  Using imaging from the Hubble Space Telescope, we derive surface brightness
profiles for ultra-compact dwarfs in the Fornax cluster and for the nuclei of
dwarf elliptical galaxies in the Virgo cluster. Ultra-compact dwarfs are more
extended and have higher surface brightnesses than typical dwarf nuclei, while
the luminosities, colors, and sizes of the nuclei are closer to those of
Galactic globular clusters. This calls into question the production of
ultra-compact dwarfs via ``threshing'' whereby the lower surface brightness
envelope of a dwarf elliptical is removed by tidal processes, leaving behind a
bare nucleus. Threshing may still be a viable model if the relatively bright
Fornax ultra compact dwarfs considered here are descended from dwarf
ellipticals whose nuclei are at the upper end of their luminosity and size
distributions.
