Milhade, L. | bioRxiv Science

bioRxiv2025

Circulating microRNAs reveal egg-brain crosstalk and a brain-specific microRNA linked to the onset of the next reproductive cycle in iteroparous salmonids

Mechanisms regulating the transition between two consecutive reproductive cycles are complex and remain poorly understood, mostly because they involve a dialog between the ovary and the central nervous system that is difficult to disentangle. In rainbow trout (Oncorhynchus mykiss), an iteroparous species spawning every year, removal of the eggs from the body cavity was used as a switch to trigger the onset of the next reproductive cycle. Changes in circulating miRNAs (c-miRNAs) levels in blood plasma and ovarian fluid were then monitored over time. Upon removal of the eggs from the body cavity we observed the dramatic down regulation of the blood plasma levels of a single c-miRNA (miR-139-5p) that is predominantly expressed in the brain. In contrast, very distinct c-miRNAs profiles were observed in blood plasma when eggs are retained in the body cavity. Among plasma c-miRNAs showing dynamic changes with egg retention, miR-135c is strongly expressed in the brain and pituitary, while miR-457a is predominant in the postovulatory ovary. In addition, egg retention in the body cavity triggers a dramatic drop in ovarian fluid levels of miR-202-5p, a miRNA known to regulate egg production in fish. Our observations reveal that the transition between two successive reproductive cycles involves a crosstalk between the eggs and the central system and that a single miRNA, miR-139, predominantly expressed in the brain, is associated with the onset of the next reproductive cycle. We identified possible miR-139-5p functional targets in rainbow trout and other iteroparous species that have been lost in semelparous salmonids. Our results offer new research perspectives to better understand the mechanisms triggering the next reproductive cycle in iteroparous fish species, including post-transcriptional regulations by miR-139 in the brain.

physiology↗

bioRxiv2021

Gene expression plasticity, genetic variation and fatty acid remodelling in divergent populations of a tropical bivalve species

Ocean warming is a particularly challenging threat for tropical marine bivalves species because many live already near their upper thermal limits. The thermal sensitivity of organisms is a strong contributor to the biogeographic boundaries of populations and species. The potential of thermal plastic response (range of thermal breadth) is typically reduced for marine populations living in stable thermal environments, e.g., in extreme latitudes organisms as traduced in the climatic variability hypothesis. However, regional-scale heterogeneity among tropical environments, such as archipelagos in French Polynesia, might also serve in modulating this plastic potential. The questions remain now, how tropical organisms are able to cope with abnormally elevated temperature on long-term (several weeks) and how environmental-variability might drive the potential of resilience? To answer these questions, we benefit from two ecologically divergent populations of a marine tropical mollusc species, Pinctada margaritifera, that usually experience either large diurnal variations (tide-pools, Marquesas archipelago) or lower temperature with stable to moderate variations (Gambier archipelago). Individuals were maintained in common garden experiment at several controlled temperature conditions (23{degrees}C, 28{degrees}C, 32{degrees}C and 34{degrees}C) over a 48 days period. We explored genetic divergence as well as thermal plastic responses by combining lipidomic and transcriptomic approaches. We show that P. margaritifera have capacities to adjust to long-term elevated temperatures that was thus far largely underestimated. Furthermore, we identified genetic variation between populations that overlapped with genes expression variations, including genes involved in the respiration machinery, a central process delimiting critical temperatures in marine invertebrates. This study is the first of a series looking at the global adaptation and acclimation mechanisms in response to climate change in Pinctada species.

ecology↗