 m to 2000 km apart. All generations were grown for two seasons in the natural environment of one of the parents. The F₁ often outperformed the parents. This F₁ heterosis reveals population structure and suggests that drift is a major contributor to population differentiation. The F₂ generation demonstrated that combining genes from different populations can sometimes have unexpected positive effects. However, the F₃ performance indicated that combining genes from different populations decreased vigor beyond that due to the expected loss of heterozygosity. Combined with previous data, our results suggest that both selection and drift contribute to population differentiation that is based on epistatic genetic divergence. Because only the F₃ consistently expressed hybrid breakdown, we conclude that the epistasis documented in our study reflects interactions among linked loci.     

Sex ratio bias in the F1 adult progeny of African rice gall midge, Orseolia oryzivora H. and G. (Dipt., Cecidomyiidae)

Abstract:  Against the background of conflicting reports of variable sex ratio distribution in different populations of the African rice gall midge (AfRGM), Orseolia oryzivora Harris and Gagne, the number and sex ratios of F₁ adult progeny produced per female from wild and screen house populations were investigated in the screen house at 27 ± 3°C, 60–70% relative humidity and 12 : 12 h (light : dark) photoperiod. Results indicated the occurrence of a single sex-biased ratio in all the F₁ adult progeny produced per female of this pest. Regardless of the source, whether from wild or screen house population, each mated female AfRGM produced a full complement of either sex only but never both. The difference between the number of eggs laid and F₁ adult progeny that emerged per female from wild and screen house populations was not significant (P > 0.05), indicating consistency in the occurrence of this phenomenon among AfRGM populations. The reason for this could not be ascertained in this study but AfRGM eggs must be fertilized for development and eclosion to occur. No F₂ adults emerged when a cohort of emergents from the same female were allowed to re-infest susceptible seedlings, indicating absence of parthenogenesis.     

Accumulation and utilization of triacylglycerol and polysaccharides in Liposcelis bostrychophila (Psocoptera, Liposcelididae) selected for resistance to carbon dioxide

Abstract: Two populations of the psocid, Liposcelis bostrychophila Badonnel, were exposed to two CO₂-enriched atmospheres (35% CO₂ + 21% O₂, and 55% CO₂ + 21% O₂, balance N₂) for 30 generations. Controls were reared in normal atmospheres. The reserves of triacylglycerol and polysaccharides were evaluated in adults of the two experimental and the control populations in generations F₁₅ and F₃₀. The utilization rate of triacylglycerol and polysaccharides in the CO₂-enriched atmospheres were also determined in generation F₃₀. The results indicated that the reserves of triacylglycerol and polysaccharides increased significantly during selection for CO₂ resistance; the higher the resistance level, the greater the reserves. Exposure of these populations to controlled atmosphere was associated with a steady utilization of the reserves. By contrast, the unselected population responded to controlled atmospheres by accelerated utilization of triacylglycerol and polysaccharides. Comparison of the utilization rates during CO₂ exposure showed that triacylglycerol is the main energy source, and polysaccharides contribute to metabolic energy supply only to a small extent.     

Correlated changes in thermotolerance traits and body color phenotypes in montane populations of Drosophila melanogaster: analysis of within- and between-population variations

Thermotolerance traits vary across geographical gradients but there is a lack of clinal variation in some Drosophila species. Thus, it is not clear whether thermotolerance or other correlated traits are the target of natural selection. In order to test selection responses, we investigated body melanization and thermotolerance traits in six altitudinal populations of Drosophila melanogaster . Based on rearing different geographical populations under uniform growth conditions at 21 °C (common garden experiments), clinal variations for cold resistance are in the direction opposite to heat resistance along an altitudinal gradient, that is darker flies from highland populations evidenced higher levels of cold resistance while lowland populations showed higher heat resistance. Phenotypic plastic responses for body melanization at 17–28 °C showed significant correlations with thermotolerance traits. At 17 °C, regression coefficients as a function of altitude are highly significant and positive for cold resistance but negative for heat knockdown. However, for flies reared at 28 °C, there is no elevational change in melanization as well as thermotolerance traits. Thus, both genetic and plastic changes of body melanization and thermotolerance traits suggest a correlated selection response. Further, within-population analyses of body melanization (based on dark, intermediate and light color phenotypes) showed significant associations with thermotolerance traits. Correlated variations in body melanization and thermal tolerances are associated with climatic thermal variability ( T _cv) but not with T _min. or T _max. along an altitudinal gradient.     

