Modulating birth weight heritability in mice

Embryo survival and fertility differ in lines divergently selected for birth weight homogeneity in mice

The selection of animals for lower environment sensitivity around the optimum trait value can also provide benefits in productivity and welfare. A divergent selection experiment for birth weight environmental variability in mice was successfully conducted over 17 generations. Animals from low variability selected line (L-line) were more robust by having a higher litter size and survival at weaning in a common breeding environment, than those from high variability line (H-line). The objective of this study was to analyze the differences between those divergently selected lines for embryo and fetal survival and for fertility and prolificacy rate. To study embryo survival and ovulation rate, a total of 98 females (34 H-line and 50 L-line) were studied in four generations of the divergent experiment. To analyze fetal survival and fertility rate, 378 female mice (138 H-line and 240 L-line) in 10 generations or the divergent experiment were studied. Ultrasound scans were performed at day 14 of gestation to establish the number of total fetal and the embryo absorptions. Mortality was addressed as the difference between litter size at birth and the number of fetuses at 14 days of gestation. The number of pregnant females in the first 3 days after mating was used to measure fertility. A linear model was also fitted to analyze embryo mortality, litter size, and the number of embryos at 14 days of gestation. A categorical model was then used to study fertility, including line, generation, and its interaction as effects. Despite the fact that there were no significant differences in the ovulation rate, litter size at birth was significantly higher in the L-line than in the H-line (9.82 vs. 8.36 pups, p < 0.001). Moreover, embryo mortality was significantly lower in the L-line than in the H-line (1.39 vs. 2.87 fetuses, p < 0.001). L-line females were more fertile (53.49% vs. 23.26% for the H-line). According to these results, the line selected for low environmental variance would be preferable for robustness and animal welfare.

Impact of selection for birth weight variability on reproductive longevity: A mice model

Uniformity, understood as a similar performance in relevant livestock traits, such as birth weight within the litter, is being included as one of the selection objectives in breeding programmes, especially for polytocous livestock species. A divergent selection experiment for birth weight within‐litter variability in mice during 23 generations showed that homogeneous animals were better for litter size, survival and feed efficiency but less heavy than heterogeneous animals. The aim of this study was to compare the reproductive longevity in both divergent lines as time to the end of the reproductive period. Two generations from both lines with an initial number of 43 females and 43 males were mated one to one and stayed together to have consecutive parturitions until the end of the reproductive life. Females were discarded when the time elapsed from the last parturition was longer than 63 days. The time to the end of the reproductive period between both lines was compared by fitting a Cox proportional hazard regression model adjusting for line, generation and its interaction. The rate of parturitions in both lines was also compared using a Prentice–Williams–Peterson model adjusted for the same effects. The low variability line was associated with a higher parturition rate, e.g., adjusted hazard ratio was 2.93 (95% CI 2.17–3.94). The Cox model showed that the low variability females also presented benefits of time to the end of the reproductive period, with an adjusted hazard ratio of 0.26 (95% CI 0.16–0.41). The median of reproductive days was 55.50 in the high variability line whilst the median was 252.50 days for the low variability females. The homogeneous line presented important reproductive advantages suggesting higher robustness and animal welfare. Further research should confirm whether the findings presented here of a better performance in the low variability line could be properly applied to some livestock species.

Genetic parameters for uniformity of harvest weight in Pacific white shrimp (Litopenaeus vannamei)

Background

Uniformity of body weight is a trait of great economic importance in the production of white shrimp (Litopenaeus vannamei). A necessary condition to improve this trait through selective breeding is the existence of genetic variability for the environmental variance of body weight. Although several studies have reported such variability in other aquaculture species, to our knowledge, no estimates are available for shrimp. Our aim in this study was to estimate the genetic variance for weight uniformity in a farmed population of shrimp to determine the potential of including this trait in the selection program. We also estimated the genetic correlation of weight uniformity between two environments (selection nucleus and commercial population).

Methods

The database contained phenotypic records for body weight on 51,346 individuals from the selection nucleus and 38,297 individuals from the commercial population. A double hierarchical generalized linear model was used to analyse weight uniformity in the two environments. Fixed effects included sex and year for the nucleus data and sex and year-pond combination for the commercial data. Environmental and additive genetic effects were included as random effects.

Results

The estimated genetic variance for weight uniformity was greater than 0 (0.06 ± 0.01) in both the nucleus and commercial populations and the genetic coefficient of variation for the residual variance was 0.25 ± 0.01. The genetic correlation between weight and weight uniformity was close to zero in both environments. The estimate of the genetic correlation of weight uniformity between the two environments (selection nucleus and commercial population) was 0.64 ± 0.06.

