A mutation creating a potential illegitimate microRNA target site in the myostatin gene affects muscularity in sheep.

Texel sheep are renowned for their exceptional meatiness. To identify the genes underlying this economically important feature, we performed a whole-genome scan in a Romanov x Texel F2 population. We mapped a quantitative trait locus with a major effect on muscle mass to chromosome 2 and subsequently fine-mapped it to a chromosome interval encompassing the myostatin (GDF8) gene. We herein demonstrate that the GDF8 allele of Texel sheep is characterized by a G to A transition in the 3' UTR that creates a target site for mir1 and mir206, microRNAs (miRNAs) that are highly expressed in skeletal muscle. This causes translational inhibition of the myostatin gene and hence contributes to the muscular hypertrophy of Texel sheep. Analysis of SNP databases for humans and mice demonstrates that mutations creating or destroying putative miRNA target sites are abundant and might be important effectors of phenotypic variation.

Genotype-level variation in lifetime breeding success, litter size and survival of sheep in scrapie-affected flocks.

Five different sheep flocks with natural outbreaks of scrapie were examined to determine associations between individual performance (lifetime breeding success, litter size and survival) and scrapie infection or PrP genotype. Despite different breed composition and forces of infection, consistent patterns were found among the flocks. Regardless of the flock, scrapie-infected sheep produced on average 34 % fewer offspring than non-scrapie-infected sheep. The effect of scrapie on lifetime breeding success appears to be a function of lifespan as opposed to fecundity. Analysis of litter size revealed no overall or genotype differences among the five sheep flocks. Survival, however, depends on the individual's scrapie status (infected or not) and its PrP genotype. Susceptible genotypes appear to perform less well in lifetime breeding success and life expectancy even if they are never affected with clinical scrapie. One possible explanation for these results is the effect of pre-clinical scrapie. Additional evidence supporting this hypothesis is discussed.

Nematode parasites and scrapie: experiments in sheep and mice.

To demonstrate the possible role of nematode parasites in the modification of host susceptibility to scrapie, experiments were conducted using sheep naturally exposed to scrapie, chosen by their genotype at the PrP gene, and infected with Teladorsagia circumcincta. Two 4-year duration experiments demonstrated that the nematode infection shortened the development of scrapie with a significant regression between the level of infection and age at first scrapie symptoms (P < 0.006). Investigations by ELISA tests in different species of nematode parasites of the digestive tract collected from scrapie infected ewes did not reveal the presence of PrPSc. In scrapie-infected C57BL mice, infected or not with Heligmosoides polygyrus at various times, parasitized animals showed a slight but significantly longer survival period. Assays on transmission by the larvae hatching from eggs collected from scrapie-infected mice were unsuccessful. We concluded that nematodes modify host susceptibility to scrapie, but their role in the horizontal transmission of the disease was not demonstrated.

A short-term divergent selection for resistance to Teladorsagia circumcincta in Romanov sheep using natural or artificial challenge.

This experiment was conducted to assess the efficiency of selection on the basis of response to artificial challenges in order to breed sheep resistant to natural infection. A short-term divergent selection process was designed to estimate the genetic parameters of these two traits. Two flocks, including 100 Romanov ram lambs each, were challenged in 1990 when they were 6 months old. One flock received three artificial infections with 20 000 third-stage Teladorsagia circumcincta larvae, at intervals of 7 weeks. Faecal egg counts (FEC) were performed on Days 22, 25 and 28 post infection (p.i.) and the animals were drenched on Day 28 p.i. The other flock was grazed for 5 months on a pasture contaminated with the same species. Faecal samples were taken from the lambs at similar ages. About 5 rams with the lowest FEC and 5 with the highest FEC were selected in each flock and mated with unselected ewes. Their offspring (200 animals) were challenged in 1992, half in the same way as their sires, and the other half by the other method. Because of a drought in the summer of 1990, it was necessary to repeat part of the experiment, and in 1992 the 5 and 8 rams with the lowest and highest FEC, respectively, were selected from the offspring challenged on the pasture in 1992 and were mated with unselected ewes. Their progeny (about 80 animals) were challenged in 1994, half by natural infection, half by artificial infection. The mean FEC of the flock increased from the first to the third artificial infection. The natural infection was highly variable in different years, reflecting the difficulty of assessing resistance using this mode of challenge. Genetic parameters were estimated using animal models and REML solutions. The repeatabilities of the FEC following artificial and natural infection were 0.49 and 0.70 respectively within a period of one week, and 0.22 and 0.41 respectively for periods separated by intervals of 7 weeks; the heritabilities of the single egg count were 0.22 and 0.38 respectively. The genetic correlation was 0.87: the FEC recorded under natural or artificial infection appear to depend on the same genetic potential.

