Hypophysectomized rats (those that have had their pituitary glands removed grow fastest when given both gh and testosterone together than either separately. what is this phenomenon called?

Based on the description above, the phenomenon that is being described is called SYNERGISM. Synergism occurs in this when both GH and Testosterone are being used and created a rather better outcome that it is given separately. The combined effect gave a better result.

  eukaryotic cells may arise from sexual reproductions, resulting in more genetic variation  i would say that is the answer