The humoral immune response is activated when circulating recognition factors are stimulated by highly conserved microbial compounds. Gram-negative-binding proteins (GNBPs) and peptidoglycan-recognition proteins (PGRPs) activate the humoral response after recognizing microbial cell-wall peptidoglycans and P-glucans. Some members of the PGRP family downregulate the immune response by degrading free peptidogly-can into non-immunogenic monomers (Lemaitre and Hoffmann, 2007).
PGRP and GNBP gene families generally evolve under purifying selection over short evolutionary time, but have undergone substantial genomic t urnover on the lineages that separate Drosophila from mosquitoes, honey bees, and Tribolium (see section 6.4.4 in this volume; Evans et al, 2006; Waterhouse et al, 2007; Zou et al, 2007). Most GNBPs and PGRPs do not appear to have experienced recent adaptive evolution in Drosophila (Jiggins and Hurst, 2003; Schlenke and Begun, 2003; Jiggins and Kim, 2006; Sackton et al, 2007), mosquitoes (Little and Cobbe, 2005), or the crustacean Daphnia (Little et al., 2004). A notable exception, however, is a Drosophila PGRP which shows strong indications of adaptive evolution. PGRP-LC, an alternatively spliced gene that sits atop the Imd signalling cascade, has sustained a two amino acid insertion in the PGRP-LCa isoform in species of the melanogaster subgroup. This insertion is predicted to alter the binding specificity of that isoform, and appears to have been positively selected in conjunction with several additional adaptive substitutions (Sackton et al., 2007). Interestingly, the alternatively spliced binding domains of PGRP-LC show evidence of either recent independent duplication or concerted evolution in D. melanogaster and A. gambiae (Christophides et al, 2002). These patterns potentially reflect lineage-specific selection for recognition of distinct microbes. In another exception, limited positive selection was also detected in GNBP genes of Nasutitermes termites (Bulmer and Crozier, 2006). In this case, it was hypothesized that adaptation of recognition capability was driven by a shift in ecology as previously herbivorous termite species adapted to feed on decaying matter, exposing them to a novel community of pathogens.
One potential explanation for the observation that PGRPs and GNBPs tend to exhibit little indication of adaptive amino acid evolution is that these proteins recognize highly conserved pathogen sugar moieties. The cell-wall components recognized by these proteins are indispensable for most microbes, and, generally speaking, may not be easily modifiable. There thus may be little pressure on these genes to adapt over short time periods. Additionally, these recognition proteins are active against molecules that are conserved across a wide range of microbial taxa. There are, however, a limited number of examples of positive selection on PGRPs and GNBPs. Coupled with the observations of gene family duplication and divergence among species, instances of positive selection may reflect bursts of diversification as recognition function is fine-tuned to species-specific selective pressures.
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