Figure 6

Onset of DNA methyltransferase expression during mammalian germ cell development. The results presented in this paper as well as observations made for the male germ line [25, 26] are depicted here in relationship to the timing of de novo methylation establishment on imprinted genes and repeats during germ cell development. The figure is adapted from [47]. The top panel illustrates the methylation dynamics of maternally and paternally methylated imprinted genes, depicted by the red and blue lines respectively. During gametogenesis the pattern of non-imprinted gene methylation closely resembles that of imprinted genes. The time at which DNA methyltransferases are expressed during germ cell development are indicated with the transcripts examined in this study highlighted in bold. Although expressed at similar stages of oogenesis, the de novo DNA methyltransferases, Dnmt3a and Dnmt3b, are grouped separately from Dnmt3L which does not have DNA methyltransferase activity. The onset of Dnmt1o expression has been previously shown in early growing oocytes [43]. Our findings show that the expression of Dnmt3a, Dnmt3b and Dnmt3L coincide with the establishment of DNA methylation on imprinted genes and repeat sequences in the female germ line. The progression of methylation imprint acquisition during oogenesis is illustrated in the bottom panel and depicted by the red shading above the female germ cells. PGC = primordial germ cell, Oog = oogonia, PL = preleptotene, L = leptotene, Z = zygotene, P = pachytene, NG = non-growing oocyte G = growing oocyte, MII = metaphase II oocyte.