Generations Of Selective Breeding Research Articles

Genetic improvement of duration of fertility in chickens and its commercial application for extending insemination intervals

The growth rate of chickens has made remarkable progress in recent decades through continuous breeding efforts. However, this advancement has also led to a decline in fertility among commercially bred chickens. Therefore, it is crucial to understand and improve factors that influence fertility to ensure the continued success of the industry. Here, we conduct a 3-generation selection experiment within 2 purebred female lines, with the aim of increasing the duration of fertility (DF). Duration of fertility refers to the length of time hens remain capable of producing fertilized eggs and is a crucial factor that directly impacts chick output. The results showed that significant genetic progress was achieved in embryo survival rates and the fertility duration day during both the peak and late laying periods. Moreover, after 3 generations of selective breeding, the disparities in embryo survival and chick health rates from setting eggs between 8-d and 5-d insemination intervals in the grandparent stock were significantly reduced. The rates decreased from 1.83% and 2.39 to 0.72% and 0.33%, respectively. Surprisingly, the hatching performances of hens with an 8-d interval were comparable to those hens that had not undergone genetic selection for DF and had a 5-d interval. We further discussed the possibility of extending the insemination interval to 8 d in parent stock for commercial practices. The parental populations exhibited remarkable performance in terms of percentages of embryo survival and healthy chicks from the setting eggs, with rates exceeding 94 and 90%, respectively. Thus, it can be inferred that an extended insemination interval is feasible by genetic selection for DF. These findings will provide valuable insights into the efficacy of genetic selection in enhancing DF and its practical application in commercial breeding programs.

Message to researchers: the characteristic absence of a posterior communicating artery is easily lost in the gerbil.

The Mongolian gerbil has historically been useful for brain ischemia experiments, owing to the gerbil's uniquely underdeveloped circle of Willis (CoW). This led to a gerbil model of cochlear ischemia being generated in our unit. However, we have found that the usual severe hearing loss seen in this model was not being induced consistently in recent experiments using the MON/Jms/GbsSlc gerbil (the sole commercially available gerbil in Japan). We set out to evaluate the posterior communicating artery (PcomA) in MON/Jms/GbsSlc, to re-establish whether this strain is appropriate for ischemia models. Having found that this unique feature is often lost, we then attempted to breed for the characteristic absent PcomA. India-ink perfusion revealed that the percentage of intact bilateral PcomA ("communicating type") in the MON/Jms/GbsSlc gerbil was 57%; unilateral only ("unilateral communicating type") was 39%; and completely absent PcomA ("non-communicating type") was 4%. We were able to obtain few examples of the indigenous old aged Japanese UNG/Mz gerbil strain (at University of Miyazaki). Unfortunately, the pure UNG/Mz female was too elderly for mating. Therefore, selective breeding crosses between MON/Jms/GbsSlc and male UNG/Mz were carried out. After five generations of selective breeding, the percentage of non-communicating type gerbils was significantly higher in the newly generated strain, MON/Jms/SlcMz (F6 generation; 63%) than in the MON/Jms/GbsSlc gerbil. Bilateral common carotid artery occlusion surgery demonstrated that the cerebral blood flow was significantly reduced in MON/Jms/SlcMz compared with MON/Jms/GbsSlc (p < 0.0001) and induced more hippocampal injuries in MON/Jms/SlcMz than in MON/Jms/GbsSlc (p < 0.01). In conclusion, the commercially available MON/Jms/GbsSlc gerbil can easily regain PcomA, and we established a new gerbil strain (MON/Jms/SlcMz) displaying non-PcomA.