Mapping of QTL on chromosome X for fat deposition, muscling and growth traits in a wild boar x Meishan F2 family using a high-density gene map

Quantitative trait loci (QTL) for fat deposition, growth and muscling traits have been previously mapped on the basis of low-density linkage maps in a wild boar × Meishan F₂ family to the chromosome X region flanked by SW2456 and SW1943 . Improved QTL resolution was possible using data for F₂ animals with a marker density of 2.7 cM distance in the SW2456 to SW1943 region, including AR , SERPINA7 and ACSL4 as candidate genes. The resolution of the QTL scan was increased substantially, as evidenced by the higher F -ratio values for all QTL. Maxima of F -ratio values for fat deposition, muscling and growth traits were 28.6, 18.2 and 16.5 respectively, and those QTL positions accounted for 7.9%, 5.0% and 4.5% of the F₂ phenotypic variance (VF₂) respectively. QTL for fatness and growth and for most muscling traits mapped near ACSL4 , with the exception of the QTL for ham traits that mapped proximally, in the vicinity of AR . An analysis performed separately for F₂ male animals showed the predominant QTL affecting fat deposition traits (up to 13.6% VF₂) near AR and two QTL for muscling traits (up to 9.9% VF₂) mapped close to ACSL4 . In the F₂ female animals, QTL affecting muscling (up to 12.1% VF₂) mapped at ACSL4 and SW2456 , and QTL for fat deposition (10% VF₂) and growth (up to 10.5% VF₂) mapped at ACSL4 .     

Reproductive trait divergence and hybrid fertility patterns between chromosomal races of the house mouse in Tunisia: analysis of wild and laboratory-bred males and females

The divergence in reproductive features and hybrid fertility patterns between two chromosomal races (2 n  = 40, 40St, and 2 n  = 22, 22Rb) of the house mouse in Tunisia were re-assessed on a larger sample of wild and laboratory-bred individuals than studied hitherto. Results showed that litter sizes were significantly smaller in 40St than in 22Rb mice, contrary to previous analyses. This suggests that variation in litter size between the two chromosomal races is more likely related to selective and/or environmental factors acting locally than to interracial reproductive trait divergence. However, the significantly reduced litter size of F₁ hybrids compared with parental individuals was confirmed, and further highlighted a sex difference in hybrid infertility, as F₁ females produced fewer litters and of smaller size than males. Histological analyses of F₁ and backcrosses showed a breakdown of spermatogenesis in males and a significantly reduced primordial follicle pool in females. The degree of gametogenic dysfunction was not related to the level of chromosomal heterozygosity per se , but a significant effect of two Rb fusions on follicle number was observed in hybrid females. These results suggest that genetic incompatibilities contribute to primary gametogenic dysfunction in hybrids between the chromosomal races in Tunisia.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 407–416.     

Postzygotic isolation between the two European subspecies of the house mouse: estimates from fertility patterns in wild and laboratory-bred hybrids

We assessed the fertility (reproductive success, litter size, testis weight, spermatocyte-to-spermatid ratio) of F₁s and backcrosses between different wild-derived outbred and inbred strains of two mouse subspecies, Mus musculus domesticus and M. m. musculus . A significant proportion of the F₁ females between the outbred crosses did not reproduce, suggesting that female infertility was present. As the spermatocyte-to-spermatid ratio was correlated with testis weight, the latter was used to attribute a sterile vs. fertile phenotype to all males. Segregation proportions in the backcrosses of F₁ females yielded 11 (inbred) to 17% (outbred) sterile males, suggesting the contribution of two to three major genetic factors to hybrid male sterility. Only one direction of cross between the inbred strains produced sterile F₁ males, indicating that one factor was borne by the musculus X-chromosome. No such differences were observed between reciprocal crosses in the outbred strains. The involvement of the X chromosome in male sterility thus could not be assessed, but its contribution appears likely given the limited introgression of X-linked markers through the hybrid zone between the subspecies. However, we observed no sterile phenotypes in wild males from the hybrid zone, although testis weight tended to decrease in the centre of the transect.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 379–393.     

Evidence for coadaptation in geographic populations of Drosophila bipectinata

In order to test whether there is genetic coadaptation in geographic populations of Drosophila bipectinata with respect to body size, reciprocal crosses were made among five strains derived from ecogeographically different localities in India. Wing length was used as an index of body size, and was measured in all the five strains, and their crosses in F and F₂ generations. The statistical analysis of the data show that there is significant interpopulation variation in body size and in all the crosses, there is an increase in body size in F₁ generation when compared with mid-parent value. Further, there is a decrease in body size in F₂ generation as compared to F₁ in most of the crosses with increased variability. These results provide evidence for genetic coadaptation in geographic populations of D. bipectinata .     

