First evidence of CpG methylation in genomic DNA of Tribolium castaneum embryos.

DNA methylation has been studied in many eukaryotic organisms, in particuIar vertebrate, and was implicated in developmental and phenotypic variations. Little is known about the role of DNA methylation in invertebrates, in particular insects which are considered as excellent models for studying the evolution of DNA methylation. The beetle Tribolium costaneum (tenenrionidae Coleoptera) is one of the most important genetic model organism and store products pest but no evidence of DNA methylation has been found in this species till now. This work describes,for the first time, a CpG methylation in Tribolium castaneum detected by methylation sensitive rertriction endonucleases. By comparing differential digestions, a CpG methylation was demonstrated to be present in embryos of Tribolium castonelum and not in larvae, pupae and adults, independently from the sex. Furthermore, a lack of CpG methylation of the major, abundant pericentromeric satellite DNA TCAST in all developmental stages, including embryos was donstrated. Satellite DNAs are the principal component of constitutive heterochromatin and DNA methylation at cytosine seems to contribute to heterochromatin formation. The absence of CpG methylation within TCAST satellite DNA, as well as from the satellite DNA from other insect such as Lepidoptera Momestra brassicae seems to argue in favour of different mechanisms involved in heterochromatin formation in these species, based probably on satellite DNA transcripts' The evolutionary significance of global DNA demethylation observed in early developing embryos of most mammals remains far from clear as well as the enzymatic machineries involved in DNA demethylation. The evidence of a global CpG methylation in embryos and of a CpG demethylation occurring during metamorphosis of Tribolium castaneum indicates possible function of this epigenetic mark in developmental regulation of insects. We propose a role of CpG methylation during the early stages of Tribolium castaneum development and' exclude a DNA methylation based mechanism of heterochromatin formation.