  We identify six ultra diffuse galaxies (UDGs) outside clusters in three
nearby isolated groups (z<0.026) using very deep imaging in three different
SDSS filters (g, r and i bands) from the IAC Stripe82 Legacy Project. By
comparing with the abundance of UDGs in rich galaxy clusters, we find that the
density of UDGs (i.e. their number per unit mass of the host structure where
they are located) decreases towards the most massive systems. This is
compatible with a scenario where UDGs are formed preferentially outside
clusters. In the periphery (D>250 kpc) of our three groups, we identify a
population of potential UDG progenitors (two of them confirmed
spectroscopically). These progenitors have similar masses, shapes and sizes but
are bluer, g-i~0.45, (and for this reason brighter, mu_g(0)<24 mag arcsec -2)
than traditional UDGs (g-i~0.76). A passive evolution of these progenitors will
transform them into regular (i.e. mu_g(0)>24 mag arcsec -2) UDGs after ~6 Gyr.
If confirmed, our observations support a scenario where UDGs are old, extended,
low surface brightness dwarfs (M*~10^8 Msun) galaxies born in the field, are
later on processed in groups and, ultimately, infall into galaxy clusters by
group accretion.
