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 yesterday. 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 over many years, he was subjected to different influences and sources of inspiration, and the canvas is full of hidden symbolic meanings.

“The ‘Mona Lisa’ must be read at various levels, not just as a portrait,’’ Vinceti said.

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.

The world-famous portrait is on display at the Louvre Museum in Paris.

The apprentice, Gian Giacomo Caprotti, known as Salai, worked with da Vinci for more than two decades, starting in 1490. Vinceti described their relationship as ambiguous, and most art historians agree Salai was a da Vinci lover.

Several da Vinci works, including “St. John the Baptist’’ and a lesser-known drawing called “Angel Incarnate,’’ were based on Salai, Vinceti told a news conference at the Foreign Press Association.

These paintings show a slender, effeminate young man with long auburn curls.

Vinceti said similarities with the “Mona Lisa’s’’ nose and mouth are striking.

“Salai was a favorite model for Leonardo,’’ he said. “Leonardo certainly inserted characteristics of Salai in the last version of the Mona Lisa.’