Since 2003, part of the Elsenburg Merino breeding herd has been subjected to structured reciprocal crosses within the breed. Prof. Dr. SWP Cloete was involved in the planning of the research, analysis and revision of the article.
ACKNOWLEDGEMENTS
General Introduction
- Introduction
- Motivation
- Study objectives
- References
A key finding was that survival of lambs in the H-line was improved relative to the L-line due to the improved survival of multiples (Cloete and Scholtz 1998; A study of the molecular divergence between the H- and L- lines using randomly amplified polymorphic lambs). DNA (RAPD) testing is described in detail in Chapter 6.
Literature Study
- Introduction
- Reproduction in sheep
- Lamb survival and maternal effects
- The Elsenburg Merino Resource flock
- Inbreeding
- Heterosis
- Molecular marker technology
- Hypotheses
- References
This initial linkage map of the sheep genome has since been revised (de Gortari et al. 1998; . The ISGC is currently in the process of creating a draft reference genome sequence of the sheep (Archibald et al. 2010). In 'Proceedings of the third international workshop on important genes and QTL in sheep and goats'.
In '6. Proceedings of the Australian Society for the Advancement of Animal Breeding and Genetics.
Genetic trends for reproduction in a Merino flock divergently selected for their ability to rear multiples
- Abstract
- Introduction
- Materials and Methods
- Results
- Discussion
- Conclusions
- References
Genetic trends in the population under consideration must be monitored to determine the effectiveness of a breeding program (Van Wyk et al. 1993). The aim of the study was to derive updated genetic trends for reproductive traits in the diversely selected Merino lines. Replacements in the low (L) line are preferably descended from ewes that reared less than one lamb per mating (barren or lost all lambs born at least once).
In line L, 46 ewes rearing one to three lambs over five calving opportunities were selected. Due to inbreeding in the H line (Cloete 2002) it was decided to also use industrial rams in this line. Genetic variation per year amounted to 1.9% and 2.1% of the respective phenotypic means for the number of lambs weaned and the corrected weight of lamb weaned per ewe in the H line, respectively.
Progress for number of lambs weaned in the H line tended to decline after 1999, although the regression did not differ from zero. Furthermore, commercial rams with lower in-flock breeding values than those of the home-bred rams were imported in 2008 to ensure that inbreeding in the H-line remains in check. The genetic trends show a decrease in the response to selection for corrected TWW and NLW in the H line.
Inbreeding trends and pedigree analysis of the Elsenburg Merino resource flock
- Abstract
- Introduction
- Materials and Methods
- Results
It is therefore important to know the effective size of a population (Ne) and the inbreeding rate (AF), which is the relative increase in inbreeding per generation (Boichard et al. 1997). Genetic trends presented in Chapter 3 show a decline in response to selection in the H-line of the Elsenburg Merino Resource herd, which may be the result of inbreeding. Ram replacements were selected to represent all bulls in their offspring group where possible (Cloete et al.
Generation intervals (GI) were calculated as the average age of parents at the birth of their offspring that were kept for reproduction (Gutierrez et al. 2010). Inbreeding coefficients were calculated for all animals in the database, using the rapid tabulation method (Groeneveld et al. 2009). The genetic contributions of founders and progenitors and summary statistics from the pedigree analysis of the H and L lines of the Elsenburg Merino resource herd are shown in Table 4.2.
However, the average inbreeding of the L-line has increased rapidly in recent years. The inbreeding percentage in the H and L lines for the period from 1979 to 2011 is shown in figure. The total number of births and inbreeding births of animals in the H and L lines of the Elsenburg Merino source herd is shown in figure.
High line
Low line
Discussion
Initially, inbreeding in the H lineage appeared to increase slightly faster than in the L lineage, until a rapid increase in the L lineage in 2011 (Fi = 9%). The implementation of MOET in the H line in 1991-1992 and in both lines in 2009 served to mitigate the effects of inbreeding. As of 2008, it has been possible to control inbreeding in the H line by introducing outbred outbreeds from the industry into the line.
All subsequent matings therefore took place within the L-line, which explains the high level of inbreeding in the L-line after 2009. The number of inbred sires and dams reached no less than 100% in the L-line in 2011, with only three bulls used to maintain the 23 dams. The effective population size is the number of individuals that would give rise to the observed or calculated inbreeding rate if bred in the manner of the idealized population (Falconer and Mackay 1996).
The concentration of animal origin is given by the effective number of founders (fe), 48 for the H line and 90 for the L line, and the effective number of ancestors (fa), 322 for the H line and 227 for the L line. A ratio greater than one indicates that bottlenecks have played a role in population formation (Fair et al. 2012). The H line is close to unity, but the L line's ratio indicates that bottlenecks may have played a role in the formation of the population.
Conclusions
Cloete S, van Niekerk F, Rust J (1998b) Use of embryo transfer to improve multiple breedability in medium wool merinos. Erasmus G, De Lange AO, Delport G, Olivier J (1991) Presence of inbreeding during a selection experiment with Merino sheep. Erckanbrack S, Knight A (1991) Effects of inbreeding on reproduction and wool production of Rambouillet, Targhee and Columbia ewes.
