| Upon exposure to infection with digenetic trematodes such as Echinostoma paraensei, the freshwater snail Biomphalaria glabrata produces increased quantities of hemolymph lectins, some of which are unique polypeptides containing both immunoglobulin superfamily (IgSF) and fibrinogen domains. These unusual lectins have been termed fibrinogen-related proteins (FREPs), and they recognize and precipitate digenean antigens. Here I report 11 distinct FREP-encoding sequences from B. glabrata, and provide the complete genomic sequence for four of these.;The unique juxtaposition of IgSF- and fibrinogen-encoding domains, previously known only from incomplete cDNAs, is now confirmed, with two complete sequences for FREPs with one IgSF-encoding domain (FREPs 2 and 4---Chapter 1) and two for FREPs with two IgSF-encoding domains (FREPs 3 and 7---Chapter 2). The IgSF-encoding domains demonstrate different genomic architecture where the numbers of introns vary between these genes, and the different domains do not all belong to the same classes of IgSF domains (V-variable, C-constant, or I-intermediate). Some minor differences in genomic fragments isolated for each of the four sequences were noted and may represent allelic variants. This may exemplify one of several ways presented here in which the diversity of FREPs is enhanced, a property that may be critical to a putative role in non-self recognition. FREP genes, particularly given their variable nature, serve as a valuable model to study the sophisticated nature of invertebrate response to parasites and pathogens.;The unexpected discovery of fibrinogen domains in snails has prompted a review of the occurrence of fibrinogen domains across the spectrum of living organisms, and a review of the function and diversification of fibrinogen domains (Chapter 3). Thus far, fibrinogen domains are known only from bilaterian phyla of animals, including invertebrate lineages in which classic fibrinogen-mediated blood clotting does not occur. Therefore, it does not seem like the primordial function of fibrinogen domains was blood clotting. In fact, the role of fibrinogen domains in blood coagulation seems to be an example of a remarkably complex innovation adopted either by non-vertebrate chordates or by vertebrates. Indeed, a role for fibrinogen domains in internal defense (non-self recognition) is increasingly recognized in results obtained from groups as disparate as molluscs, arthropods, ascidians and mammals. |
