Exploring the potential for an evolutionarily conserved role of the taste 1 receptor gene family in gut sensing mechanisms of fish.

In this study, we investigated the transcriptional spatio-temporal dynamics of the taste 1 receptor () gene family repertoire in seabream ( []), during larval ontogeny and in adult tissues. In early larval development, expression arises heterochronously, i.e. the extraoral taste-related perception in the gastrointestinal tract (GIT) anticipates first exogenous feeding (at 9 days post hatching [dph]), followed by the buccal/intraoral perception from 14 dph onwards, supporting the hypothesis that the early onset of the molecular machinery underlying expression in the GIT is not induced by food but rather genetically hardwired. During adulthood, we characterized the expression patterns of within specific tissues ( = 4) distributed in oropharingeal, GIT and brain regions substantiating their functional versatility as chemosensory signaling players to a variety of biological functions beyond oral taste sensation. Further, we provided for the first time direct evidences in fish for mRNA co-expression of a subset of genes (mostly , i.e. the common subunit of the heterodimeric T1R complexes for the detection of "sweet" and "umami" substances), with the selected gut peptides ghrelin (), cholecystokinin (), hormone peptide yy () and proglucagon (). Each peptide defines the enteroendocrine cells (ECCs) identity, and establishes on morphological basis, a direct link for T1R chemosensing in the regulation of fish digestive processes. Finally, we analyzed the spatial gene expression patterns of 2 taste signaling components functionally homologous to the mammalian subunit gustducin, namely and , and demonstrated their co-localization with the in EECs, thus validating their direct involvement in taste-like transduction mechanisms of the fish GIT. In conclusion, data provide new insights in the evolutionary conservation of gut sensing in fish suggesting a conserved role for nutrient sensors modulating entero-endocrine secretion.