Invertebrates lack adaptive immune systems homologous to those of vertebrates, yet it is becoming increasingly clear that they can produce diversified antigen recognition molecules. We have previously noted that the snail Biomphalaria glabrata produces a secreted lectin, fibrinogen-related protein 3 (FREP3), unusual among invertebrate defense molecules because it is somatically diversified by gene conversion and point mutation. Here we implicate FREP3 in playing a central role in resistance to a major group of snail pathogens, digenetic trematodes. FREP3 is up-regulated in three models of resistance of B. glabrata to infection with Schistosoma mansoni or Echinostoma paraensei, and functions as an opsonin favoring phagocytosis by hemocytes. Knock-down of FREP3 in resistant snails using siRNA-mediated interference resulted in increased susceptibility to E. paraensei, providing a direct link between a gastropod immune molecule and resistance to trematodes. FREP3 up-regulation is also associated with heightened responsiveness following priming with attenuated digenetic trematodes (acquired resistance) in this model invertebrate immune system. host-parasite interactions | evolution | immunology | parasitology | schistosomiasis

Schistosomiasis, a neglected tropical disease, owes its continued success to freshwater snails that support production of prolific numbers of human-infective cercariae. Encounters between schistosomes and snails do not always result in the snail becoming infected, in part because snails can mount immune responses that prevent schistosome development. Fibrinogen-related protein 3 (FREP3) has been previously associated with snail defense against digenetic trematode infection. It is a member of a large family of immune molecules with a unique structure consisting of one or two immunoglobulin superfamily domains connected to a fibrinogen domain; to date fibrinogen containing proteins with this arrangement are found only in gastropod molluscs. Furthermore, specific gastropod FREPs have been shown to undergo somatic diversification. Here we demonstrate that siRNA mediated knockdown of FREP3 results in a phenotypic loss of resistance to Schistosoma mansoni infection in 15 of 70 (21.4%) snails of the resistant BS-90 strain of Biomphalaria glabrata. In contrast, none of the 64 control BS-90 snails receiving a GFP siRNA construct and then exposed to S. mansoni became infected. Furthermore, resistance to S. mansoni was overcome in 22 of 48 snails (46%) by pre-exposure to another digenetic trematode, Echinostoma paraensei. Loss of resistance in this case was shown by microarray analysis to be associated with strong down-regulation of FREP3, and other candidate immune molecules. Although many factors are certainly involved in snail defense from trematode infection, this study identifies for the first time the involvement of a specific snail gene, FREP3, in the phenotype of resistance to the medically important parasite, S. mansoni. The results have implications for revealing the underlying mechanisms involved in dictating the range of snail strains used by S. mansoni, and, more generally, for better understanding the phenomena of host specificity and host switching. It also highlights the role of a diversified invertebrate immune molecule in defense against a human pathogen. It suggests new lines of investigation for understanding how susceptibility of snails in areas endemic for S. mansoni could be manipulated and diminished.

Last year, we discovered extremely rich microvertebrate locality early Campanian Allison Member of the Menefee Formation in northwestern New Mexico. This site has yielded a highly diverse paleofauna, including fish (Lepisosteidae and Amiidae) teeth, sharks (Lissodus, Cretodus, Carcharias, Cretolamna, Ischyrhiza, Onchosaurus, Squatina, and Squatirhina), rays (Ptychotrygon, Pseudohypolophus mcnultyi, Protoplatyrhina renae, Protoplatyrhina hopii, Dasyatis and Myledaphus), the earliest confirmed New Mexico mammals ( Paracimexomys group of Eaton and Cifelli [2001]), maniraptoran (Richardoestesia isosceles) and dromaeosaurid (Saurornitholestes langtoni and S. sp.) theropod dinosaur teeth, hadrosaurid dinosaur teeth, Brachychampsa-like crocodilian teeth, lissamphibian teeth and jaw fragments referable to the salamander Albanerpeton, and squamate lizard scales. The aqueous fauna is by far the most abundant and diverse assemblage in the unit and provides information on the aqueous paleoecology of the Menefee Formation. Taphonomic study of the site indicates little or no transport of the fossils, and therefore little or no information addition, and thus this site yields is an accurate representation of the aqueous paleofauna of the Menefee. The taxa identified here are principally freshwater forms, and our analysis of the sedimentology of the source area confirms this hypothesis. There is enough information here to reconstruct a basic food web of the freshwater vertebrate fauna of the Menefee. This is done largely on the basis of tooth morphology and comparison with extant forms, as well as the use of ecological principles. We propose that dominant aqueous taxa in the Menefee were the chondrichthyans Pseudohypolophus, Ptychotrygon, Lissodus, and Onchosaurus. The dominance of these forms indicates an abundant invertebrate fauna of crustaceans and mollusks in the Menefee that was not preserved.