PrP(Sc) accumulation in placentas of ewes exposed to natural scrapie: influence of foetal PrP genotype and effect on ewe-to-lamb transmission.

Placentas from scrapie-affected ewes are known to be infectious. Nevertheless, placenta infectivity in such ewes is not systematic. Maternal transmission to lambs is highly suspected but contamination of the foetus in utero has not been demonstrated. Using ewes from a naturally scrapie-infected flock, it was demonstrated that abnormal prion protein (PrP(Sc)) accumulation in the placenta (i) is controlled by polymorphisms at codons 136, 154 and 171 of the foetal PrP gene and (ii) is restricted mainly to placentome foetal trophoblastic cells. In order to go deeper into the role of the placenta in scrapie transmission, the pattern of PrP(Sc) dissemination was established in susceptible lambs (genotype VRQ/VRQ) sampled from 140 days post-insemination to the age of 4 months from either VRQ/VRQ ewes with PrP(Sc)-positive placentas or ARR/VRQ ewes with PrP(Sc)-negative placentas. In both VRQ/VRQ lamb groups, PrP(Sc) spatial and temporal accumulation patterns were similar, suggesting post-natal rather than in utero contamination.

Segregation of a major gene influencing ovulation in progeny of Lacaune meat sheep.

Inheritance of the ovulation rate (OR) in the Lacaune meat breed was studied through records from a small nucleus of 36 hyper-prolific ewes screened on farms on the basis of their natural litter size, and from progeny data of three selected Lacaune sires. These sires were chosen at the AI centre according to their breeding values estimated for the mean and the variability of their daughters' litter size. Non-carrier Lacaune dairy ewes were inseminated to produce 121 F1 daughters and 27 F1 sons. Twelve sons (four from each sire) were used in turn to inseminate non-carrier Lacaune dairy ewes providing 260 BC progeny ewes. F1 and BC progeny were brought from private farms and gathered after weaning on an experimental farm where ovulation rates were recorded in the first and second breeding seasons. With an average of 6.5 records each, the mean OR of hyper-prolific ewes was very high (5.34), and 38.4% of records showed a rate of 6 or more. F1 data showed high repeatability of OR (r = 0.54) within ewe, with significant variability among ewes. High OR (> or = 4) were observed in each family. A segregation analysis provided a significant likelihood ratio and classified the three founders as heterozygous. BC ewes also displayed high repeatability of OR (r = 0.47) and the mean OR varied considerably between families (from 1.24 to 1.78). Seven of the 12 BC families presented high-ovulating ewes (at least one record > or = 4) and segregation analysis yielded a highly significant likelihood ratio as compared to an empirical test distribution. The high variability of the mean ovulation rate shown by a small group of daughters of BC ewes inseminated by putative carrier F1 rams, and the very high ovulation rate observed for some of these ewe lambs, confirmed the segregation of a major gene with two co-dominant alleles borne by an autosome. The difference between homozygous non-carriers and heterozygous ewes was about one ovulation on the observed scale and 2.2 standard deviations on the underlying scale.