Figure 7

Proposed mechanism for the increase in the number of transposons following injection-mediated Tol2 remobilization. One-cell embryos harvested from an outcross of 12M2, with a single transposon integration (hbr), were injected with Tol2 mRNA. GFP-positive tadpoles were raised to adulthood and outcrossed to identify germline transmission of the remobilized transposon. Analysis of progeny from remobilized frog 12M2♀5 indicated that many of the offspring had inherited multiple copies of the Tol2XIG transposon. The Tol2 transposase uses a simple 'cut-and-paste' transposition mechanism and the apparent increase in transposon content in the progeny was unexpected. The likely explanation for the increase in number of Tol2 transposons is the remobilization of the substrate following DNA replication, but prior to mitosis (reviewed in [42]), in the blastomeres in the early embryo. DNA replication (S phase) results in two copies of the Tol2 substrate in each cell and one, or both, copies can be remobilized by the injected Tol2 transposase enzyme. During mitosis the transposon alleles are randomly segregated into the daughter cells. In the example depicted in the cartoon (a), the resident Tol2 transposon on the hypothetical blue chromosome is replicated during S phase and one copy is remobilized by the Tol2 transposase to a new location on a hypothetical grey chromosome. If the targeted blastomere gives rise to a germ cell, then the resulting gametes may contain different combinations of the transposon alleles (b) and the resulting tadpoles may have one or both of the integration events.