Gutierrez J, Goyache F, Cervantes I (2010) ENDOG v4.8 A computer program for monitoring the genetic variability of populations using pedigree information - User's guide. Universidad Complutense de Madrid: Madrid, Spain). Naidoo P, Cloete S, Fossey A (2005) South African Merinos are divergently selected for multiple rearing ability: a preliminary investigation of divergence based on RAPD markers. Norberg S, Sorensen A (2007) Inbreeding trend and inbreeding depression in the Danish populations of Texel, Shropshire and Oxford Down.
Scholtz A, Cloete S, van Wyk J, Kruger A, Van der Linde TdK (2010) Influence of divergent selection for reproduction on the incidence of breech impaction in mature Merino ewes. Van Wyk J, Fair M, Cloete S (2009) Case study: The effect of inbreeding on the production and reproductive traits of the Elsenburg Dormer sheep stud. Evaluation of reciprocal crosses and pure lines of Merino lambs and reproductive Merino ewes divergently selected for.
Evaluation of reciprocal crosses and pure lines of Merino lambs and reproducing Merino ewes divergently selected for
- Abstract
- Introduction
- Material and Methods
- Results
- Discussion
- Conclusions
- References
Further, previous studies of Elsenburg Merino stock flocks divergently selected for the ability of ewes to multiply per union have shown that lamb mortality differed between selection lines (Cloete and Scholtz 1998; Cloete et al. 2009a ). The purpose of this investigation is to evaluate reciprocal crosses between H and L lines of the Elsenburg Merino stock flock in terms of lamb survival, lamb weight traits, reproductive performance and heterosis. Traits that were evaluated included: number of lambs born and weaned per mated ewe, total weaned lamb weight per mated ewe, and mean weaned lamb weight at ~100 days of age, treated as a ewe trait .
To estimate possible heterosis, linear contrasts were calculated between the midparent value (i.e. the average performance of the H and L lines) and the average performance of the reciprocal cross between them according to Harvey (1990). When the contrast between the two purebred lines and the reciprocal cross was considered, there was a tendency (P for the mean performance of the reciprocal cross to be slightly below the midparent value of the two purebred lines). The lower birth weight of the offspring of H-line ewes mated with L-line rams compared to the three other genotypes (including the pure L-line) cannot be clearly explained.
This study found no evidence of heterosis for birth coat score in the reciprocal crosses of the lines. The mean survival of the two crossing lines was remarkably superior to the midparent value in absolute terms, although the contrast did not reach significance. This result is consistent with previous studies of the Elsenburg Merino resource flock (Cloete and Scholtz 1998; Cloete 2002).
Molecular divergence of the Elsenburg Merino flock based on RAPD markers
- Abstract
- Introduction
- Materials and methods
- Results
- Discussion
- Conclusions
- References
To elucidate the phenotypic divergence between the lines, the records of the two most recent groups of ewes present for three lambing occasions (considered a proxy for lifetime reproduction for this study) were analysed. 10 μl of purified sample DNA and 2 μl of loading buffer were loaded into each subsequent well. DNA purity was evaluated by pellet color in the rehydration step of the DNA purification process.
RAPD assays performed included selection of RAPD primers; subsequent amplification of dominant loci and electrophoresis to generate RAPD profiles. The population genetics analysis package POPGENE (Yeh et al. 1999) was used for the analysis of the RAPD assay data, using the tools for evaluating diploid dominant data of single and multiple populations. The Gentra Puregene DNA Purification Kit was found to be most effective on frozen blood samples and was then used for most extractions.
However, none of the RAPD bands amplified in this study corresponded to the alleles reported by Cushwa and Medrano (1996). A more precise determination of the molecular genetic differentiation between H and L lines could be performed using SNP chip technology, as well as possible detection of the effect of selection on reproductive traits at the molecular level. In 'Proceedings of the 36th National Congress of the South African Society for Animal Science' 36, 65-68.
General Discussion, Conclusions and Recommendations
Evaluations of the mutual crosses and pure lines of the variously selected Elsenburg Merinos are provided in Chapter 5. Furthermore, it is assumed that traits with lower heritability such as reproduction and fitness may exhibit higher levels of heterosis. Lamb survival appears to benefit from individual heterosis, although this result was not significant in this study.
The number of lambs born per mated ewe is a good representation of flock productivity and significant levels of heterosis for this trait, as determined in this experiment. It is acknowledged that the sample size used in this experiment (Chapter 5) is relatively small, and a larger data set would allow a more accurate estimate of the intersections; however, it is not expected to significantly change the results obtained. It is also noted that since inbreeding levels were within acceptable levels for the period in which the crosses were evaluated, it is unlikely to have played a role in this experiment.
Returning to the third hypothesis presented at the beginning of this study (Do reciprocal crosses perform better than the average of pure lines?) it is now possible to state that moderate levels of heterosis were observed for lamb mortality and lamb numbers hatched for mated sheep. Total lamb weaning weight of reciprocal crosses also tended to be slightly better than the median value, but did not reach significance. Although the derived levels of within-breed heterosis did not always reach significance, there is ample evidence from the interbreeding literature that such effects occur in sheep.