Cherry Shrimp Neocaridina davidi (Bouvier 1904) (Crustacea: Decopoda: Atyidae)

Neocaridina davidi is a species of freshwater ornamental shrimp that belongs to the Atyidae family. Neocaridina davidi originates from Taiwan, but their popularity in the aquarium trade has led to them being available in other countries. This shrimp has different common names based on its color. Wild type shrimp are generally transparent or brown in color, however, generations of selective breeding have produced a variety of colors and patterns.&#x0D; Also available on the Featured Creatures website at http://entnemdept.ufl.edu/creatures/MISC/cherry_shrimp.html

Genome‐wide SNP analysis reveals an increase in adaptive genetic variation through selective breeding of coral

Marine heat waves are increasing in magnitude, duration, and frequency as a result of climate change and are the principal global driver of mortality in reef-building corals. Resilience-based genetic management may increase coral heat tolerance, but it is unclear how temperature responses are regulated at the genome level and thus how corals may adapt to warming naturally or through selective breeding. Here we combine phenotypic, pedigree, and genomic marker data from colonies sourced from a warm reef on the Great Barrier Reef reproductively crossed with conspecific colonies from a cooler reef to produce combinations of warm purebreds and warm-cool hybrid larvae and juveniles. Interpopulation breeding created significantly greater genetic diversity across the coral genome compared to breeding between populations and maintained diversity in key regions associated with heat tolerance and fitness. High-density genome-wide scans of single nucleotide polymorphisms (SNPs) identified alleles significantly associated with larval families reared at 27.5°C (87-2,224 loci), including loci putatively associated with proteins involved in responses to heat stress (cell membrane formation, metabolism, and immune responses). Underlying genetics of these families explained 43% of PCoA multilocus variation in survival, growth, and bleaching responses at 27.5°C and 31°C at the juvenile stage. Genetic marker contribution to total variation in fitness traits (narrow-sense heritability) was high for survival but not for growth and bleaching in juveniles, with heritability of these traits being higher at 31°C relative to 27.5°C. While based on only a limited number of crosses, the mechanistic understanding presented here demonstrates that allele frequencies are affected by one generation of selective breeding, key information for the assessments of genetic intervention feasibility and modelling of reef futures.

Growth Curve of Selectively Bred Tambaqui (Colossoma macropomum) Reared in Different Environments

The objective of this study was to obtain the growth curve of selectively bred tambaqui (Colossoma macropomum) reared in different environments. The experiment was carried out in the municipalities of Santo Antônio de Leverger (Mato Grosso – MT) and Campo Grande (Mato Grosso do Sul – MS), Brazil, over 431 days. Weight and morphometric traits of two families (A and B) from the second generation of selective breeding (G2) were measured every 30-45 days. The Gompertz regression model was used to obtain the growth curves. The production performance of both families and the interaction between families and locations (genotype × environment) were evaluated by analysis of variance considering the family (A and B), location (MT and MS), family × location interaction and error as variation factors. The asymptotic value (parameter A) obtained for weight and morphometric traits (except head length) was higher (P&lt;0.05) in MT (weight of families A and B: 2279.6 g) than in MS (weight of family A: 1400.0 g; weight of family B: 1600.0 g). Family B showed better production performance in MS. There was a genotype × environment interaction effect on weight, body length and standard length. The two families have distinct growth patterns in different production environments. Family B has better growth performance in the environment with lower temperatures (MS).

Evaluation of Genetic Breeding for the Color of Shell Nacre in the Freshwater Pearl Mussel Hyriopsis cumingii (Lea, 1852) by Raman Spectroscopy

The color of pearl, one of the important quality traits, is determined by the shell nacre color of host mussel; however, the effective method to evaluate the effect of genetic breeding on the purity and uniformity of the nacre color was still limited to the freshwater mussel Hyriopsis cumingii. In this study, the characteristics of the υ1 band of polyene pigments in the colorful shell nacre and pearl samples from different selective generations of H. cumingii were detected by Raman spectroscopy. The results of this study showed that the number of peaks at the υ1 band of polyene pigments decreased gradually from 6–8 in the shell nacre and pearls with the color scheme of purple for wild mussels to 1 in those from the mussels of the F4 generation by genetic breeding. Similar results were also found in the samples with the color scheme of orange. But, no obvious peak of the υ1 band was detected in the samples with white color after four generations of selective breeding. Thus, it could be speculated that the single peak of the υ1 band in the Raman spectroscopy of shell nacre could be used as the sign of color uniformity to reflect the degree of purification in shell nacre color by genetic breeding. Meanwhile, the results showed that the samples with the same color scheme have a similar main peak at the υ1 band (ca. 1,508 cm–1 for the purple color scheme and ca. 1,528 cm–1 for the orange color scheme), suggesting that the polyene pigments in shell nacre and pearl with the same color scheme have the same length of C = C double bond. The length of C = C double bond were 12 and 9 in purple and orange samples, respectively. In summary, the results of this study support the conclusion that Raman spectroscopy is an efficient method to evaluate the genetic breeding for nacre color in H. cumingii.

