The Tetraodontidae family encompasses several species which attract scientific interest in terms of their ecology and evolution. The silver-cheeked toadfish (Lagocephalus sceleratus) is a well-known ‘invasive sprinter’ that has invaded and spread, in less than a decade, throughout the Eastern and part of the Western Mediterranean Sea from the Red Sea through the Suez Canal. In this study, we built and analysed the first near-chromosome level genome assembly of L. sceleratus and explored its evolutionary landscape. Through a phylogenomic analysis, we positioned L. sceleratus closer to T. nigroviridis, compared to other members of the family, while gene family evolution analysis revealed that genes associated with the immune response have experienced rapid expansion, providing a genetic basis for studying how L. sceleratus is able to achieve highly successful colonisation. Moreover, we found that voltage-gated sodium channel (Nav 1.4) mutations previously connected to tetrodotoxin resistance in other pufferfishes are not found in L. sceleratus, highlighting the complex evolution of this trait. The high-quality genome assembly built here is expected to set the ground for future studies on the species biology.

By evaluating genetic variation across the entire genome, one can address existing questions in a novel way while new can be asked. Such questions include how different local environments influence both adaptive and neutral genomic variation within and among populations, providing insights not only into local adaptation of natural populations, but also into their responses to global change and the exploitation-induced evolution. Here, under a seascape genomic approach, ddRAD genomic data were used along with environmental information to uncover the underlying processes (migration, selection) shaping European sardines (Sardina pilchardus) of the Western Mediterranean and adjacent Atlantic waters. This information can be relevant to the (re)definition of fishery stocks, and their short-term adaptive potential. We found that studied sardine samples form two clusters, detected using both neutral and adaptive (outlier) loci suggesting that natural selection and local adaptation play a key role in driving genetic change among the Atlantic and the Mediterranean sardines. Temperature and especially the trend in the number of days with sea surface temperature (SST) above 19oC was crucial at all levels of population structuring with implications on species’ key biological processes, especially reproduction. Our findings provide evidence for a dynamic equilibrium where population structure is maintained by physical and biological factors under the opposing influences of migration and selection. Given its dynamic nature, such a system postulates a continuous monitoring under a seascape genomic approach that can benefit by incorporating a temporal as well as a more detailed spatial dimension.

Understanding the molecular basis of stress is of long standing interest in biology and fish science. We tackled this question by modifying the epiGBS (epiGenotyping By sequencing) technique to screen for cytosine methylation and explore the genome-wide epigenomic response to a repeated acute stress challenge in the European sea bass (Dicentrarchus labrax). Following a minimally invasive sampling using nucleated red blood cells (RBCs), our modified epiGBS protocol retrieved 501,108,033 sequencing reads after trimming, with a mean mapping efficiency of 73.0% (unique best hits). Sequencing reads mapped across all linkage groups (LGs). A total of 47,983 CpG coordinates with a minimum 30X read depth was retained for differential methylation analysis between pre- and post-stress fish. A family effect was demonstrated, and 57 distinct differentially methylated cytosines (DMCs) distributed on 17 of 24 LGs were found between RBCs of pre- and post-stress individuals and located close to 51 distinct stress-related genes. Thirty-eight of these genes were previously reported as differentially expressed in the brain of zebrafish, most of them involved in stress coping differences. Some DMC-related genes appear as good candidates to study the stress response, especially a set of them associated to the Brain Derived Neurotrophic Factor (BDNF), a protein that favors stress adaptation and fear memory. Limits to our study and future directions are presented, including the use of RBCs as a surrogate to other target tissues, to provide with classical physiological measurements a more complete picture of the stress response in fish.