Effect of pre-ovulatory follicle size on oocyte transcript abundance in beef cows

Abstract

Inadequate oocyte competence is a potential explanation for reduced pregnancy rates and(or) increased late embryonic mortality when small dominant follicles are induced to ovulate prematurely with gonadotropin releasing hormone (GnRH) in beef cows. Previous studies have shown that follicle size at GnRH- induced ovulation may affect oocyte competence, as higher fertilization rates and higher embryo quality were reported after induction of ovulation in large ([greater than or equal to]12.5mm) compared to small (12.5 mm; no estrus expression), and Spontaneous (11.6-13.9 mm; estrus expression and endogenous gonadotropin surge). Cumulus-oocyte complexes were collected after trans-vaginal aspiration, and oocytes were fully denuded of surrounding cumulus cells. RNA was later extracted from pools of 4 oocytes (n= 6 oocyte pools from both small and large follicles; n=5 oocyte pools from the spontaneous (control) group) and submitted for sequencing on an Illumina HiSeq 2000 (single reads, 100 bases per read) to generate an average of 8,363,823 raw reads per pool that were aligned to a Bos Taurus transcriptome from NCBI, with additional annotations, and the bovine genome (Cow_3.1_btau_4.6.1_Y_ncbi).