Genetic analysis of superovulation and embryo transfer traits in Holstein cattle

Abstract Text: The objectives of this study were to estimate variance components and investigate genomic regions of interest associated with superovulation and embryo transfer in dairy cattle. Superovulation and embryo transfer are methods commonly used by dairy producers to increase the rate of genetic gain achievable from superior females. A limiting factor of these reproductive technologies remains the variability of animal response to treatment. If some of this variability is attributable to genetics, selection for traits related to superovulation and embryo transfer may allow for further improvement. Data were collected from a Holstein dairy operation in Florida from 2008 through 2015, including 926 superovulation records (total number of structures recovered, total number of good embryos), 628 in vitro fertilization records (number of ooctyes recovered, number of cleaved embryos, number of high- and low-quality embryos, number of transferred embryos), and 12,399 embryo transfer records (pregnancy success). Two transformations of count data were compared: Anscombe and logarithmic. Univariate repeatability animal models were fitted for each trait of interest, with the exception of pregnancy success, for which a threshold liability model was used. For traits where a significant genetic component was estimated (total structures collected and number of good embryos), single-step genomic BLUP analyses were conducted using AI-REMLF90 (version 1.116).  postGSf90 (version 1.35) was used to calculate SNP effects and 10-SNP window variances. The two transformation methods produced very similar results. Significant genetic components were estimated for total number of structures recovered and number of good embryos in the superovulation dataset, with heritabilities of 0.31±0.07 and 0.21±0.06, respectively. Genetic components estimated from the in vitro fertilization dataset were not significantly different from zero. Heritability of recipient pregnancy success after embryo transfer was estimated to be 0.024 (SD = 0.01). The region explaining the largest proportion of variance for total structures collected in the superovulation data was located on chromosome 8, at 55,663,248 basepairs with additional peaks located on chromosomes 5, 13, 14, and 21. Similar regions were identified for total number of good embryos, with the largest proportion of variance explained by a region on chromosome 14 at 26,713,734 basepairs. Results indicate that these traits have a genetic component. Significant genomic regions can be further investigated for genes putatively associated with these traits.

Keywords: embryo transfer, genetic analysis, superovulation