Trends in Soybean Trait Improvement over Generations of Selective Breeding

Improvement of agronomic and seed quality traits in soybean [Glycine max (L.) Merr.] occurs in breeding programs as a result of crossing elite soybean genotypes and selection of superior offspring in each cycle. The objective of this study was to evaluate changes in traits over 100 yr of selection in Ontario, Canada, in two soybean breeding programs at the University of Guelph: Guelph Campus (Maturity Group [MG] 0 and I) and Ridgetown Campus (MG II) programs. Fifty‐six Guelph cultivars (released between 1983 and 2016), 46 Ridgetown cultivars (released between 1987 and 2015), and 37 Guelph‐pedigree related historical cultivars (from 1913 to 1985) were grown in field trials over 3 yr (Guelph Campus) and 2 yr (Ridgetown Campus) at two locations per breeding program to measure 13 traits. Yield increased in the Guelph Campus cultivars by 17.1 kg ha−1 yr−1 and by 15.7 kg ha−1 yr−1 in the Ridgetown Campus cultivars, while yield per day of maturity per year of release significantly increased in elite cultivars. Protein in the Ridgetown Campus cultivars increased at 0.76 g kg−1 yr−1, whereas the Guelph Campus cultivars showed no trend. In Ridgetown Campus cultivars 100‐seed weight significantly increased at 0.1 g yr−1. Within the historical cultivars, 100‐seed weight was significantly correlated with both protein (r = 0.50) and oil (r = −0.42). Genotype × trait biplot analyses revealed differential patterns of trait relationships between the elite and historical cultivars. Oil and yield were associated within the Guelph Campus cultivars. The association of 100‐seed weight and protein was only found in the historical cultivars. Yield has increased in both breeding programs while seed traits have been altered over time.

Field performance of various Pinus radiata breeding families established on a drought-prone site in central Chile

BackgroundPinus radiata D.Don is in its third generation of selective breeding on contrasting site types in central Chile, creating interest in its responses to selection and any differential adaptation to site types. We studied the phenotypic variability of growth traits, survival and six ecophysiological traits in 30 open- and control-pollinated families, representing two breeding regions and three breeding generations, in a 2-year-old P. radiata field trial on a sandy soil in the Mediterranean drylands of Central Chile.FindingsGrowth, survival and the ecophysiological traits did not differ between the regional origin of the breeding populations. However, breeding-generation effects were significant for height and diameter. As expected, growth traits showed progressive improvement with successive generations. Individual-family effects were evident for all traits except one ecophysiological trait.ConclusionsBreeding has evidently improved early growth performance in the field, despite no clear effect of regional breeding population, but no such effects were observed for the ecophysiological traits. Despite the current study demonstrating considerable family variation at age 2 years, it is unclear how these results relate to performance of mature trees in the field.

Economic values of growth and feed efficiency for fish farming in recirculating aquaculture system with density and nitrogen output limitations: a case study with African catfish (Clarias gariepinus).

