Comparative and Functional Analysis of Exosomal microRNAs During Semelparous Reproduction in Ayu Fish (<i>Plecoglossus altivelis</i>)

As a life history strategy, some semelparous organisms, such as the ayu fish (<i>Plecoglossus altivelis</i>), reproduce only once in their lifetime and then die. They invest heavily in their single reproductive event, producing many offspring. However, the regulatory mechanisms that trigger mortality after reproduction are not well understood. Exosomes serve as an essential pathway for intercellular communication, with internal microRNA (miRNA) playing a crucial role in regulating physiological activities within the organism. This study aimed to elucidate the function of exosomal miRNA during the semelparous reproduction period in <i>P. altivelis</i>. Exosomes were successfully extracted from <i>P. altivelis</i> plasma during reproduction, and abundant miRNA molecules were discovered through small RNA sequencing. The miRNA expression patterns in ayu fish during reproduction exhibited notable differences between females and males. Furthermore, it was observed that key cellular processes and signalling pathways associated with intercellular transmission and intracellular stress are regulated by miRNA expression, and these changes in regulation may be responsible for post-breeding mortality. This study enhances our understanding of the function of exosomal miRNA during semelparous reproduction in ayu fish and provides further insight into the intrinsic mechanisms of ageing and mortality in organisms.