Dairy cattle reproduction is a tightly regulated genetic process: Highlights on genes, pathways, and biological processes

Herd fertility is a key factor for the sustainability of cattle farming. However, since the 1980s, fertility in dairy cattle has steadily decreased and has become one of the main causes of culling and cow replacement in developed countries (Lucy, 2001; Barbat et al., 2010). Many studies have been performed in the last decades to investigate the genetic bases of this decline, considering the reproduction process as a whole or focusing on some particular stages of reproduction. A better knowledge of molecular mechanisms was expected to help in identifying key genes and explaining antagonistic interactions between fertility and other traits exemplified by the negative genetic correlation between milk yield and fertility in dairy cattle. For instance, spermatozoa or oocyte quality and their interactions with the maternal environment, as well as embryo and fetus developmental capabilities, have been assessed using the so-called “Omics” in various dairy cattle breeds. Omics refers to the highthroughput technologies such as microarrays or new sequencing technologies (NGS) used to explore the genome structure (genomics) and function (transcriptomics, proteomics, and metabolomics). Epigenetics studies also provide some insight into the role of heritable changes in gene expression due to cytosine methylation as well as long and small non-coding RNAs. Both gene or quantitative trait loci (QTL) mapping programs and functional genomics studies have been performed to search for key genes associated with reproduction and fertility, and many results have been obtained. Unfortunately, no clear picture has emerged. However, incorporating reproductive traits in genomic selection programs has succeeded in reversing the declining trend in reproductive success in many countries such as the USA, where the genetic merit for dairy cattle fertility has now reached 1985 levels. Consequently, reproductive success in dairy cattle is now seen as the result of complex and dynamic interactions between the genome and environmental factors. The aim of this review is to provide an overview of genes and functions associated with male and female reproduction, highlighting whenever possible candidate genes and variants responsible for genetic variations in fertility. Due to the huge amount of data and literature, we focused mainly on studies not described in previous reviews. As a result, this review is not meant to be exhaustive but will demonstrate the value of genetics and genomics to understand reproduction processes and finely decipher their molecular bases in cattle.