In fish farming, economic values (EV) of breeding goal traits are lacking, even though they are key parameters when defining selection objectives. The aim of this study was to develop a bioeconomic model to estimate EV of 2 traits representing production performances in fish farming: the thermal growth coefficient (TGC) and the feed conversion ratio (FCR). This approach was applied to a farm producing African catfish (Clarias gariepinus) in a recirculating aquaculture system (RAS). In the RAS, 2 factors could limit production level: the nitrogen treatment capacity of the biofilter or the fish density in rearing tanks at harvest. Profit calculation includes revenue from fish sales, cost of juveniles, cost of feed, cost of waste water treatment, and fixed costs. In the reference scenario, profit was modeled to zero. EV were calculated as the difference in profit per kilogram of fish between the current population mean for both traits (µt) and the next generation of selective breeding (µt+Δt) for either TGC or FCR. EV of TGC and FCR were calculated for three generations of hypothetical selection on either TGC or FCR (respectively 6.8% and 7.6% improvement per generation). The results show that changes in TGC and FCR can affect both the number of fish that can be stocked (number of batches per year and number of fish per batch) and the factor limiting production. The EV of TGC and FCR vary and depend on the limiting factors. When dissolved NH3-N is the limiting factor for both µt and µt+Δt, increasing TGC decreases the number of fish that can be stocked but increases the number of batches that can be grown. As a result, profit remains constant and EVTGC is zero. Increasing FCR, however, increases the number of fish stocked and the ratio of fish produced per kilogram of feed consumed ("economic efficiency"). The EVFCR is 0.14 €/kg of fish, and profit per kilogram of fish increases by about 10%. When density is the limiting factor for both µt and µt+Δt, the number of fish stocked per batch is fixed; therefore, extra profit is obtained by increasing either TGC, which increases the annual number of batches, or by decreasing FCR, which decreases annual feed consumption. EVTGC is 0.03 €/kg of fish and EVFCR is 0.05-0.06 €/kg of fish. These results emphasize the importance of calculating economic values in the right context to develop efficient future breeding programs in aquaculture.

Microsatellite analysis of variation among wild, domesticated, and genetically improved populations of blunt snout bream (Megalobrama amblycephala).

In the present study, the genetic diversity of one selected strain (Pujiang No. 1), two domesticated populations (GA and HX) and four wild populations (LZ, YN, SS and JL) of blunt snout bream (Megalobrama amblycephala) was analyzed using 17 microsatellite markers. The results showed that an average of 4.88-7.65 number of alleles (A); an average of 3.20-5.33 effective alleles (Ne); average observed heterozygosity (Ho) of 0.6985-0.9044; average expected heterozygosity (He) of 0.6501-0.7805; and the average polymorphism information content (PIC) at 0.5706-0.7226. Pairwise FST value between populations ranged from 0.0307-0.1451, and Nei's standard genetic distance between populations was 0.0938-0.4524. The expected heterozygosities in the domesticated populations (GA and HX) were significantly lower than those found in three wild populations (LZ, SS and JL), but no difference was detected when compared with the wild YN population. Likewise, no difference was found between the four wild populations or two domesticated populations. The expected heterozygosity in Pujiang No. 1 was higher than the two domesticated populations and lower than the four wild populations. Regarding pairwise FST value between populations, permutation test P-values were significant between the GA, HX and PJ populations, but not between the four wild populations. These results showed that the expected heterozygosity in the selected strain of blunt snout bream, after seven generations of selective breeding, was lower than that of wild populations, but this strain retains higher levels of genetic diversity than domesticated populations. The genetic differences and differentiation amongst wild populations, domesticated populations and the genetically improved strain of blunt snout bream will provide important conservation criteria and guide the utilization of germplasm resources.

Genetic variation during four generations of selective breeding in the pearl oyster &lt;I&gt;Pinctada fucata&lt;/I&gt;

Genetic variation during four generations of selective breeding in the pearl oyster &lt;I&gt;Pinctada fucata&lt;/I&gt;

Establishment and WSSV resistant characteristics of selective breeding families for resistance to the white spot syndrome virus of Litopenaeus vannamei

