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Background  Aspartic proteases are known to play an important role in the biology of nematode parasitism. This role is best characterised in blood-feeding nematodes, where they digest haemoglobin, but they are also likely to play important roles in the biology of nematode parasites that do not feed on blood. In the present work, we investigate the evolution and expression of aspartic proteases in  Strongyloides ratti , which permits a unique comparison between parasitic and free-living adult forms within its life-cycle.    Results  We identified eight transcribed aspartic protease sequences and a further two genomic sequences and compared these to homologues in  Caenorhabditis elegans  and other nematode species. Phylogenetic analysis demonstrated a complex pattern of gene evolution, such that some  S. ratti  sequences had a one-to-one correspondence with orthologues of  C. elegans  but that lineage-specific expansions have occurred for other aspartic proteases in these two nematodes. These gene duplication events may have contributed to the adaptation of the two species to their different lifestyles. Among the set of  S. ratti  aspartic proteases were two closely-related isoforms that showed differential expression during different life stages: ASP-2A is highly expressed in parasitic females while ASP-2B is predominantly found in free-living adults. Molecular modelling of the ASP-2 isoforms reveals that their substrate specificities are likely to be very similar, but that ASP-2B is more electrostatically negative over its entire molecular surface than ASP-2A. This characteristic may be related to different pH values of the environments in which these two isoforms operate.    Conclusions  We have demonstrated that  S. ratti  provides a powerful model to explore the genetic adaptations associated with parasitic versus free-living life-styles. We have discovered gene duplication of aspartic protease genes in  Strongyloides  and identified a pair of paralogues differentially expressed in either the parasitic or the free-living phase of the nematode life-cycle, consistent with an adaptive role for aspartic proteases in the evolution of nematode parasitism.

bmc genomics

Identification of novel aspartic proteases from Strongyloides ratti and characterisation of their evolutionary relationships, stage-specific expression and molecular structure