Question

what is transposon methylation in plants, how does this occur and what is the effect the phenotype of plants and further what is the effects this has on darwinian selection?

Answer 1

DNA methylation is the used for transcriptional silecing. Transcriptional silencing is generally achieved through epigenetic modifications by DNA methylation and histone modifications. In DNA methylation, methylation of Cytosine bas takes place. Methylation of transposons supresses the activity of transposons, affects gene expression and, in turn influence the phenotype.

DNA methylation, by RNA-directed DNA methylation (RdDM) pathway. RdDM begins when the plant-specific RNA polymerase Pol IV transcribes a transposon. Then formed single-stranded RNA is processed into 24 nt siRNAs by RDR2 and DCL3. Resultantly 24 nt siRNAs guide protein complexes to homologous DNA sequences that are then targeted for cytosine methylation.

When DNA methylation occur, at least two mechanisms takes place to maintain it

(1) a positive feedback loop- Pol IV and Pol V, the RNA polymerases involved in RdDM, preferentially act on methylated DNA and imposing silencing.

(2) maintening CG and CHG (H = A, C, or T) methylation during DNA replication and cell division. Once a transposon is targeted for DNA methylation the host genome employs feedbacks to ensure that the transposon reaches and maintains a quiescent state.

If a transposon to insert into the centre of such gene that is crucial for plants survival, that gene become non-functional and the cell will die. Therefore, transposon-silencing methylation has an clear survival advantage (survival of fittest). Survival of the important phenomena of is evolution.