Molecular technologies have allowed the generation of reagents and diagnostic kits previously unavailable for livestock. This includes reagents for diagnostic enzyme-linked immunosorbent assay kits, as well as reverse transcription-polymerase chain reaction and standard PCR. The latter two have been used for identification of viral and microbe infection (e.g., screening pig semen for porcine reproductive and respiratory syndrome virus), as well as genetic diagnosis of embryonic sex prior to embryo transfer.
Cell Biology Technology
Cell biology is being used to examine sperm membrane compositional changes during capacitation and the acrosome reaction to assess sperm quality and to enhance the cryopreservation of livestock sperm and embryos.
Fluorescence-activated cell sorting was used for the commercialization of “sexed” semen, allowing the dairy industry to obtain greater numbers of female offspring for milk production, and helping the beef industry obtain more male offspring for the efficient production of nutrient-dense meat for human consumption.
Although the complete genomic sequences for many livestock species are yet to be completed or released, considerable effort is being directed toward identifying quality trait loci and single nucleotide polymorphisms that can be used to enhance genetic selection. At commercial artificial insemination companies, the use of single nucleotide polymorphism chip analysis of sires is becoming routine.
Genetic sex determination currently is utilized, as well as limited “marker assisted selection” for production traits, such as muscle development and intramuscular fat deposition (i.e., meat quality). With second-generation, solid-state DNA sequencing now available, complete transcriptome analysis of various production efficiency traits is on the horizon.
|Representation of somatic cell nuclear transfer. Somatic cells are transferred into the perivitelline space of an MII stage-enucleated oocyte, fused and activated, and either cultured or immediately transferred into a recipient.
The generation of lines of transgenic livestock has not met the expectations initially anticipated, primarily because of low technical efficiency and long generation intervals. It was hoped that transgenic lines could be developed that were either disease-resistant; that produced valuable products (e.g., pharmaceuticals) that could be harvested from milk, blood or eggs in large quantities or that exhibited decreased excretion of compounds detrimental to the environment.
Somatic cell nuclear transfer initially was applied to livestock, and although it has not had the anticipated commercial application, it has provided considerable insight into factors regulating early embryo development. Genetic manipulation of the “donor” cells is more efficient than standard transgenic approaches and has allowed the generation of SCNT-transgenic lines. Some of these are being developed as models for human disease and/or xenotransplantation.
For example, CFTR-null pigs, generated by SCNT, appear to be a better model for human cystic fibrosis than available mouse models, because the CFTR-null piglets develop more of the hallmark pathologies associated with cystic fibrosis in humans.