Experience of genomic selection in the US

Genetic improvement of livestock during the second half of the 20th century using pedigree and performance data has been very successful, particularly in dairy cattle populations. The improvement of dairy cattle has depended heavily on the use of artificial insemination (AI) to maximize the impact of elite bulls globally. Historically, progeny testing, or the characterization of these AI bulls by measuring and comparing performance of daughters, has been a critical step in identifying the very best bulls for widespread use. However, traditional genetic improvement schemes in dairy cattle have been limited by time required and expense of the progeny test paradigm. This process remained relatively slow because of the substantial time needed to accumulate sufficient daughter phenotypes to compute genetic evaluations with high accuracy. The recent development of genomic selection programs based on single-nucleotide polymorphism genotypes was expected to increase rates of genetic gain in several ways, including shortened generation interval(s) (GI) and increased reliability of predicted breeding value(s) (PBV). A doubling of rates of genetic gain was predicted when comparing genomic evaluations and traditional progeny testing schemes. These advantages have been demonstrated in simulations, and increased accuracies have been documented in the US Holstein population, but response to the incorporation of genomic data into dairy cattle evaluations has not been characterized. 

Rendel and Robertson described a four-path model of genetic improvement in which genetic progress occurs with differing selection dynamics, partitioned into improvement due to genetic changes in sire(s) of bulls (SB), sire(s) of cows (SC), dam(s) of bulls (DB), and dam(s) of cows (DC). The objective of this study was to measure the impact of genomic selection on SD and GI in US Holstein cattle using this four-path model, and to compare these observed results with those results predicted by theory. 

Genetic trends improved substantially, resulting in rapid genetic improvement in these fitness traits. It is possible, or even likely, that relative selection emphases changed as genomic selection altered rates of genetic gain. These results clearly demonstrate the positive impact of genomic selection in US dairy cattle, even though this technology has only been in use for a short time. It is unlikely that a state of equilibrium has been reached, and these estimates may not reflect the full impact of genomic selection. Based on the four paths of selection model, rates of genetic gain per year increased from 50–100% for yield traits and from three- to fourfold for lowly heritable traits. 

References: Changes in genetic selection differentials and generation intervals in US Holstein dairy cattle as a result of genomic selection/Adriana García-Ruiz, John B. Cole, Paul M. VanRaden, George R. Wiggans, Felipe J. Ruiz-López and Curtis P. Van Tassell//PNAS 2016 July, 113 (28) E3995-E4004. 

Rendel J.M., Robertson A./Estimation of genetic gain in milk yield by selection in a closed herd of dairy cattle. J Genet 50.  1950.  (1):1–8. 

 

Yulmeteva Yu.R. - Candidate of Biological Sciences, Senior Researcher 

02.03.2018, 120 просмотров.