Transcriptomes of male and female bovine embryos in the early elongation stage - environmental mirrors or indicators of future vitality?

Abstract Authors: Michael Hoelker¹; Urban Besenfelder³; Vitezslav Havlicek³, Eva Held-Hoelker^1,2; Dawit Tesfaye⁴; Dessie Salilew-Wondim^1,2

1.         Department of Animal Sciences, University of Goettingen, Goettingen, Germany

2.         Institute of Animal Sciences, University of Bonn, Bonn, Germany

3.         Institute for Animal Breeding and Genetics, University of Veterinary Medicine Vienna, Vienna, Austria

4.         Department of Biomedical Sciences, Colorado State University, Fort Collins, USA

Abstract Text:

Various studies have reported significant differences in developmental kinetics and sensitivity to environmental conditions in male and female bovine embryos. However, little is known about dimorphic gene expression patterns in bovine embryos at the initiation of elongation, one of the critical stages of development. Furthermore, to the best of our knowledge, there is little or no data available on the sexually dimorphic gene expression patterns of bovine embryos in relation to maternal environmental conditions at this stage. Building on this, our group has recently discovered sexually dimorphic expression of short non-coding RNAs in early elongation-stage embryos in different developmental environments. A better understanding of the bovine embryo's transcriptome signatures could improve our understanding of fertility problems in cattle and would also be valuable as a model for early human embryo development. The main objective of our studies was therefore to determine the sexually dimorphic gene expression profiles of male and female bovine embryos at the initiation of elongation on day 13 of gestation, in response to the maternal environment provided by lactating dairy cows or non-lactating nulliparous heifers, using RNA-seq.

These studies identified 159 genes, including those involved in steroid biosynthesis and gastrulation, as being differentially expressed exclusively between male and female cow embryos, which were used as a model for an adverse developmental environment. The expression pattern of 61 of these genes, including CYP39A1, CYP2R1 and CYP27B1, was found to be upregulated, while the expression pattern of 98 of these genes, including HSD17B1, HSD17B10 and aromatase (CYP19A1), was found to be downregulated in male embryos compared to female embryos. Chromosomal analysis showed that approximately 31.2% of the differentially expressed genes (DEGs) are located on the X chromosome, with 96% of these genes being upregulated in female embryos. Similarly, 254 genes were differentially expressed exclusively between male and female embryos developed in heifers, which served as a model for a favourable development environment. These genes were involved in female sex differentiation, placenta development, transmembrane transport and cell adhesion. Of these, 108 genes were upregulated and 146 were downregulated in males compared to females, with 17.3% of the DEGs located on the X chromosome, and 75% of these genes being upregulated in females. Conversely, 38 genes, including SLC30A10, SLC10A4, ATP6AP1 and KDM5C, exhibited a highly conserved sexually dimorphic expression pattern, independent of the maternal environment, suggesting their potential significance in sex-specific embryonic development. Interestingly, a higher proportion of genes showing sexually dimorphic expression in embryos developed in cows than in heifers were found to be located on the X chromosome, predominantly being upregulated in female embryos. This suggests that X-chromosome inactivation in female bovine embryos may be less effective in the reproductive tract of postpartum lactating cows than in heifers.

Taken together, these studies revealed that embryos developing in rather unfavourable in vivo environments exhibited sexually dimorphic expression of genes involved in steroid biosynthesis, gastrulation, and energy metabolism. This outcome was further supported by follow-up studies revealing sexually dimorphic profiles of mitochondrial bioenergetics in early elongation-stage bovine embryos. Additionally, embryos that developed in favourable in vivo environments exhibited sexually dimorphic expression of genes associated with sex differentiation and transmembrane transport, suggesting that contrasting reproductive tract environments impact the sex-specific gene expression patterns of embryos differently.