Heifer reproduction strategies: molecular insights into early embryonic development and viability

Abstract

Current heifer development strategies aim to produce the most fertile female possible. However, in western states heifers commonly transition from high to relatively low nutritional planes following breeding. The effects of maternal nutritive status on the embryonic system are not defined, though this transition is likely contributing to the high rates of early embryonic mortality observed in these animals. This study aimed to decipher the impact of maternal nutrition on day 14 conceptuses collected from beef heifers maintained on ?100% or 70% NRC nutrient requirements. Conceptus RNA was isolated and sequenced to identify differentially expressed genes (DEGs). The DEG set included 771 annotated and 132 novel genes. The biological processes representing the most significant differential expression between conceptuses collected from Low- vs- High groups were hormone metabolic processes (Padj = 0.015), inner ear development (Padj = 0.015), inner ear morphogenesis (Padj = 0.015), hematopoietic progenitor cell differentiation (Padj = 0.015), and tissue morphogenesis (Padj = 0.018). KEGG analysis identified the biological pathways most affected between Low- and High-derived conceptuses. Analysis revealed viral fusion proteins, oocyte meiosis, mineral absorption, and sphingolipid metabolism were significantly affected. Interestingly, the pathways representing the highest number of DEGs were peptidases and inhibitors (n = 22 DEGs), MAPK signaling pathway (n = 17 DEGs), and viral fusion proteins (n = 17). Deeper analysis of the KEGG pathway and gene ontology results linked many of the DEGs to processes related to the cellular differentiation, formation, and function of extraembryonic tissues. While more advanced embryonic patterning and fetal development presented as areas of concern in the literature search, at this time they are not likely factors contributing with the most weight to the high rates of embryonic mortality observed in beef cattle. A majority of the DEGs were related to pathways that control early cell specification events that are necessary for the formation of the trophectoderm and the yolk sac. Data indicates that a decreased plane of maternal nutrition affects the expression of genes associated with critical embryonic events, and likely contributes to increased rates of embryonic mortality by altering the function of extraembryonic tissues.