Schistosoma mansoni mucin gene (SmPoMuc) expression: epigenetic control to shape adaptation to a new host.
PLoS Pathog 2013;
9:e1003571. [PMID:
24009504 PMCID:
PMC3757033 DOI:
10.1371/journal.ppat.1003571]
[Citation(s) in RCA: 22] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/11/2013] [Accepted: 06/27/2013] [Indexed: 11/28/2022] Open
Abstract
The digenetic trematode Schistosoma mansoni is a human parasite that uses the mollusc Biomphalaria glabrata as intermediate host. Specific S. mansoni strains can infect efficiently only certain B. glabrata strains (compatible strain) while others are incompatible. Strain-specific differences in transcription of a conserved family of polymorphic mucins (SmPoMucs) in S. mansoni are the principle determinants for this compatibility. In the present study, we investigated the bases of the control of SmPoMuc expression that evolved to evade B. glabrata diversified antigen recognition molecules. We compared the DNA sequences and chromatin structure of SmPoMuc promoters of two S. mansoni strains that are either compatible (C) or incompatible (IC) with a reference snail host. We reveal that although sequence differences are observed between active promoter regions of SmPoMuc genes, the sequences of the promoters are not diverse and are conserved between IC and C strains, suggesting that genetics alone cannot explain the evolution of compatibility polymorphism. In contrast, promoters carry epigenetic marks that are significantly different between the C and IC strains. Moreover, we show that modifications of the structure of the chromatin of the parasite modify transcription of SmPoMuc in the IC strain compared to the C strain and correlate with the presence of additional combinations of SmPoMuc transcripts only observed in the IC phenotype. Our results indicate that transcription polymorphism of a gene family that is responsible for an important adaptive trait of the parasite is epigenetically encoded. These strain-specific epigenetic marks are heritable, but can change while the underlying genetic information remains stable. This suggests that epigenetic changes may be important for the early steps in the adaptation of pathogens to new hosts, and might be an initial step in adaptive evolution in general.
Schistosoma mansoni is a parasitic worm and agent of a disease that causes a considerable economic burden in African and South American countries. The propagation of the parasite requires passage through a freshwater snail of Biomphalaria genus. In the field, actually very few snails are infected. This is due to the fact that specific strains of the parasite can infect only specific strains of the snail. Comparative studies have shown that this so-called compatibility is based on the expression of a family of genes that are called SmPoMucs. We have shown previously that all parasites strains possess the repertoire of all SmPoMuc genes but every strain and even every individual parasite expresses only a subset. These differences could be due to DNA sequence differences in the regions that control gene expression, but here we show that these regions are nearly identical. Instead, the chromatin structure shows strain-specific characteristics. This means that the parasite can adapt to different snail strains simply by changing its chromatin structure and not necessarily the DNA sequence. If this holds true for other parasites, then we have to rethink the way parasite evolution is currently imagined but this also provides a new potential entry point to control the spread of diseases.
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