Dscam from Lophotrochozoa: The Option of Multiple Isoforms Is Ruled out
摘要
The cell surface receptor Down syndrome cell adhesion molecule(Dscam) subjected to mutually exclusive splicing in Arthropoda showsan extraordinary degree of potential immune recognition specificity.We have tested the hypothesis on the existence of similar polymorphismin the homologous receptors from sister-clade protostomian animals,Spiralia, comprising Mollusca and Annelida. Two new members of Dscamfamily from bivalve mollusks, Mytilusedulis (MeDscam) and Crassostreagigas (CgDscam) were cloned from ganglia mRNA and theirdifferential expression was estimated in immune cells. The expressionof MeDscam and CgDscam in haemocytes is weak, constitutive and wasnot modulated by immune inducers neither in vivo nor in vitro experiments.Based on the sequencing results, we did not observe diversity inthe Dscam transcripts in both ganglia and haemocytes. Our screeningof C. gigas genome and genomesof other Spiralia animals, gastropod mollusk Lottiagigantea and polychaete worm Capitellateleta, revealed their Dscam homologues genes do notcontain the hypervariable exon arrays and all exons are found at uniquepositions. This type of gene structure leaves no chance for thegeneration of multiple splice variants. The immune function of Dscampattern recognition receptor based on the vast repertoire of isoformsis an evolutionary innovation developed exclusively in Arthropodalineage and could not be attributed to other non-arthropod Protostomia.