The developmental transcriptome of contrasting Arctic charr (Salvelinus alpinus) morphs

Abstract

Species showing repeated evolution of similar traits can help illuminate the molecular and developmental basis of diverging traits and specific adaptations. Following the last glacial period, dwarfism and specialized bottom feeding morphology evolved rapidly in several landlocked Arctic charr (Salvelinus alpinus) populations in Iceland. To initiate study of the genetic divergence between small benthic morphs and larger morphs with limnetic morphotype, we conducted an RNA-seq transcriptome analysis of developing charr. We sequenced mRNA from whole embryos at four stages in early development of two stocks with contrasting morphologies, the small benthic (SB) charr from Lake Thingvallavatn and Holar aquaculture (AC) charr.\n\nThe data reveal significant differences in expression of several biological pathways during charr development. There was also an expression difference between SB- and AC-charr in genes involved in energy metabolism and blood coagulation genes. We confirmed expression difference of five genes in whole embryos with qPCR, including lysozyme and natterin like which was previously identified as a fish-toxin of a lectin family that may be a putative immunopeptide. We also verified differential expression of 7 genes in developing heads, that associated consistently with benthic v.s. limnetic craniofacial morphology (studied in 4 morphs total). Comparison of Single nucleotide polymorphism (SNP) frequencies reveals extensive genetic differentiation between the SB- and AC-charr (60 fixed SNPs and around 1300 differing by more than 50% in frequency). In SB-charr the high frequency derived SNPs are in genes related to translation and oxidative processes. Curiously, three derived alleles in the otherwise conserved 12s and 16s mitochondrial ribosomal RNA genes are found in benthic charr.\n\nThe data implicate multiple genes and molecular pathways in divergence of small benthic charr and/or the response of aquaculture charr to domestication. Functional, genetic and population genetic studies on more freshwater and anadromous populations are needed to confirm the specific loci and mutations relating to specific ecological or domestication traits in Arctic charr.