Species-Specific Proteins in the Oviducts of Snail Sibling Species: Proteotranscriptomic Study of Littorina fabalis and L. obtusata

SIMPLE SUMMARY: Genitalia and reproduction-associated proteins are often species-specific and might evolve rapidly. The situation in which the morphology of the reproductive system is the only difference between two or several closely related species has been reported on multiple occasions. Nevertheless, the reasons for such rapid divergence of the reproductive system is still poorly investigated. To shed some light on the issue, we performed a transcriptomic and proteomic comparison of pallial oviducts from the two sibling species of gastropods Littorina obtusata and L. fabalis. The main identified differences were associated with three functional groups of genes: transposable elements, which enhance genome variation and promote the evolution of new genes, receptor proteins potentially involved in friend or foe recognition, and various enzymes. We hypothesize that these functional groups reflect both the mechanism (transposable elements) and the directions (friend or foe recognition and reproductive physiology) of the rapid evolution of the reproductive system. ABSTRACT: Genus Littorina subgenus Neritrema (Mollusca, Caenogastropoda) includes the “obtusata” group of closely related species (Littorina obtusata and L. fabalis). The anatomy of the adult reproductive system (pallial oviduct) is the only reliable feature used for species identification in females of these species. Reproductive system anatomy and reproduction-associated proteins often diverge between sibling species. Despite being of high evolutionary interest, the molecular basis of this divergence remains poorly understood. We performed proteotranscriptomic comparison of oviducts of L. obtusata and L. fabalis by RNA-seq on Illumina HiSeq 2500 and two-dimensional protein electrophoresis (2D DIGE) with MS/MS identification of the species-specific proteins. The interspecies differences in the oviduct were associated with (1) metabolic proteins reflecting overall physiological differences between L. obtusata and L. fabalis, (2) receptor proteins, and (3) transcripts related to transposable elements (TEs). Various receptors identified may recognize a wide variety of ligands from pathogen-associated molecular patterns to specific carbohydrates on the sperm surface. Therefore, these may participate in immune defense as well as in sperm storage and regulation. Species-specificity of multiple TE sequences (coding for reverse transcriptase and ribonuclease H) may indicate the important role of these genomic elements in the Littorina species divergence, which has not been reported previously.