Morphological and functional modifications of the gastrointestinal tract during metamorphosis in Atlantic halibut (Hippoglossus hippoglossus)

Metamorphosis in flatfish is the transition from a symmetrical larva to an asymmetrical juvenile stage, allowing the organism to utilize new resources in a different habitat and thus providing an evolutionary advantage. In addition to the overt changes in external morphology, coordinated maturation of many tissues and organs occur, including the gastrointestinal (GI-tract). In flatfish species, the GI-tract is extensively remodelled during larval development and its appropriate development is crucial for adapting to the shift in habitat and diet that accompanies the transition into a juvenile. Thyroid hormones (THs) play a central role in this process, translating the environmental cues into a coordinated program that remodels the organism. This thesis uses Atlantic halibut (Hippoglossus hippoglossus), a valuable aquaculture species, to provide new insights into flatfish metamorphosis. The transition from larvae to juvenile is here explored based on transcriptional changes during metamorphosis, functional development of the GI-tract and the establishment of neuroendocrine pathways for appetite-regulation. To study the main molecular mechanisms associated with Atlantic halibut metamorphosis, reference transcriptomes of the GI-tract, head and skin were generated using 454 pyrosequencing. The resulting large set of good quality reads has been assembled into a significant number of contigs and successfully annotated using a multi Blast step approach. Functional analyses revealed that the most prominent biological processes are equally common between the three regions analysed (GI-tract, head and skin) despite significant differences in tissue complexity. Additionally, unique sets of biological processes associated with tissue-specific morphology and function were identified for each region. For the GI-tract, the focal organ of the present thesis, a total of 206 GO-terms were found to be unique, including gastric acid secretion process. Using SOLiD sequencing technology, it was revealed that during Atlantic halibut development hundreds of genes are significantly (p<0.05) upor down-regulated at the whole larvae level, indicating that many key transcriptional modifications underlie the significant changes in tissue that occur between premetamorphic and juvenile stages. It is well established that initiation of metamorphosis in Atlantic halibut is associated with a surge of THs, thus THs levels increase until the metamorphic climax and decrease in