Conclusions

The existence of genetic variance for weight uniformity suggests that genetic improvement of this trait is possible. Selection for weight uniformity should not decrease weight, given the near zero genetic correlation between these two traits. The strong genetic correlation of weight uniformity between the two environments indicates that response to selection for uniformity in the nucleus will be at least partially transmitted to the commercial population if this trait is included in the breeding goal.

Genetic parameters for canalization analysis of morphological traits in the Pura Raza Español horse

Measurements from 13 different morphological traits of importance in the Pura Raza Español (PRE) horse were used to estimate genetic and environmental parameters following a heteroscedastic model in which data were assigned to stallions. Data sets used ranged from 20,610 (height at withers) to 48,486 measurements (length of shoulder), and the number of animals analysed in the pedigrees varied from 17,662 (height at withers) to 23,962 (dorsal-sternal diameter). Results of heritabilities of the traits varied from 0.09 (width of chest and upper neck line) to 0.30 (muscular development). Further, genetic correlations between traits and their environmental variability were estimated, obtaining values from -0.56 (muscular development) to 0.69 (height at withers). Also, predicted breeding values for the mean and for the environmental variability were obtained for all horses in the pedigrees, providing individual information about not only the expected phenotypic value of their offspring but also the expected heterogeneity among them. Results proved the possibility of improving morphological traits and reducing the heterogeneity of offspring at a time by the selection of animals and levels of systematic effects.

Selection Response in a Divergent Selection Experiment for Birth Weight Variability in Mice Compared with a Control Line

Birth weight (BW) in animal production is an economically important trait in prolific species. The laboratory mouse (Mus musculus) is used as an experimental animal because it is considered a suitable model for prolific species such as rabbits and pigs. Two mouse lines were divergently selected for birth weight variability with a third line of non-selected control population of the same origin as the animals starting the experiment. The objective of this study was, therefore, to compare and evaluate the differential response of each line. The animals were from the 17th generation of both low and high BW variability lines of the divergent selection experiment, including in addition animals from the control line. The dataset contained 389 records from 48 litters of the high line, 734 records from 73 litters of the low line, and 574 records from 71 litters of the control line. The studied traits were as follows: the BW, the BW variance, the BW standard deviation, the BW coefficient of variation within-litter, the weaning weight (WW), the litter size at birth and at weaning, the weight gain, and the preweaning survival. The model included the line effect jointly with the parturition number and its interaction, the linear and quadratic LS as covariates except for the LS trait itself when analyzing litter traits, as well as the pup sex when analyzing individual traits. The low line had a lower BW and WW, but a higher litter size, and greater robustness owing to a higher survival at weaning. As a model of livestock animals, the findings from this experiment led to a proposal of selection for pig production that would combine an increase in litter size with higher survival and welfare. Compared with the control line, a much higher response was observed in the low variability line than in the high line, making it extremely satisfactory given that homogeneity provides advantages in terms of animal welfare and robustness.

Calving date and its variability as a potential trait in the breeding objective to account for reproductive seasonality in alpacas

The low fertility and offspring survival indicators in alpacas can be partially due to their particularity seasonal reproduction that reduces the opportunities of the females to become pregnant within a season, with the survival of the offspring concerned by the availability of food and exposure to diseases that depends on the calving date. Optimizing the date of delivery and reducing its variability are shown as eligible criteria that could be used as selection criteria within the genetic improvement programmes in alpacas, the calving date being a much more appropriate trait to measure and optimize fertility unlike of age at first calving and the calving interval, this due to the reproductive seasonality in camelids. For this study, 6,533 birth date records were taken between 2001 and 2018 of Peruvian alpacas, to estimate the genetic parameters. Models assuming heterogeneity in the residuals were fitted besides classical homogeneous models to address, not only the possibility of forwarding or delaying the calving date, but also the trend to have parturitions in similar dates. The heritability and repeatability ranged from 0.07 to 0.20 for a homogeneity model and from 0.08 to 0.23 for a heterogeneity model, and suggest the possibility of advancing or delaying the calving date. It should be taken into account that the gestation length of camelids makes it difficult to adapt many reproductive traits, and trying to centre the calving date could delay it. It was concluded the feasibility to genetically select the calving date, also in the production of camels and dromedaries, which have the same reproductive characteristics as alpacas. This selection can be combined with other traits. The heterogeneity model was shown to provide a better fit.

