A male apprentice, longtime companion and possible lover of Leonardo da Vinci was the main influence and a model for the "Mona Lisa" painting, an Italian researcher said Wednesday.

But the researcher, Silvano Vinceti, said the portrait also represents a synthesis of Leonardo's scientific, artistic and philosophical beliefs. Because the artist worked on it at various intervals for many years, he was subjected to different influences and sources of inspiration, and the canvas is full of hidden symbolic meanings.

This is one of many theories that have circulated over the decades about the identity of "Mona Lisa" and the meaning for her famously enigmatic smile. Others have said the painting was a self-portrait in disguise, or the depiction of a Florentine merchant's wife — the latter drawing a consensus among scholars....