INTRODUCTION
As potent neurotoxin, Tetrodotoxin (TTX) and Saxitoxin (STX) can specifically bind to sodium channels of animals by sharing one binding site with high affinities4,5,6, which could lead to poisoning and death of animals. However, some animals can accumulate TTX or STX in their body to defense the external environment since they possibly could be resistant to toxins by target site insensitivity, such as in fish, frog and mullusca1,2,3,32. Understanding how TTX is accumulated in animals and how animals resist to TTX in genetic level would be particularly critical for adaptive evolution research and toxin poisoning cure in medicine.
Some proteins which were possibly related with TTX binding and transportation were found in puffer fish2,17. Takashi et al (2000)34 firstly purified some TTX-binding proteins in puffer fish. Yotsu-Yamashita et al (2010 and 2013)2,17 reported the distribution and localization of TTX-binding protein and their homologous proteins in puffer fish, which possibly be related with TTX transportation. Feroudj et al (2014)19 employed DNA microarray analysis revealed some gene candidates related with TTX accumulation in puffer fish19. But the detailed accumulation mechanism of TTX is needed to be well understood in a wide range of animals. Except pufferfish, STX and TTX resistance also appears in various kinds of animals, such as snails7, xanthid crab8, blue-ringed octopus 9, and and frogs3,10-12. It has been reported that the mutation in sodium channel genes13 can block the binding of toxin and thus result in the toxin resistance1,2,3,14-16. Understanding how organisms resist TTX on genetic level would contribute to investigate adaptive evolution in ecological communities.
Nassarius , a species-rich genus of Nassariinae, is important in maintaining the balance of marine ecological system. For a long time,Nassarius is consumed as popular food in Asian countries as economic species21. However, eating nassariids ofNassarius have caused hundreds of food poisoning and death incidents in the last several years, especially in China. Our previous studies detected that the food poisoning incident was due to TTX accumulated in Nassarius sp’s body and the toxicity of differentNassarius species and different populations within oneNassarius species was different22, which means that some Nassarius species and populations can accumulate TTX in their body but some others do not accumulate. This is possibly related with the sea environments they live which could lead to adaptive evolution for different communities.
Currently the genetic basis involving TTX accumulation and resistance inNassarius is unclear. Here we performed transcriptional analysis for both toxic and non-toxic specimens in two Nassarius species (Nassarius succinctus and Nassarius variciferus ), aiming to reveal their genetic patterns for TTX accumulation and resistance in genetic expression and mutation level for better understanding the adaptive evolution for toxin.