Transcriptomic analysis of heifers according to antral follicle count

Abstract

While antral follicle count (AFC) has been associated with higher pregnancy rates, at present, our understanding of it as a reproductive parameter remains incomplete. This study aimed to characterize gene expression profile of oocytes from crossbred Bos taurus x Bos indicus heifers with high and low AFCs. Crossbred Nelore-Angus heifers (n = 50) with a mean (SD) age of 9.6 ± 0.55 months, a weight of 295.4 ± 32.6 kg, and a BCS of 3.44 ± 0.41 were studied in a feedlot system. The heifers received a hormonal protocol based on injectable progesterone and estradiol cypionate administered 12 days apart, and ovarian ultrasonography (US) was performed 12 days after to assess the AFC. Based on AFC, heifers were divided into low (≤14 follicles) and high (≥31 follicles) AFC, groups.Forty-five days after US, 14 heifers were slaughtered, and their ovaries were collected for morphological analysis and follicle aspiration. Cumulus-oocyte complexes (COCs) from the high and low AFC groups were graded according to their quality. Only best-quality COCs were stored for RNA-seq analysis. No differences were found in the presence or diameter of the dominant follicle and corpus luteum in the US, nor in the volume of the dominant follicle postmortem. The quantity of COCs recovered from high-AFC heifers was higher than that from low-AFC heifers (P < 0.05), and a tendency (P = 0.07) toward a higher amount of grade II COCs was observed. Thirty-two genes were differentially expressed between the groups, of which 30 were up-regulated and two down-regulated in the low AFC group. Among these, 22 % (7/32) were associated with fertility (CAB39, SLC2A6, CITED2, FDX1, HSD11B2, CD81, and PLA2G12B). Moreover, 9 and 2 exclusive genes were identified in the high and low AFC groups, respectively. Enrichment analyses showed that genes exclusive to oocytes from low-AFC heifers were associated with fundamental cellular processes, such as biosynthesis/biogenesis of ribosomes, peptides, amides, and nucleotides, and also with autophagy, mitophagy and mTOR signalling pathways.On the other hand, only one pathway was enriched in the high AFC group, but this cannot be related to the events studied No differences were observed in the ovarian structures after pre-synchronization of the estrus cycle of young Crossbred Nelore-Angus heifers. However, a tendency of a higher amount of grade II COCs was observed in heifers with high AFC than in those with low AFC. RNA sequencing results indicated that the main differences between high and low AFC heifers were not reflected in the genes directly related to fertility.