Identification of salt stress response genes using the Artemia transcriptome

Abstract

Habitat salinity is a major abiotic factor governing the activity, physiology, biology and distribution of aquatic animals. Salinity changes cause salt stress, affecting crustaceans reared in aquaculture both on an ecological and economic level. Current salt stress research in aquatic animals is mainly focused on salt stress in the gills at relatively low salinity ranges. Knowledge about whole-body salinity response in crustaceans and other organisms is lacking, especially in hypersaline conditions.Artemia franciscana is a small halophilic model crustacean able to withstand high salinities up to 300 g/l and strong osmotic shocks thanks to its mitigating strategies for fluctuating salinity levels, such as its unique larval salt gland and osmoregulatory capacity. This study aims to identify the genes responsible for Artemia's unique hypersalinity tolerance by differential expression analysis.First, the full transcriptome of A. franciscana in different metabolic and life cycle stages was assembled de novo (assembly statistics: N50 = 1,430; GC content = 35.63%; transcript number = 64,972) and functionally annotated (annotated transcripts = 36%). Then, naupliar RNA-Seq reads generated under respectively hypersaline and marine conditions were pseudo-aligned to the A. franciscana transcriptome. Expression levels in both conditions were finally compared and 177 differentially expressed, functionally annotated transcripts were identified, of which 113 transcripts with GO annotations. Signalling genes, such as EIF and several genes from the glutathione and the chitin metabolic pathways were induced in Artemia under hypersaline conditions. Hypersalinity also activated gene regulation mechanisms (expression, transcription and post transcription) in the nucleus for DNA repair, ubiquitination, and also for cell cycle arrest through La-related protein. Several lipid metabolic genes and lipid transporters were upregulated, potentially to provide energy for ion balance and to maintain membrane structure integrity. Transport of metal ions and other ions was upregulated as well to maintain ion homeostasis. Lastly, known crustacean stress response genes such as Heat shock 70 kDa protein cognate were upregulated. This work shows that salt stress in Artemia nauplii, through signal transduction, gene regulation, lipid metabolism, transport and stress response genes, has an important influence on known and novel homeostasis-repairing mechanisms in Artemia.