Heterogeneous variances and genetics by environment interactions in genetic evaluation of crossbred lambs.

Accounting for environmental heteroscedasticity and genetics by environment interaction (G×E) in genetic evaluation is important because animals may not perform predictably across environments. The objectives of this study were to evaluate the presence and consequences of heteroscedasticity and G×E on genetic evaluation. The population considered was crossbred lambs sired by terminal sires and reared under commercial conditions in the UK. Data on 6325 lambs sired by Charollais, Suffolk and Texel rams were obtained. The experiment was conducted between 1999 and 2002 on three farms located in England, Scotland and Wales. There were 2322, 2137 and 1866 lambs in England, Scotland and Wales, respectively. A total of 89 sires were mated to 1984 ewes of two types (Welsh and Scottish Mules). Most rams were used for two breeding seasons with some rotated among farms to create genetic links. Lambs were reared on pasture and had their parentage, birth, 5 week, 10 week, and slaughter weights recorded. Lambs were slaughtered at a constant fatness, at which they were ultrasonically scanned for fat and muscle depth. Heteroscedasticity was evaluated in two ways. First, data were separated into three subsets by farm. Within-farm variance component estimates were then compared with those derived from the complete data (Model 1). Second, the combined data were fitted, but with a heterogeneous (by farm) environmental variance structure (Model 2). To investigate G×E, a model with a random farm by sire (F×S) interaction was used (Model 3). The ratio of the F×S variance to total variance was a measure of the level of G×E in the population. Heterogeneity in environmental variability across farm was identified for all traits (P<0.01). Rank correlations of sire estimated breeding value between farms differed for Model 1 for all traits. However, sires ranked similarly (rank correlation of 0.99) for weight traits with Model 2, but less so for ultrasonic measures. Including the F×S interaction (Model 3) improved model fit for all traits. However, the F×S term explained a small proportion of variation in weights (<2%) although more in ultrasonic traits (at least 10%). In conclusion, heteroscedasticity and G×E were not large for these data, and can be ignored in genetic evaluation of weight but, perhaps, not ultrasonic traits. Still, before incorporating heteroscedasticity and G×E into routine evaluations of even ultrasonic traits, their consequences on selection response in the breeding goal should be evaluated.

Genetic evaluation of days to harvest in crossbred lambs.

Days to harvest (DTH) is the number of days a lamb is fed before reaching a target level of fatness. Although economically relevant, this trait has not been thoroughly evaluated in sheep. Most lambs harvested in the United Kingdom are crossbreds sired by purebred terminal sires, with Charollais, Suffolk, and Texel most commonly used. Sires from these breeds were selected on an index designed to increase lean growth while constraining fat. The purpose of this research was to 1) evaluate the effects of index selection in terminal sires on DTH and 2) evaluate the feasibility of incorporating DTH into genetic evaluation programs. Charollais, Suffolk, and Texel sheep had participated in sire referencing schemes where genetic links among flocks were established by sharing rams. Rams with high or low index scores were chosen from these schemes and mated to crossbred ewes at 3 farms in the United Kingdom. Lambs were harvested at a target 11% subcutaneous fat. Records on DTH from 6,350 lambs were analyzed in 2 ways: 1) as time to harvest fitting a survival model and 2) as a normally distributed variable in a bivariate analysis with weight at harvest. The survival analysis was stratified by rearing type (single or twin). In both approaches, sires were fitted using a multivariate normal distribution with a relationship matrix. Regardless of model fitted, sire index did not affect DTH (P>0.10). However, Texel-sired lambs reached harvest faster (P<0.01) than either Charollais- or Suffolk-sired lambs although DTH in those 2 breed types did not differ (P>0.1). Ewe lambs reached harvest faster than wethers (P<0.01). Lambs from older ewes were harvested faster (P<0.001). The heritability of DTH was 0.21 from the survival model and 0.20 from the bivariate model. Rank correlation of sire EBV between methods was 0.9, suggesting strong agreement. The use of high or low index sires did not extend DTH in lambs harvested at a target fatness. Importantly, there is no antagonism between improving carcass merit and extending the grazing season. Furthermore, DTH is moderately heritable. If economically justified within a breeding program, it could be reduced through genetic selection.

Index selection in terminal sires improves lamb performance at finishing.