Artificial hybridization within Saxifraga pentadactylis (Saxifragaceae)

Saxifraga pentadactylis subsp. almanzorii , an endemic to the subalpine nucleus of Sierra de Gredos (central Spain), differs from its closest relative, subsp. willkommiana , by its less showy petals. An artificial crossing program was carried out in order to assess the degree of reproductive isolation between the subspecies. To facilitate interpretation of the results, the program was extended to 10 other interspecific hybrid combinations within sect. Saxifraga . All the data gathered are congruent with the occurrence of two evolutionary scenarios. Interspecific crossings, rendering moderate to high seed-set (in obtaining the F₁), and vigorous but relatively sterile F₁ offspring, reveal reproductive barriers at the level of the F₁ fertility, probably originated as a byproduct of divergent evolution. In contrast, intraspecific crossings within S. pentadactylis resulted in seed-set values lower than expected (in obtaining the F₁), in a majority of weak non-viable F₁ offspring but also in a few fertile F₁ hybrid specimens which were able to originate F₂ offspring. This second pattern reveals reproductive barriers at the level of the F₁ vitality, probably arisen in a quite abrupt fashion. The lower P/O for subsp. almanzorii as compared to subsp. willkommiana , together with the rest of the evidence suggest that the reproductive barriers between them might be the product of active selection against hybridization achieved by incrementing the levels of autogamy in the former.     

Mapping of quantitative trait loci for clinical–chemical traits in swine

Clinical–chemical traits are diagnostic parameters essential for characterization of health and disease in veterinary practice. The traits show significant variability and are under genetic control, but little is known about the fundamental genetic architecture of this variability, especially in swine. We have identified QTL for alkaline phosphatase (ALP), lactate (LAC), bilirubin (BIL), creatinine (CRE) and ionized sodium (Na⁺), potassium (K⁺) and calcium (Ca⁺⁺) from the serum of 139 F₂ pigs from a Meishan/Pietrain family before and after challenge with Sarcocystis miescheriana , a protozoan parasite of muscle. After infection, the pigs passed through three stages representing acute disease, subclinical disease and chronic disease. Forty-two QTL influencing clinical–chemical traits during these different stages were identified on 15 chromosomes. Eleven of the QTL were significant on a genome-wide level; 31 QTL were chromosome-wide significant. QTL showed specific health/disease patterns with respect to the baseline values of the traits as well as the values obtained through the different stages of disease. QTL influencing different traits at different times were found primarily on chromosomes 1, 3, 7 and 14. The most prominent QTL for the investigated clinical–chemical traits mapped to SSC3 and 7. Baseline traits of ALP, LAC, BIL, Ca⁺⁺ and K⁺ were influenced by QTL regions on SSC3, 6, 7, 8 and 13. Single QTL explained up to 21.7% of F₂ phenotypic variance. Our analysis confirms that variation of clinical–chemical traits is associated with multiple chromosomal regions.     

Population divergence in the amphicarpic species Amphicarpaea edgeworthii Benth. (Fabaceae): microsatellite markers and leaf morphology

Comparative analyses of the genetic differentiation in microsatellite markers ( F _ST) and leaf morphology characters ( Q _ST) of Amphicarpaea edgeworthii Benth. were conducted to gain insight into the roles of random processes and natural selection in the population divergence. Simple sequence repeat analyses on 498 individuals of 19 natural populations demonstrate that a significant genetic differentiation occurs among populations (mean F _ST = 0.578), and A. edgeworthii is a highly self-fertilized species (mean selfing rate s  = 0.989). The distribution pattern of genetic diversity in this species shows that central populations possess high genetic diversity (e.g. population WL with H _E = 0.673 and population JG with H _E = 0.663), whereas peripheral ones have a low H _E as in population JD (0.011). The morphological divergence of leaf shape was estimated by the elliptical Fourier analysis on the data from 11 natural and four common garden populations. Leaf morphology analyses indicate the morphological divergence does not show strong correlation with the genetic differentiation ( R  = 0.260, P  = 0.069). By comparing the 95% confidence interval of Q _ST with that of F _ST, Q _ST values for five out of 12 quantitative traits are significantly higher than the average F _ST value over eight microsatellite loci. The comparison of F _ST and Q _ST suggests that two kinds of traits can be driven by different evolutionary forces, and the population divergence in leaf morphology is shaped by local selections.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 505–516.     

