Schistosoma Mansoni - Genome

Genome

Schistosoma mansoni has 8 pairs of chromosomes (2n = 16)—7 autosomal pairs and 1 sex pair. The female schistosome is heterogametic, or ZW, and the male is homogametic, or ZZ. Sex is determined in the zygote by a chromossomal mechanism. The Schistosoma genome is approximately 270 MB with a GC content of 34%, 4–8% highly repetitive sequence, 32–36% middle repetitive sequence and 60% single copy sequence. Numerous highly or moderately repetitive elements have been identified, and their frequency in genomic sequence data also suggests at least 30% repetitive DNA. Chromosomes range in size from 18 to 73 MB and can be distinguished by size, shape and C banding. There are estimated to be 15–20 thousand expressed genes.

In 2000, the first BAC library of Schistosome was constructed. In June 2003, a ~5x whole genome shotgun sequencing project was initiated at the Sanger Institute. Together with the shotgun data being generated by TIGR, an ~8x coverage of the genome will be obtained, assembled and annotated. Also in 2003, 163,000 ESTs (expressed sequence tags) were generated (by a consortium headed by the University of São Paulo) from six selected developmental stages of this parasite, resulting in 31,000 assembled sequences and an estimated 92% of the 14,000-gene complement.

In 2009 the genomes of both S. mansoni and S. japonicum were published, with each describing 11,809 and 13,469 genes respectively. Analysis of the S. mansoni genome highlighted expansions in protease families and deficiencies in lipid anabolism; both observations can be directly related S. mansoni's parasitic lifestyle. The former included the invadolysin (host penetration) and cathepsin (blood feeding) gene families, while the latter encompassed several enzymes required for the de novo synthesis of fatty acids and sterols (so the worm must rely on its host for these products). The results open the way for research on new targeted treatments.

In 2012 an improved version of the S. mansoni genome was published, with only 885 scaffolds and more than 81% of the bases organised into chromosomes. In the same study the authors have also used transcriptome sequencing (RNA-seq) from four time points in the parasite’s life cycle to refine 45% gene predictions and profile their expression levels.