Whole-Transcriptome Landscape Deciphers a ceRNA-Mediated "De-Repression-Activation" Circuit Governing Continuous Estrus in Ovine.

(1)

Background

Seasonal breeding constraints represent a long-standing barrier to efficient sheep production worldwide. Small Tail Han sheep, a Chinese indigenous breed that exhibits year-round estrus independent of photoperiod, offers a unique natural model for investigating the regulatory mechanisms underlying estrus cyclicity-yet the molecular basis of this remarkable trait remains largely unexplored. This study therefore aimed to compare ovarian whole-transcriptome profiles between Small Tail Han sheep (year-round estrus) and Ujumqin sheep (seasonal estrus) during estrus and diestrus. Specifically, we sought to characterize breed-specific transcriptional response strategies, identify key miRNAs associated with continuous estrus, and construct ceRNA networks that mediate ovarian estrous cycle regulation. (2)

Methods

To address these objectives, we profiled ovarian transcriptomes of Small Tail Han sheep (year-round estrus) and Ujumqin sheep (seasonal estrus) during both estrus and diestrus (12 samples total) using whole-transcriptome sequencing, systematically characterizing the expression landscapes of mRNAs, miRNAs, lncRNAs and circRNAs. Through differential expression analysis, GO/KEGG functional enrichment, multi-tiered miRNA screening, and ceRNA network construction, we compared the transcriptional response strategies between the two breeds. (3)

Results

Our findings revealed two unexpected and divergent patterns. Small Tail Han sheep showed 1881 differentially expressed mRNAs between estrus and diestrus, 82-fold more than the 23 in Ujumqin sheep, suggesting a broader mRNA-level transcriptional response in the year-round estrus breed. circRNAs were predominantly up-regulated (80-85%) in both breeds, while mRNAs and lncRNAs were predominantly down-regulated, uncovering a conserved "circRNA-up, mRNA-down" regulatory logic across breeds. Multi-tiered screening identified 38 "brake-releaser" miRNAs (high in diestrus, down-regulated in estrus) and 21 "accelerator" miRNAs (low in diestrus, up-regulated in estrus), which together constitute a "de-repression-activation" dual regulatory mode. ceRNA network analysis further revealed a clear functional stratification: conserved core miRNAs targeted "sensor" pathways (cAMP, PI3K-Akt, MAPK), whereas accelerator miRNAs specifically targeted "executor" pathways (estrogen signaling, progesterone-mediated oocyte maturation, Hippo signaling). Network topology analysis pinpointed miR-199a-5p, miR-204-3p and miR-199b as the core hubs bridging the two regulatory modules. (4)

Conclusions

Taken together, our findings redefine the molecular basis of year-round estrus-moving beyond single-gene mutations or overexpression to an enhanced global transcriptomic responsiveness of the ovary. This work provides a fresh conceptual framework for understanding the divergence of reproductive strategies in sheep and offers actionable candidate targets for genetic improvement of continuous estrus traits.