Analysis of the expression pattern of the defensin gene in the lepidopteran Mamestra brassicae.

Southern blotting experiments performed on M. brassicae genomic DNA after digestion with methylation-sensitive restriction enzymes indicated that defensin gene is methylated at CpG targets in the promoter region. However, defensin gene is actively transcribed despite the presence of methylation. Experiments performed by genome demethylation indicated that demethylated defensin gene resulted in altered expression after bacterial induction. In particular, if defensin gene is demethylated it has not possible to observed any increase in gene expression after induction with Gram positive bacteria. The present results are very intriguing since they indicate not only that in M. brassicae DNA methylation is not involved in gene silencing but also that cytosine methylation could be essential to assure the expression of specific genes. Finally, the above reported data apparently argue against a unifying and evolutionary conserved role of cytosine methylation from invertebrates to vertebrates. In fact, it appears that the DNA methylation/gene silencing correlation, which is typically reported in vertebrates, appears not to hold true for insects.