Mapping quantitative trait loci regulating chicken body composition traits

Genome scans were conducted on an F₂ resource population derived from intercross of the White Plymouth Rock with the Silkies Fowl to detect QTL affecting chicken body composition traits. The population was genotyped with 129 microsatellite markers and phenotyped for 12 body composition traits on 238 F₂ individuals from 15 full-sib families. In total, 21 genome-wide QTL were found to be responsible for 11 traits, including two newly studied traits of proventriculus weight and shank girth. Three QTL were genome-wide significant: at 499 c m on GGA1 (explained 3.6% of phenotypic variance, P  < 0.01) and 51 c m on GGA5 (explained 3.3% of phenotypic variance, P  < 0.05) for the shank & claw weight and 502 c m on GGA1 (explained 1.4% of phenotypic variance, P  < 0.05) for wing weight. The QTL on GGA1 seemed to have pleiotropic effects, also affecting gizzard weight at 490 c m , shank girth at 489 c m and intestine length at 481 c m . It is suggested that further efforts be made to understand the possible pleiotropic effects of the QTL on GGA1 and that on GGA5 for two shank-related traits.     

Genetic and in vitro molecular hybridization of malate dehydrogenase isozymes in interspecific bass (Micropterus) hybrids

Supernatant malate dehydrogenase (MDH) isozymes (as visualized by starch gel electrophoresis) are encoded by two distinct gene loci in both the largemouth and smallmouth bass. When an interspecific F₁ hybrid is formed between these two fish, a unique MDH isozyme is generated. The results of freeze-thaw molecular hybridization (which is the first application of this technique to MDH) indicate that this unique isozyme in the F₁ hybrid is a heterodimer composed of one subunit of each parental type. The F₁ hybrids produced F₂ hybrids which in turn formed the F₃ hybrid population. The inheritance of alleles at the MDH-B locus is consistent with a single Mendelian autosomal locus. Furthermore, there is no evidence of linkage between the lactate dehydrogenase-E locus and the MDH-B locus.     

Increasing tolerance to Cry1Ac cotton from cotton bollworm, Helicoverpa armigera, was confirmed in Bt cotton farming area of China

Abstract.  1. Changes in the frequency of Cry1Ac resistance genes and shifts in tolerance of cotton bollworm, Helicoverpa armigera , to the Cry1Ac toxin were assessed using bioassays of F₁ and F₂ offspring of isofemale lines from Anci County of Hebei Province (a multiple-crop system including corn, soybean, peanut, and Bt cotton) and Xiajin County of Shandong Province (an intensive Bt cotton planting area) in Northern China during 2002–2005.
2. A conservative analysis of the overall results indicated that there was a small increase in the frequency of major, non-recessive resistance genes over time.
3. The relative average development ratings [RADR – growth rate of a line on a Bt diet in proportion to the growth rate on a non-Bt (NBT) diet] of the bollworm larvae in F₁ tests increased significantly from year to year, indicating a gradual trend towards higher tolerance to Cry1Ac in the field populations.
4. There were also significant positive correlations between RADR of the lines in the F₁ generation and the RADR of their F₂ offspring, indicating that the tolerance was genetically based.
5. Quantitative genetic simulation analysis showed that resistance of H . armigera to Bt cotton in Xiajin could evolve to a high level in 11–15 years if no effective resistance management measures are carried out.     

A comprehensive analysis of QTL for abdominal fat and breast muscle weights on chicken chromosome 5 using a multivariate approach

Quantitative trait loci (QTL) influencing the weight of abdominal fat (AF) and of breast muscle (BM) were detected on chicken chromosome 5 (GGA5) using two successive F₂ crosses between two divergently selected 'Fat' and 'Lean' INRA broiler lines. Based on these results, the aim of the present study was to identify the number, location and effects of these putative QTL by performing multitrait and multi-QTL analyses of the whole available data set. Data concerned 1186 F₂ offspring produced by 10 F₁ sires and 85 F₁ dams. AF and BM traits were measured on F₂ animals at slaughter, at 8 (first cross) or 9 (second cross) weeks of age. The F₀, F₁ and F₂ birds were genotyped for 11 microsatellite markers evenly spaced along GGA5. Before QTL detection, phenotypes were adjusted for the fixed effects of sex, F₂ design, hatching group within the design, and for body weight as a covariable. Univariate analyses confirmed the QTL segregation for AF and BM on GGA5 in male offspring, but not in female offspring. Analyses of male offspring data using multitrait and linked-QTL models led us to conclude the presence of two QTL on the distal part of GGA5, each controlling one trait. Linked QTL models were applied after correction of phenotypic values for the effects of these distal QTL. Several QTL for AF and BM were then discovered in the central region of GGA5, splitting one large QTL region for AF into several distinct QTL. Neither the 'Fat' nor the 'Lean' line appeared to be fixed for any QTL genotype. These results have important implications for prospective fine mapping studies and for the identification of underlying genes and causal mutations.