Genomic imprinting is the parent-of origin-dependent expression of an allele from one but not both chromosomes. In mice, this epigenetic programming is erased in germ cells by DNA demethylation by the Tet enzymes when germ cells arrive at the genital ridge. DNA methylation is re-established after the determination of germ cell sex, by DNA methyltransferase (Dnmt) proteins. Dnmt3a and Dnmt3b are de novo DNA methyltransferases, Dnmt1 maintains methylation patterns during cell replication and Dnmt3l has no enzymatic activity but instead acts as a co-factor for Dnmt3a and Dnmt3b. In the mouse, Tet1 and Tet2 and Dnmt3a and Dnmt3l are the key players in germ cell DNA de- and re-methylation respectively. Control of the Dnmt gene family is complex, with alternative promoter usage and distinct expression patterns. Almost all of our knowledge on the roles of Dnmt and Tet proteins during germ cell development has come from the mouse. Marsupials diverged from eutherians around 160 million years ago, so examining DNMT and TET expression and localisation during marsupial germ cell development will inform us about the conservation of germ cell imprinting mechanisms in mammals.
We examined the expression and localisation of DNMT and TET members during germ cell development in the tammar wallaby using RT-PCR, qPCR and immunohistochemistry. The tammar uses two of the three DNMT1 promoters known to be active in the mouse and tammar DNMT3A2 expression replaces DNMT3A expression during female but not male germ cell development. DNMT and TET proteins show dynamic localisation patterns during the different phases of germ cell development (proliferation, mitotic arrest, meiosis, oocyte growth and spermatogenesis) and some of these patterns are similar to the mouse whilst others differ. This adds to the suggestion that several aspects of genomic imprinting differ between marsupials and eutherians, especially in the germ cells.