Based on one generation of individual selective breeding and four generations of family selective breeding of Litopenaeus vannamei,120 selective breeding families of resistance to WSSV were produced from 2002 to 2007.Experimental WSSV challenging tests showed that the survival rates of generation 2(G2),G3,G4 and G5 were 5.57%±9.83%,8.66%±11.52%,9.52%±8.84% and 13.79%±12.86%0%,and the variation coefficient of survival rates were 1.77,1.40,0.97and 0.87,respectively.Each generation of selective breeding families of resistance to WSSV was divided into three groups:relatively high resistant families,mid-resistant families and susceptible families post WSSV infection.The proportions of susceptible families in G2 to G5 were 76.5%,55.2%,51.4% and 33.3%,and the survival rates were 0.44%±1.09%,0.78%±1.70%,2.27%±2.76% and 2.44%±3.09%,respectively,with an acute mortality peak occurring on 2nd-3rd day post infection.The proportions of mid-resistant families in G2 to G5 were 0,20.7%,31.1% and 38.5%,and the survival rates were 0,9.08%±1.46%,10.7%±1.41%and 11.36%±3.30%,respectively,with two mortality peaks at post WSSV infection,and the 1st peak value was greater than that of the 2nd peak.The proportions of relatively high resistant families in G2 to G5 were 23.5%,24.1%,17.1% and 28.2%,and the survival rates were 22.23%±5.21%,22.7%±12.30%,24.45%±6.56% and 28.98%±8.09%,respectively,with two mortality peaks at post WSSV infection,and the 1st peak value was less than that of the 2nd peak,which showed that these families in this group possessed strong resistance to WSSV infection.So the eruption time of WSSV was delayed and the valuable opportunity could be used to take effective measure to prevent and cure.However,the average of spawning quantity of each generation was declining(P0.05)year after year.

Genomic mapping of social behavior traits in a F2 cross derived from mice selectively bred for high aggression

BackgroundRapid response to selection was previously observed in mice selected for high levels of inter-male aggression based on number of attacks displayed in a novel social interaction test after isolation housing. Attack levels in this high aggression line (NC900) increased significantly within just four generations of selective breeding, suggesting the presence of a locus with large effect. We conducted an experiment using a small (n ≈ 100) F2 cross between the ICR-derived, non-inbred NC900 strain and the low aggression inbred strain C57BL/6J, genotyped for 154 fully informative SNPs, to determine if a locus with large effect controls the high-aggression selection trait. A second goal was to use high density SNP genotyping (n = 549,000) in the parental strains to characterize residual patterns of heterozygosity within NC900, and evaluate regions that are identical by descent (IBD) between NC900 and C57BL/6J, to determine what impacts these may have on accuracy and resolution of quantitative trait locus (QTL) mapping in the F2 cross.ResultsNo evidence for a locus with major effect on aggressive behavior in mice was identified. However, several QTL with genomewide significance were mapped for aggression on chromosomes 7 and 19 and other social behavior traits on chromosomes 4, 7, 14, and 19. High density genotyping revealed that 28% of the genome is still segregating among the six NC900 females used to originate the F2 cross, and that segregating regions are present on every chromosome but are of widely different sizes. Regions of IBD between NC900 and C57BL/6J are found on every chromosome but are most prominent on chromosomes 10, 16 and X. No significant differences were found for amounts of heterozygosity or prevalence of IBD in QTL regions relative to global analysis.ConclusionsWhile no major gene was identified to explain the rapid selection response in the NC900 line, transgressive variation (i.e. where the allele from the C57BL/6J increased attack levels) and a significant role for dominant gene action were hallmarks of the genetic architecture for aggressive behavior uncovered in this study. The high levels of heterozygosity and the distribution of minor allele frequency observed in the NC900 population suggest that maintenance of heterozygosity may have been under selection in this line.

Stability and potential inheritance of infanticidal behavior in prairie voles

Naïve female prairie voles show significant variability in their behavioral response to newborns. We investigated whether that behavioral response (a) was related to the quality of postpartum maternal behavior; (b) was affected by postpartum maternal experience; and (c) could be selectively bred. The behavior of females was recorded in three conditions: as naïve in a nonreproductive context, as single lactating (no male present), and as experienced mother in a nonreproductive context. Finally, females and males with similar behavioral response to newborns were selectively bred for three generation. Males were removed before the offspring was born. Our results revealed that (a) naïve females that attacked pups, spent more time distant from them after parturition than those that were maternal or ignored the pups (p < .05); (b) postpartum maternal experience did not reverse infanticidal behavior; and (c) at the third generation of selective breeding, 90% of the offspring of females that were nonmaternal as virgins, behaved as their mothers. These findings suggest that the infanticidal behavioral response is a stable behavioral trait and might be passed to the offspring.

