I have understood that IOTA uses a constant Seed and a variable index to generate Winternitz private and keys, to sign transactions.
PK^-1 = Hash (Seed + index)
So we can have practically endless PK pairs. This is something beyond the Lamport, Winternitz, Merkle, and some further OT-Signature schemes like CMSS, XMSS, and so on. SO: I have reviewed a few papers and some pages on the internet, and still do not know who came up with this first? IOTA Foundation? Winternitz? Merkle? I am trying to cite.