Abstract
BackgroundCytosine DNA methylation has been detected in many eukaryotic organisms and has been shown to play an important role in development and disease of vertebrates including humans. Molecularly, DNA methylation appears to be involved in the suppression of initiation or of elongation of transcription. Resulting organismal functions are suggested to be the regulation of gene silencing, the suppression of transposon activity and the suppression of initiation of transcription within genes. However, some data concerning the distribution of methylcytosine in insect species appear to contradict such roles.Principal FindingsBy comparison of MspI and HpaII restriction patterns in genomic DNA of several insects we show that stick insects (Phasmatodea) have highly methylated genomes. We isolated methylated DNA fragments from the Vietnamese Walking Stick Medauroidea extradentata (formerly known as Baculum extradentatum) and demonstrated that most of the corresponding sequences are repetitive. Bisulfite sequencing of one of these fragments and of parts of conserved protein-coding genes revealed a methylcytosine content of 12.6%, mostly found at CpG, but also at CpT and CpA dinucleotides. Corresponding depletions of CpG and enrichments of TpG and CpA dinucleotides in some highly conserved protein-coding genes of Medauroidea reach a similar degree as in vertebrates and show that CpG methylation has occurred in the germline of these insects.ConclusionsUsing four different methods, we demonstrate that the genome of Medauroidea extradentata is strongly methylated. Both repetitive DNA and coding genes appear to contain high levels of methylcytosines. These results argue for similar functions of DNA methylation in stick insects as those already known for vertebrates.
Highlights
Variable proportions of cytosine residues in eukaryotic genomes are methylated
Using four different methods, we demonstrate that the genome of Medauroidea extradentata is strongly methylated
Both repetitive DNA and coding genes appear to contain high levels of methylcytosines. These results argue for similar functions of DNA methylation in stick insects as those already known for vertebrates
Summary
Variable proportions of cytosine residues in eukaryotic genomes are methylated. The percentage of methylcytosines ranges from 0– 10% in insects, about 3–10% in mammals and birds, about 10% in fish and amphibians up to 50% in some plants [1,2]. There have been many studies on DNA methylation, to establish its primary role that has led to its widespread distribution in higher organisms has proved to be controversial. It might be the suppression of activity of transponible elements [5,6]. It has been proposed that DNA methylation primarily acts to stabilize patterns of endogenous gene activity by maintaining gene silencing that was build by other means [7]. This could be used to transmit determined and differentiated states in cell lineages [8]. Some data concerning the distribution of methylcytosine in insect species appear to contradict such roles
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