Lamb meat is often perceived by consumers as fatty, and consumption has decreased in recent decades. A lean growth index was developed in the UK for terminal sire breeds to increase carcass lean content and constrain fat content at a constant age end point. The purposes of this study were 1) to evaluate the effects of index selection of terminal sires on their crossbred offspring at finishing and 2) to evaluate its effectiveness within terminal sire breeds. Approximately 70% of lambs marketed in the UK have been sired by rams of breeds typically thought of as specialized terminal sires. The most widely used are Charollais, Suffolk, and Texel. These breeds participated in sire referencing schemes from the early 1990s by sharing rams among flocks selected on the lean growth index. From 1999 to 2002 approximately 15 "high" and 15 "low" lean growth index score rams were selected from within their sire referencing schemes and mated to Welsh and Scottish Mule ewes. Their crossbred offspring were commercially reared on 3 farms in the UK. Lambs were finished to an estimated 11% subcutaneous fat by visual evaluation. At finishing, lambs were weighed, ultrasonically scanned, and assessed for condition score and conformation. Records were obtained for 6356 lambs on finishing BW (FWT), ultrasonic muscle depth (UMD), ultrasonic fat depth, overall condition score (OCS), and conformation of gigot, loin, and shoulder. Ultrasonic fat depth was log transformed (logUFD) to approach normality. High-index-sired lambs were heavier at finishing (1.2±0.2 kg) with thicker UMD (0.7±0.2 mm) and less logUFD (0.08±0.01 mm; P<0.05). There were no differences in OCS or conformation based on the sire index or breed (P>0.08). Suffolk-sired lambs were heavier than Charollais (1.0±0.3 kg), which were heavier than Texel (0.9±0.3 kg; P<0.001). Texel-sired lambs had thicker UMD than Charollais (0.7±0.2 mm; P<0.001) but were not different than Suffolk. Charollais-sired lambs had greater logUFD than both Texel (0.098±0.016 mm) and Suffolk (0.061±0.017 mm) sired lambs (P<0.001). Within a breed, high- and low-index-sired lambs differed in performance with the exceptions of FWT and UMD in Suffolks. Index selection produced heavier and leaner lambs at finishing. Producers have flexibility in choosing the terminal sire that best fits their production system.

Index selection in terminal sires improves early lamb growth.

The use of terminal sires (TS) for crossbreeding is integral to the UK sheep industry where approximately 71% of market lambs are sired by TS rams. Early growth of these crossbred lambs affects profitability. The objectives of this study were i) to evaluate the effectiveness of index selection among TS on BW and ADG of their crossbred offspring; and ii) to compare the efficacy of that selection within TS breeds. The most widely used TS breeds in the United Kingdom are Charollais, Suffolk, and Texel. These participated in sire referencing schemes in which they were evaluated on a lean growth index designed to increase carcass lean weight at a given age. From 1999 to 2002, approximately 15 high and 15 low lean growth index rams per breed (93 in total, differing in index on average by 4.6 SD) were selected from within their sire referencing schemes and mated to Welsh and Scottish Mule ewes. Their crossbred offspring were reared commercially on 3 experimental farms in England, Scotland, and Wales. A total of 6,515 lambs were born between 2000 and 2003. Lambs were weighed at birth (BWT), 5 wk (5WT), and 10 wk (10WT), and their ADG from birth to 10 wk was calculated. Lambs sired by high index rams were on average, across breeds, heavier at all ages (P < 0.01) with 0.07 ± 0.03, 0.3 ± 0.1, and 0.4 ± 0.1 kg greater BWT, 5WT, and 10WT, respectively. Their ADG was 5.1 ± 1.9 g/d greater than low-index-sired lambs (P < 0.01). Suffolk-sired lambs were on average heavier at all ages, with greater ADG, whereas Charollais-sired lambs were lightest with smallest ADG. Overall, there was no significant interaction between sire index and sire breed (P > 0.10). Within Suffolk-sired lambs, there was little difference between high and low index sires for the traits studied (P > 0.3). High and low index Charollais-sired lambs differed in BWT (0.09 ± 0.04 kg) and 5WT (0.3 ± 0.1 kg), and Texel-sired lambs differed in 5WT (0.5 ± 0.1 kg), 10WT (0.9 ± 0.2 kg), and ADG (10.2 ± 3.3 g/d; P < 0.01). Lambs from Scottish Mule dams were heavier, with greater ADG, than lambs born to Welsh Mules (P < 0.01). Lambs reared in Scotland were heavier at all ages (P < 0.01). The results suggest that using index selection in TS can improve the growth of their commercial offspring reared on grass.

Genetic diversity and population structure in lines of chickens divergently selected for high and low 8-week body weight.

A long-term selection experiment for high or low 8-wk BW in White Plymouth Rock chickens was conducted to study effects of selection on BW and correlated characters. Two lines [high (HWS), low (LWS) weight] were established and have undergone 48 generations of selection. The lines were managed to curtail inbreeding and to maintain similar population structures; such is necessary for equitable comparison of selection response between lines. Our objective was to test the success of that breeding strategy by characterizing genetic diversity and inbreeding in these lines. A pedigree of 5,998 individuals was assembled, with 68 founders, 2,962 HWS chickens, and 2,968 LWS chickens. Inbreeding coefficients (F) were calculated for each line. Maximum F was 0.53 and 0.61, mean F was 0.26 (SD 0.15) and 0.30 (SD 0.17), and change in F was 1.3 and 1.6% per generation in LWS and HWS lines, respectively. The effective population size was 38.3 in LWS and 32.1 in HWS lines. The effective number of founders was 15.7 in both lines, and the effective number of ancestors was 17.5 and 15.5 in LWS and HWS lines, respectively. Thirty ancestors accounted for 90% of the genetic makeup of both lines. Seven male and eight female founders still contributed to both lines at generation 48, although some contributed more to one line than the other. Family sizes were similar for males and females of each line, with males having larger family sizes with greater variance. Accumulated inbreeding was high and effective population size was low, as expected in closed lines. Effective number of founders was relatively low compared with actual number of founders, indicating some contributed more than others to the last generation. Family size statistics indicated that fewer males than females were used, leading to the observed levels of inbreeding. Given their similarity in genetic diversity and family size, it can be concluded that breeding decisions throughout the project resulted in similar population structures in the lines.