The mysterious steps in carcinogenesis: addendum

Sir, In the past 100 years, many explanations have been proposed for the process of carcinogenesis but none of them has proved to be totally persuasive. For this reason, we deliberately did not offer a modern synthesis in our review article (Brash and Cairns, 2009). However, in the last few years, thanks to certain experiments, a possible interpretation has emerged, which could be of practical importance. We now see that all cells (bacterial, yeast and mammalian cells) are more far-sighted than we had imagined. Confronted by stressful or damaging changes in their environment, populations of cells activate a programme that raises their mutation rate for several generations but temporarily masks the mutant phenotypes. This greatly increases the likelihood that some of them will be able to flourish in the new environment. Two important observations suggest that induction of this ‘stress response' might be the crucial initiating event in cancer. (1) When cells are exposed to chemical or physical initiators in vitro, every cell can be initiated so that it yields transformed descendants, which implies that initiation is the long-term activation of a programme rather than the production of mutations in certain genes (Kennedy et al, 1984). (2) Inactivation of one of the genes involved in the stress response protects mice against various experimental cancers (Dai et al, 2007). If the formation of most cancers is initiated by activation of a programme that depends on the interplay of several gene products, then defects in some of these products (although evolutionarily deleterious) might prevent most cancers; therefore, it may be useful to look for polymorphisms that protect against cancer rather than, as has become usual these days, concentrate solely on those that increase the risk. This could not easily be done with humans (whose lifetime risk of cancer is only about 50%), but could be done with mice. Even within inbred strains, mice are known to vary in susceptibility to skin cancer, and only a few generations of selective breeding can produce mice that are largely insusceptible (Boutwell, 1964). So the project would be to look for the genetic changes that accompany such selection and then, if found, study the frequency of changes in the equivalent human genes in relation to the risk of cancer, using the DNA samples that have already been collected for the many studies of genetic susceptibility.

Association of a locus on rat chromosome 4 with anxiety-related behaviors in two selectively bred rat lines

The Floripa H and L rat lines, selected for high and low locomotion in the central aversive area of an open field, a widely used emotionality test, were proposed as a model for studying the genetic basis of anxiety. The present study aimed to verify if the QTL Ofil1, mapped to rat chromosome 4 and previously identified as being related to emotionality in another population of rats, contributes to the behavioral variability observed in the Floripa rat lines. To this purpose, rats of five generations of selective breeding were genotyped for two polymorphic markers, D4RAT59 and D4MGH27, flanking Ofil1. Changes in genotype and allele frequencies throughout generations were evaluated in both H and L lines, in order to assess if the bidirectional selection based on behavioral scores induced divergent changes in the genotype of this genome region. There were significant changes in genotype frequencies for both molecular markers, however, only the genotype variations of the D4RAT59 marker were significantly correlated with the variations in the selected phenotype. This result suggests that the region of the genome near D4RAT59 contains one or more genes contributing to the interindividual variation in central locomotion in the open field test.

Selective breeding for catalepsy changes the distribution of microsatellite D13Mit76 alleles linked to the 5‐HT1A serotonin receptor gene in mice

Catalepsy (pronounced motor inhibition) is a natural defensive reaction against predator. Recently, the quantitative trait locus for catalepsy was mapped on mouse chromosome 13 near the 5-HT(1A) serotonin receptor gene. Here, the linkage between catalepsy and the 5-HT(1A) receptor gene was verified using breeding experiment. Selective breeding for high predisposition to catalepsy was started from backcross BC[CBA x (CBA x AKR)] generation between catalepsy-prone (CBA) and catalepsy-resistant (AKR) mouse strains. CBA and AKR strains also differed in the 5-HT(1A) receptor functional activity. A rapid increase of cataleptic percentage from 21.2% in the backcrosses to 71% in the third generation of selective breeding (S3) was shown. The fragment of chromosome 13 including the 5-HT(1A) receptor gene was marked with D13Mit76 microsatellite. Breeding for catalepsy increased the concentration of CBA-derived and decreased the concentration of AKR-derived alleles of microsatellite D13Mit76 in the S1 and S2. All mice of the S9 and S12 were homozygous for CBA-derived allele of D13Mit76 marker. Mice of the S12 showed CBA-like receptor activity. These findings indicate that selective breeding for behavior can involve selection of polymorphic variants of the 5-HT(1A) receptor gene.