Challenging the selection for consistency in the rank of endurance competitions

Background

Control of the environmental variability by genetic selection offers possibilities for new selection objectives for productive traits. This methodology aims at reducing heterogeneity in productive traits and has been applied to several traits and species for which animal homogeneity is profitable. In horse breeding programmes, rank in competitions is a common selection objective but has been challenging to model. In this study, the parameters of environmental variability for the rank of a horse were computed to analyse the capability of a horse to maintain the best ranking across competitions that consist of long-distance races in which the adapted physical condition of the horse is essential. The genetic component of the environmental variance for the rank in endurance competitions was evaluated, which resulted in proposing a new transformation of horse scores in competitions.

Results

Homogeneous and heterogeneous variance models were compared by assaying three random effects that affect both the rank and its variability, using endurance ride data consisting of 2863 records. The pedigree relationship matrix contained 5931 animals. The rank trait was transformed into a normalized variable to prevent false estimates of the genetic correlation by inappropriate artificial skewness. The models included the number of participants in the race, sex, and age as systematic effects. The rider, the rider-horse interaction, or an environmental permanent effect were tested as random effects, in addition to additive genetic and residual effects. The models were analysed using the GSEVM program. Estimates of heritability for rank ranged from 0.12 to 0.15. The heterogeneous variance model that fitted the rider was assessed as the best model based on the deviance information criterion. Estimates of genetic variance for rank variability ranged from 0.12 to 0.13. The genetic correlation between the rank and its environmental variability was low and did not differ from 0.

Conclusions

These results offer an opportunity to select animals for canalization by reducing the variability of race results and achieving the best positions, which could be a new selection objective by weighting estimated breeding values for rank and its variability in a selection index.

The Statistical Scale Effect as a Source of Positive Genetic Correlation Between Mean and Variability: A Simulation Study

The selection objective for animal production is the highest income with the lowest production cost, while ensuring the highest animal welfare. A selection experiment for environmental variability of birth weight in mice showed a correlated response in the mean after 20 generations starting from a crossed panmictic population. The relationship between the birth weight and its environmental variability explained the correlated response. The scale effect represents a potential cause of this correlation. The relationship between the mean and the variability implies: the higher the mean, the higher the variability. The study was to quantify by simulation the genetic correlation between a trait and its environmental variability. This can be attributable to the scale effect in a range of coefficients of variation and heritabilities between 0.05 and 0.50. The resulting genetic correlation ranged from 0.1335 to 0.7021 being the highest for the highest heritability and the lowest CV. The scale effect for a trait with heritability between 0.25 and 0.35 and CV between 0.15 and 0.25 generated a genetic correlation between 0.43 and 0.57. The genetic coefficient of variation (GCV) affecting residual variability was modulated by the strength reducing the impact of the scale effect. GCV ranged from 0.0050 to 1.4984. The strength of the scale effect might be in the range between 0 and 1. The scale effect would explain many reported genetic correlation and the additive genetic variance for the variability. This is relevant when increasing the mean of a trait jointly with the reduction of its variability.

Effect of feed restriction on the environmental variability of birth weight in divergently selected lines of mice

BACKGROUND

Selection of mice for decreased environmental variability of birth weight has achieved higher survivability and larger litter size as a correlated response to canalized selection, which suggests higher welfare and robustness, and animals that are more homogeneous. However, in these studies, animals were not exposed to an environmental challenge. To demonstrate the advantages of this mouse line with a low environmental variability of birth weight, animals from two divergent lines (high and low variability of birth weight) were subjected to feed restriction. The objective of this study was to use these divergent lines to compare their response in terms of robustness against an environmental challenge. At weaning, 120 females, i.e. four full-sib females from 10 random litters of three consecutive generations of selection, were chosen from these divergent lines. The total number of females was divided into four groups, which were subjected to a feeding regimen by imposing different levels of feed restriction (i.e. 75, 90 and 85% of full ad libitum feed across three generations, respectively) in different combinations during the growth and reproduction periods.

RESULTS

Animals from the "low" line were less sensitive to a change in feed level than those from the "high" line. Regarding reproduction, the "low" line performed better in terms of number of females having parturitions, number of parturitions, and litter size. Imposing a feed restriction on female mice during their growth period did not affect the birth weight of their pups. The "low" line was preferred because of its higher reproductive efficiency and survival under an environmental challenge.

CONCLUSIONS

Selection for decreased environmental variability of birth weight produces animals that are less sensitive to environmental conditions, which can be interpreted as having greater robustness.