The point is that $+Q$ and $-Q$ are not new charges being placed there. They just indicate differences in charge.
If you in the wire
- have a lot of electrons added to one end, then there is a huge total negative charge at that end.
- There doesn't have to be any positive charges at the other end - simply the fact that there are less negative charges at that other end is enough.
A lot of negative charges gathered at the same spot will strongly repel each other. So any other point nearby with from which the negative charges are repelled less from, makes them want to move there.
All such other places are then defined as points positive charge; not because they have any positively charged particles, but because there is less negative charge here than at the other end. We will symbolise that less negative point as $+Q$, while the negative end is symbolised $-Q$.
- The negative charge will push each other towards the other end from which they are less repelled. Soon, they are balanced out, and the total charge throughout the wire causing a repulsion force from each point is the same at each point along the wire. No charges flow, no current.
- If you now add more charges to one end, then we again have a charge imbalance. Then again there is a higher repulsion from this point that from the other end. So, negative charges will as before want to flow to the other end from which they again are repelled less.