gas infall enhances SF or even triggers starbursts (see sect.\[dIrrs\]) and should not only affect dIrrs.

Dwarf spheroidal galaxies {#dSphs}
=========================

The possible cycle of morphological mutations, i.e. from gas-rich objects to gas-poor systems by means of gas expulsion and back to a significant gas content by gas accretion, can be explored in the local environment, namely, in the galaxies around the Milky Way, their satellites. Except the Magellanic Clouds most of these can be characterized as gas-free spheroidal systems which manifest the faint end of dEs. Since these dSph are gathered around massive galaxies like our MWG and M31 orbiting them as satellites, go down to the lightest and most metal-poor end of galaxies, they have attracted increasing attention over the last years with the advent of more advanced observing facilities. Understanding their formation and evolution is of substantial relevance for our astrophysical picture of cosmological structure formation and of galaxy evolution. Four main questions are addressed:

1.  How and when did they form? They all harbour a very old stellar population [@tol09] and, therefore, seem to have been unaffected by the re-ionzation era [@gre04].

2.  Is their existence as satellite system typical for all massive galaxies? Their origin 