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Heterosis has been extensively exploited in chicken breeding to improve laying traits in commercial hybrid stock. However, the molecular mechanisms underlying it remains elusive. This study characterizes the miRNAome in the pre-hierarchical follicles of purebred and hybrid laying hens, and investigate the functions of miRNAs with non-additive expression in the pre-hierarchical follicles as they modulate heterosis for egg number and clutch size. To achieve that aim, White Leghorn and Rhode Island Red chicken lines were reciprocally crossed to generate hybrids. The crossbreds demonstrated heterosis for egg number and clutch size, and pre-hierarchical follicles from 4 birds of each genotype were collected at 53 weeks of age. Mode of miRNA expression was characterized after miRNA sequencing. A total of 50 miRNAs including 30 novel ones, were found to exhibit non-additive expression. Dominance was the predominant mode of expression exhibited by majority of the miRNAs. Functional analysis of target genes of the known miRNAs with non-additive expression revealed Gene Ontology terms related to regulation of transcription, metabolic processes and gene expression. KEGG and REACTOME pathways including hedgehog, cellular senescence, wnt, TGF-β, progesterone-mediated oocyte maturation, oocyte meiosis, GnRH signaling, signal transduction and generic transcription, which can be linked to primordial follicle activation, growth and ovulation, were significantly enriched by target genes of miRNAs with non-additive expression. Majority of the genes enriched in these biological pathways were targeted by gga-miR-19a, gga-miR-19b, gga-miR-375, gga-miR-135a, and gga-miR-7 and 7b, thus, revealing their synergistic roles in enhancing processes that could influence heterosis for egg number and clutch size in hybrid hens.

Calanus finmarchicus is a key secondary producer in the North Atlantic. Shortly prior to the spring bloom the animals ascend from diapause at depth to surface waters, where the females spawn partly, based on winter lipid reserves. C. finmarchicus eggs are an important prey of first feeding fish larvae inhabiting North Atlantic shelves during early spring and are thus essential for larval survival. Comprehensive late April surveys have been carried out on and around the Faroe shelf, which is located between the Northeast Atlantic and the Nordic Seas, for more than two decades. One aim is to investigate the critical match-mismatch between the spring bloom development, zooplankton reproduction and occurrence of first feeding fish larvae. In this study, we examine spatial and temporal changes in pre-bloom reproductive activity of C. finmarchicus on and around the shelf using a unique dataset of more than 8,000 examined females sampled during the period 1997–2020. Enhanced productivity was observed on the north-western side of the shelf, where the main flow of oceanic water to the inner permanently well mixed shelf takes place. We attribute this increased productivity to enhanced food (phytoplankton) availability in the seasonally stratified outer shelf, slightly upstream of the main egg production area. Both individual egg production rates and the fraction of spawning females declined throughout the Faroe shelf during the examined period. This decline could not be explained by the employed environmental parameters. The declining pre-bloom egg production may have consequences for first feeding fish larvae.