Sheep GEMS News Brief 7 – October 2024
Genetic diversity is what allows for adaptation and evolution in response to change, whether that change is environmental or consumer driven. As we look across the array of domestic sheep breeds throughout the U.S., the genetic diversity is clear. From fine-wool to long-wool, hair to dairy, and all in-between, there is a wide variety of diversity that exists in sheep breeds. While the differences among breeds can be stark, shared characteristics among individuals are what define a breed. Yet within a breed, no two individuals are the same as considerable variation exists among individuals. That individual variation allows breeders to select animals with traits that are better suited for their production system and environment. Selection is a powerful tool which can transform generations, but there is delicate balance between the desire to have uniform animals to produce a consistent product and the need to have enough genetic variability from which to select those “best” animals.
Being able to “see” the genetic diversity that exists across breeds is straightforward. Tools, such as estimated breeding values (EBV) produced by the National Sheep Improvement Program, allow producers to take advantage of individual differences within a breed to select optimal breeding candidates. By considering an animal’s relationships and their phenotypes, EBV help distinguish those animals predicted to be superior in their performance. A higher resolution is being able to “see” genetic diversity at the molecular level, which can be achieved through evaluating genomic markers in the form of single nucleotide polymorphisms (SNP). Integrating these SNP into evaluation tools has led to the development of genomically-enhanced estimated breeding values (GEBV).
Conservation of genetic diversity is usually a priority among breeders of rare breeds or those with low population numbers. In these cases, maintaining multiple genetic lines, focusing on degree and rate of inbreeding, and carefully planning mate selection may be more important than genetic selection for improving production traits. In most major breeds with large population sizes, the loss of genetic diversity driven by selection is rarely considered until detrimental effects appear. While adopting genomic selection has the potential to double the rate of genetic gain, it also has the potential to double the rate of genetic loss in the population. Additionally, the use of GEBV in other species has led to major improvements in production traits yet negative effects on correlated traits, such as health and reproduction. An awareness of these pitfalls allows the sheep industry an opportunity to make better decisions with the use of GEBV.
Due to the potential loss of genetic variation from selection, the Sheep GEMS consortium has characterized the current genetic diversity of the Suffolk, Polypay, Rambouillet, and Katahdin breeds. Evaluation of diversity for these breeds includes both pedigree- and molecular-based methods to fully describe the populations. These baselines of diversity will be references for future diversity evaluations to assess if there has been any significant loss of variation over generations. Stay tuned for an upcoming article describing the extent of genetic diversity we have found in these breeds currently.
For further information contact Dr. Carrie Wilson (carrie.wilson@usda.gov).
Acknowledgements. We thank U.S. sheep associations and breed organizations, the National Sheep Improvement Program, and sheep producers, for their contributions to this research. This work was supported by the Organic Agriculture Research and Extension Initiative (grant 2016-51300-25723/project accession no. 1010329), and by the Agriculture and Food Research Initiative Competitive Grant (grant 2022-67015-36073/project accession no. 1027785), from the USDA National Institute of Food and Agriculture. The USDA is an equal opportunity provider and employer. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the USDA.