Genetic diversity and population structure of American Red Angus cattle.

The objective of this study was to characterize the population structure and genetic diversity of registered American Red Angus cattle. Inbreeding and average relationship coefficients, effective population size, effective number of founders, and effective number of herds supplying grandparents to the population were calculated from the recorded pedigree. Inbreeding in 1960 was 10.7% and decreased until 1974 at a rate of 0.2% per year, whereas in 1975 inbreeding was 3.2% and increased until 2005 at a rate of 0.02% per year. The numerator relationship coefficients of the 10 individual paternal grandsires (PGS; sires of sires), paternal granddams (PGD; dams of sires), maternal grandsires (MGS; sires of dams), and maternal granddams (MGD; dams of dams) that had the greatest number of registered grandprogeny, with all other registered animals, increased with their birth year from 1960 on. Average numerator relationships of these with all other PGS, PGD, MGS, MGD, bulls, and sires were greater for paternal (PGS, PGD) than maternal (MGS, MGD) pathways. The effective population size was 445, with 649 effective founders. The effective numbers of herds supplying PGS, PGD, MGS, and MGD were 435, 369, 453, and 459, respectively. Inbreeding is at a low level and the effective population size is large. The effective number of founders and effective number of herds supplying grandparents is small in relation to the total number of animals and herds, indicating the disproportionate influence of a few founders and herds on the genetics of the breed. The calculated parameters indicate satisfactory genetic diversity in American Red Angus cattle.

Quantitative trait loci with effects on feed efficiency traits in Hereford x composite double backcross populations.

Two half-sib families of backcross progeny were produced by mating F(1) Line 1 Hereford (L1) x composite gene combination (CGC) bulls with L1 and CGC cows. Feed intake and periodic weights were measured for 218 backcross progeny. These progenies were genotyped using 232 microsatellite markers that spanned the 29 BTA. Progeny from L1 and CGC females was analysed separately using composite interval mapping to find quantitative trait loci (QTL) affecting daily dry matter intake (DMI), average daily gain (ADG), feed conversion (FCR) and residual feed intake (RFI). Results from both backcrosses were pooled to find additional QTL. In the backcross to L1, QTL were detected for RFI and DMI on BTA11, FCR on BTA16, and ADG on BTA9. In the backcross to CGC, QTL were detected for RFI on BTA10, FCR on BTA12 and 16 and ADG on BTA15 and 17. After pooling, QTL were detected for RFI on BTA 2, 6, 7, 10, 11, 13 and 16; for FCR on BTA 9, 12, 16, 17 and 21; for ADG on BTA 9, 14, 15, 17; and for DMI on BTA 2, 5, 6, 9, 10, 11, 20 and 23.

[Relationship between HLA antigens and the HIV infection in patients from the state of Zulia]. / Estudio de la relación entre antígenos HLA y la infección por VIH en pacientes del estado de Zulia.

PURPOSE: Some genetic factors associated to the HLA system phenotypes may allegedly predispose to the development of infection in patients exposed to the human immunodeficiency virus (HIV). So the aim of this study was to assess if certain HLA antigens are positive or negative risk factors in the development of AIDS in Zulia State. PATIENTS AND METHODS: A total of 62 samples were studied, 31 from HIV seropositive subjects and 31 form healthy individuals. The patients were subclassified into four groups in accordance with Atlanta's CDC guidelines. Tests for histocompatibility including HLA-A-B-C, DR and DQ typing were performed with Terasaki's technique. VIH positivity was determined by ELISA and confirmed by Western Blot. The statistical evalub1p4n was performed with the chi 2 test for antigen frequency comparison, the relative risk (RR) was estimated with the Ryder and Svelgaard test, and the inferential analysis was made by means of non-parametric statistics. RESULTS: Most patients were included in CDC's groups II and IV, 48.4% and 29.0%, respectively. Increased B35 and DQw2 and decreased B39 and DR2 antigens were found when comparing the HLA distribution in the sample and the antigenic frequency of the population. RR > 1 was observed in the infected patients A for A1, A3, A10, A11, B5, B7, B12, B14, B35, B61, CW4, DR4, DRW52 and DQW2 HLA antigens. A positive association between symptomatic infected patients and antigen B35 was present (X = 7.045). CONCLUSION: The findings reported here suggest that antigen B35 is a major risk factor for the development of AIDS.