  The domination game is played on a graph G. Vertices are chosen, one at a
time, by two players Dominator and Staller. Each chosen vertex must enlarge the
set of vertices of G dominated to that point in the game. Both players use an
optimal strategy---Dominator plays so as to end the game as quickly as possible
while Staller plays in such a way that the game lasts as many steps as
possible. The game domination number of G is the number of vertices chosen when
Dominator starts the game and the Staller-start game domination number of G
when Staller starts the game.
  In this paper these two games are studied when played on trees and spanning
subgraphs. A lower bound for the game domination number of a tree in terms of
the order and maximum degree is proved and shown to be asymptotically tight. It
is shown that for every k, there is a tree T with game domination number k and
Staller-start game domination number k+1, and it is conjectured that there is
no tree with game domination number k and Staller-start game domination number
k-1. A relation between the game domination number of a graph and its spanning
subgraphs is considered. It is proved that for any positive integer n, there
exists a graph G and its spanning tree T such that the game domination number
of G is at least n more than the game domination number of T. Moreover, there
exist 3-connected graphs G having a spanning subgraph such that the game
domination number of the spanning subgraph is arbitrarily smaller than that of
G.
