Transposon defense in Drosophila somatic cells: a model for distinction of self and non-self in the genome.

Genomes need an immune system much like entire organisms, because their integrity is threatened by selfish genetic elements which transpose and proliferate at the cost of the host. Unlike bacteria or viruses, these DNA parasites do not have any particular feature that helps to detect them as foreign sequences within the genome-they have no "antigen" so to speak. Nonetheless, sequence-specific defense mechanisms have evolved: The germ-line piRNAs rely on previous exposure that has left degenerate copies of many transposon-families in certain genomic loci from which small RNA sentinels are produced. In addition, the somatic cells of Drosophila deploy transposon-complementary endo-siRNAs to repress their activity. It was unclear how their precursors are generated or which mechanism leads to preferential targeting of transposons. Several publications now report progress in our understanding of endo-siRNA biogenesis and propose the first models for how "self"-DNA might be distinguished from selfish DNA.