  Faint extended elliptically-shaped ultra-diffuse galaxies and slightly
brighter and more compact dwarf elliptical and lenticular stellar systems are
common in galaxy clusters. Their poorly constrained evolutionary paths can be
studied by identifying young ultra-diffuse galaxy and dwarf elliptical analogs
populated with bright, massive stars. Using data mining we identified 11 such
low-mass ($2\times10^8<M_*<2\times10^9 M_{\odot}$) galaxies with large
half-light radii ($2.0<r_e<5$ kpc) and recently quenched star formation in the
Coma and Abell 2147 galaxy clusters. All galaxies happen to have
ram-pressure-stripped tails with signs of current or recent star formation.
Deep spectroscopic observations revealed rotating stellar discs containing
70-95% dark matter by mass. A large fraction of the disc stars (10-60%) formed
in intense star bursts 180-970 Myr ago, probably triggered by ram pressure.
Observed global gradients of stellar age corroborate this scenario. Passive
evolution in the next 10 Gyr will transform 9 of the 11 galaxies into
ultra-diffuse galaxies. If we assume a constant rate of galaxy infall,
44$\pm$16% of the most luminous present-day ultra-diffuse galaxies in Coma must
have formed via ram pressure stripping of disky progenitors.