Mice congenic for a locus that determines phenotype in the neuroma model of neuropathic pain

A common strategy for identifying physiological processes responsible for pain is to compare animal strains that show high versus low pain phenotype. However, this approach yields only weak inference because most strains differ from one another at a large number of genetic loci. We undertook a congenic breeding program aimed at transposing the gene(s) that confers dominant (low) pain phenotype in the neuroma model of neuropathic pain from mice of the C58/J strain onto a genetic background that normally expresses high pain phenotype (C3H/HeN). Successful transposition, and the observation of equal numbers of phenotypically high and low offspring in each of 11 generations of selective breeding, provides evidence that the trait is largely controlled by a single gene (or linked cluster) of major effect.

Who--or what--are the rats (and mice) in the laboratory.

This paper explores the many meanings attached to the designation, "the rodent in the laboratory" (rat or mouse). Generations of selective breeding have created these rodents. They now differ markedly from their wild progenitors, nonhuman animals associated with carrying all kinds of diseases. Through selective breeding, they have moved from the rats of the sewers to become standardized laboratory tools and (metaphorically) saviors of humans in the fight against disease. This paper sketches two intertwined strands of metaphors associated with laboratory rodents. The first focuses on the idea of medical/scientific progress; in this context, the paper looks at laboratory rodents often depicted (in advertising for laboratory products) as epitomizing medical triumph or serving as helpers or saviors. The second strand concerns the ambiguous status of the laboratory rodent who is both an animal (bites) and not an animal (data). The paper argues that, partly because of these ambiguous and multiple meanings, the rodent in the laboratory is doubly "othered"--first in the way that animals so often are made other to ourselves and then other in the relationship of the animal in the laboratory to other animals.

Susceptibility and resistance of rats to stress-induced decreases in swim-test activity: a selective breeding study

In this study, selective breeding was used to generate two populations of rats that differed in their susceptibility to showing decreased struggling activity in a swim test after being exposed to uncontrollable electric tail-shock. After five generations of selective breeding, we obtained a population that displayed large decreases in swim-test struggling after shock (swim-test susceptible) and a population that displayed no decrease in struggling after shock (swim-test resistant). Males of this fifth generation from the two selectively-bred populations were then compared for differences in non-swim behavioral measures (home-cage 24-h spontaneous ambulatory activity and food/water intake) and several aspects of brain catecholaminergic activity, including electrophysiological activity of locus coeruleus (LC) neurons, catecholamine/metabolite concentrations in various brain regions, and in vivo tyrosine hydroxylase activity. Interestingly, swim-test resistant rats displayed larger decreases in home-cage ambulatory activity and water intake after exposure to shock than did swim-test susceptible animals. Marked differences were also seen in measures of brain noradrenergic activity. Compared to the susceptible rats, resistant rats showed higher levels of evoked activity of LC neurons, larger shock-induced depletions of norepinephrine (NE) and 3-methoxy-4-hydroxyphenylglycol (MHPG) in the LC, lower in vivo tyrosine hydroxylase (TH) activity in ventral bundle projection areas such as the hypothalamus, and larger amounts of NE in dorsal bundle projection areas. Finally, swim-test resistant rats had much higher concentrations of dopamine (DA) and dihydroxyphenylacetic acid (DOPAC) in striatum and nucleus accumbens than susceptible rats. These results appear to be explainable on the basis that differences in swim-test struggling behavior for which the two populations were selectively bred were a consequence of differences in forebrain DA whereas stress-induced differences in other behavioral measures (i.e. spontaneous ambulation and intake) occurred because swim-test resistant animals showed greater disturbance of the LC-NE system after uncontrollable shock.