DISCUSSION
More and more researches pointed that animals could accumulate toxin, like PSP, TSX and TTX in their body and thus produce toxin resistance31,32. It is significant to reveal the genetic basis of toxin accumulation and resistance in various of animals accumulating toxin, not only for adaptive evolution understanding but also for food-poisoning cure in medicine. It has been reported that the toxin resistance could be due to the mutation of sodium channel genes in animals where the toxin specially bind to1,13,31-,32. But the accumulation mechanism of TTX in animals was uncertain. Different environments can lead to the adaptive evolution of animals with TTX. Species in Nassarius have toxic and non-toxic communities from different sea areas. In this study, as typical TTX-resistant species, N. succinctus and N. variciferuswere used to reveal their genetic expression and mutation patterns from toxic and non-toxic communities.
First of all, N. succinctus and N. variciferusdemonstrated similar DEG patterns. Generally the toxic communities produced more up-regulated genes than the non-toxic communities. ForN. variciferus , the up-regulated genes of toxic specimens was significantly more than that of non-toxic specimens. For N. succinctus , two toxic specimens showed more up-regulated genes. This may suggest that special mechanism activities associated with TTX accumulation and resistance occur in toxic samples. The PCA analysis of DEGs indicated that the gene expression patterns of toxic specimens was different from that of non-toxic specimens for both species. Furthermore, we selected the most common DEGs (both upregulated and downregulated genes) between the toxic and non-toxic groups for both species. Compared with the non-toxic groups, the heat shock protein and cytochrome c oxidase subunit were the most significant upregulated genes from toxic groups. The heat shock protein involved the pathway of protein processing in endoplasmic reticulum. One of the most important functions of endoplasmic reticulum is detoxification, which is the removal of all the toxic materials such as metabolic wastes or drugs35-36. This strongly suggest that the TTX inNassarius ’s body is possibly removed by special mechanism. Cytochrome c oxidase subunit I and II involve the pathways of various diseases like Oxidative phosphorylation, Metabolic pathways, Cardiac muscle contraction, Non-alcoholic fatty liver disease (NAFLD), Alzheimer disease, Parkinson disease and Huntington disease. The cytochrome c oxidase subunit plays key role in respiration. Thus, it is reasonable that the cytochrome c oxidase subunit is highly upregulated in specimens accumulating TTX.
All the transcriptome unigenes were clustered against the sodium channel gene references to get the sodium channel genes of Nassarius. We obtained D2 and D3 domains of N. succinctus with length of 3425bp where one amino acid site “L” was different from that of all other animals possessing TTX resistance. Whether this new amino acid “L” is a potential mutational site with TTX resistance should be further confirmed by electrophysiology. Unfortunately, the D1 and D4 domains ofNassarius could not be obtained by unigene blast, reads blast or RACE-PCR. We suggest the reasons may be that the RNA-seq sequences do not cover the sodium channel regions due to RNA degradation or that the sodium channel genes of Nassarius are complicated for assembly with RNA-seq. In the future studies, we would employ comprehensive samples and more advanced sequence technologies to explore the complete sodium channel genes of various toxic and non-toxic communities in different Nassarius species.