Genetic Architecture Of Carcass Composition In Sheep As Inferred From Quantitative Trait Loci Meta-analysis And Cloning, Spatiotemporal Expression Profiles Analysis Of Genes Related To The Trait

Carcass composition is one of the most important economical traits in sheep breeding.It is also relatively easy to score when compared to other physiological traits that require heavy experimentation,so that several studies have addressed QTL mapping populations.Synthesizing this information provides a unique opportunity to understand the genetic variation of carcass composition within a large range of diversity.Genetic architecture of carcass composition in sheep was addressed by synthesizing a total of 156 quantitative trait Loci(QTL) available for this trait. These were analyzed first with a projection,and then with a meta-analysis method that yielded a synthetic genetic model with 12 consensus QTL.Meta-analysis led in this case to an increase in the precision in QTL position estimation,when compared to initial QTL position within the corresponding region.The MQTL were compared first to the positions of the few carcass composition candidate genes that have been mapped in sheep.We then projected cattle candidate genes onto the sheep genome using a comparative mapping,analyzed the association of QTL with candidate genes.A total of 156 QTL from 13 studies.Collectively,these studies include data from 9 different breeds and 9127 individual animals.12 were carcass weight(CW),50 were fatness (FAT),33 were live weight(LW),61 were muscle(MUS).A total of 156 QTL have been projected onto the reference map.The QTL for carcass composition are not distributed within the whole sheep genome.All the QTL project to 1,2,3,4,5,6,8,11,18,20 and 21 chromosomes.There was a significant imbalance in QTL breakdown between the 11 chromosomes.Chromosome 2 and 18 together accounted for 56%of QTL.Each 2 and 18 chromosome contained more than 25 QTL, but otherwise chromosome 4,8,11 and 21 contained no more than three QTL.Individual QTL clearly be pooled in one cluster.Of the 156 QTL that could be projected on the reference map,10 were alone in the linkage group,so 146 were candidates for aggregation in the so-called MQTL using a meta-analysis approach.Meta-analysis resulted in 12 MQTL.The most accurate MQTL were located on 2 and 18 chromosomes,with CI values of 4.37 cM,8.98 cM and 4.36 cM, respectively.With respect to the reduction in the length from mean initial to meta-QTL CI,the gain in accuracy ranged from 0.85 cM-72.65 cM.Therefore increases the precision of QTL mapping,which facilitates the identification of relevant candidate genes.Chromosome 1 has been identified two MQTL,MQTL1(215.86 cM) and MQTL2(294.37 cM).Chromosome 2 has been identified three MQTL,MQTL3(95.41 cM),MQTL4(143.44 cM) and MQTL5(244.58 cM). Chromosome 3 has been identified two MQTL,MQTL6(19 cM) and MQTL7(243 cM). Chromosome 5 and Chromosome 6 have been identified only one MQTL,MQTL8(0.23 cM) and QTL9(1.06 cM),respectively.Chromosome 18 has been identified two MQTL,MQTL10(76.18 cM)å’ŒMQTL 11(100.4 cM).Chromosome 20 has been identified one MQTL,QTL 12(65.26 cM). Loci involved in carcass composition defined by the meta-analysis method are associated with only two sheep genes already know.We then projected 13 cattle candidate genes onto "Virtual Sheep Genome".This yielded 3 associations between sheep QTL and genes involved in carcass composition in cattle.The Myostatin(MSTN) which was associated the MQTL4 and the myogenic regulator factors(MRFs) which were not associated the MQTL were selected as candidate genes. To investigate the muscle developmental and organizational expression patterns of Myostatin (MSTN),myogenic determining factor(MYOD),myogenin(MYOG),myogenic factor5(MYF5) and myogenic factor5(MYF5),Real-time fluorescence quantitative PCR was applied.Understanding how these factors function requires a detailed expression of these factors in postnatal sheep muscle. Results showed that:the expression level of MSTN mRNA in skeletal muscle was significantly higher than cerebrum,heart and liver(P<0.05).The expression level of MSTN mRNA in liver was significantly lower than cerebrum and skeletal muscle(P<0.05).The expression level of MSTN mRNA in cerebrum and heart were no significantly difference.After birth,MSTN mRNA abundance gradually decreased to its lowest level of expression when sheep were 60 days,but returned back to peak between 60 days and 105 days.And then plateaued between 105 days and 195 days.Thereafter,mRNA decreased again.We here report that MYOD,MYOG and MYF5 transcripts are expressed at high levels in the longissimus of newborn lamb and their level of expression continuously declines throughout postnatal life.MYF6 transcript,on the other hand,is present at a constant level through the life span of the study.This result indicated that this level of MYF6 expression is maintained in the adult.Differences in the expression patterns for MYF5, MYOD,MYOG and MYF6 between skeletal muscle development suggest that the relative timing of expression for each muscle regulatory factor may control the distinct phenotypes associated with myotomal myocytes and multinucleate myofibers.And we obtained the corresponding complete CDS sequences in sheep by RT-PCR method.The homology of MYOG and MYF6 nucleotide sequences between Bos Taurus and sheep were 96%and 99%,respectively. These results highlight five possible candidate genes which associated with MQTL for sheep carcass composition.Results suggest that the combination of meta-analysis within a species of interest and comparative genome map can be an efficient strategy for identifying new candidate genes for trait variation.Markers at this MQTL should prove helpful in marker-assisted selection.