In an article appearing online in Science this week, and discussed in The Scientist, Watson et al. (PubMed) implicate the Drosophila Dscam gene in host defense. They detect secreted forms of the protein in hemolymph and show that the gene enhances phagocytosis of bacteria by hemocytes. They also demonstrate conservation of the potential for extreme isoform diversity across insect taxa, an extension of earlier work from the Graveley lab (Graveley et al. 2004; PubMed, RNA journal). Isoform diversity due to alternative splicing is therefore implicated in the generation of adaptive variation in host defense molecules. It is interesting that isoform diversity due to alternative splicing of Toll-like proteins has likewise been implicated in plant defense (reviewed by Kazan 2003 and Jordan et al. 2002; an example is Zhang & Gassmann 2003).
What kind of adaptation does this make possible? Certainly, extreme variability allows rapid adaptation on a population level. Furthermore, the presence of membrane bound and secreted forms of the same molecule presents the possibility of adaptive immunity through clonal selection of hemocytes that see antigen. Louisa Wu pointed me to an article in Nature Immunity (Little, Hultmark and Read 2005) making the point that neither memory nor specificity has been ruled out in invertebrate immunity. True adaptive immunity in insects would be very exciting, but we're a long way from that. How could variation in isoform production among hemocytes in Dscam isoforms be heritable? Through epigenetic silencing of splicing factors? We're just at the beginning of this story.
The authors say this:
broad conservation of receptor diversity strongly suggests important
functions and future studies will have to further address
whether the presence of diverse immune receptors in
invertebrates increases the effectiveness of immune responses
of individual animals. Alternatively, given the relative short
life span of many invertebrates, it may be that immune
receptor diversity is less important ontogenetically but rather
enhances the adaptive potential of animal populations to
changing environmental and pathogenic threats.