Mechanisms of species invasion in apple snails: proteome of the egg perivitelline fluid, and proteomic responses of the adults to abiotic stressors

The Pomacea apple snails are amphibious operculate freshwater gastropods. They exhibit different stress tolerant abilities as some of them are invasive (e.g., Pomacea maculata and Pomacea canaliculata) and others are non-invasive species (e.g., Pomacea diffusa). By using a comparative approach, I have profiled the perivitelline fluid of P. maculata, compared the physiological and proteomic responses of P. canaliculata and P. diffusa to thermal and hypoxic stressors, and examined their sequence divergences. 74 proteins were identified from the PVF of P. maculata. Comparison of gene expression levels between albumen gland (the organ that secrets PVF) and other organs showed that 24 PVF genes were specifically expressed in albumen gland. Base substitution analysis of PVF and housekeeping orthologs between P. maculata and its congener P. canaliculata showed that these reproductive genes have higher evolutionary rate. The LT50 of P. canaliculata was significantly higher than that of P. diffusa. More than 3,350 proteins were identified from the hepatopancreas of the snails exposed to acute and chronic thermal stress using iTRAQ-coupled mass spectrometry. Chronic exposure caused differential expression of more proteins, with many of them related to restoration of damaged molecules, ubiquitinating dysfunctional molecules, and utilization of energy reserves in both species; but only in P. diffusa there was a shift from carbohydrate to lipid catabolism. Analysis of orthologous genes encoding the differentially expressed proteins revealed nine candidate genes may have undergone positive selection (Ka/Ks > 0.5). A 72 h mortality experiment showed that P. canaliculata is much more tolerant to hypoxia than P. diffusa. The two species were then exposed to four levels of dissolved oxygen (6.7, 4.1, 2.0 and 1.0 mg O2 L-1) for 8 h, and their gill proteins were analyzed using iTRAQ-coupled LC-MS/MS. Compared with P. diffusa, P. canaliculata had more up-regulated signal transduction proteins and down-regulated proteins which are involved in glycolysis and the tricarboxylic acid cycle. Evolutionary analysis indicated that selection has acted on some of the hypoxia responsive genes. The comparative method and results provide a framework for studying the genetic basis of species invasion and predicting their further expansion in a changing climate for non-model species.