The manual for Super Mario Brothers states that Princess Toadstool was the daughter of the Mushroom King.
While it is good that the lovely Princess Toadstool wasn't created by a process of sporing, but indeed has parents like the rest of us, nothing else is mentioned about the good king. We don't even know if he is alive.
It's just think kind of little detail that enchanted a generation. A character is brought up, and never mentioned again, but all through the game, the player never knows when he is going to turn up. It is little teasing mentions of such thing that led school children to spread rumors of the mysterious negative world across the school yards of America for five years. I also heard rumors that if you beat the game a certain amount of times, the Mushroom King, instead of the Princess, would appear at the end of Level 8-4. My sister once played the game through somewhat like 2 dozen times, trying to see if he would show up.
The only thing better then cinematic plot, character and plot development and revelations about the origin of life on earth is a good background story that lends mystery to every single thing. And the only type of background story that can do that mentions things that never appear.