A man takes a photograph of Leonardo da Vinci's masterpiece the Mona Lisa in the Louvre museum in Paris. (Amel Pain/Associated Press)

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.

But the researcher, Silvano Vinceti, said Wednesday 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.

"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 Leonardo for more than two decades starting in 1490. Vinceti described their relationship as "ambiguous," and most art historians agree Salai was a Leonardo lover.

Several Leonardo 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."

Skepticism from some scholars

It was not the first time that Salai's name had been associated with the Mona Lisa, though some scholars expressed skepticism. Pietro Marani, art historian and Leonardo expert, called the theory "groundless."

Vinceti said other influences may have affected Leonardo. He does not rule out that Lisa Gherardini, wife of Florentine merchant Francesco del Giocondo, may have provided an early inspiration.

Equally, Vinceti said further inspiration may have come from noblewoman Beatrice D'Este, who was married to Ludovico Sforza, the duke of Milan at whose court Leonardo worked in the late 15th century. Vinceti said that Leonardo often would see the woman while he was painting The Last Supper for the Monastery of Santa Maria delle Grazie in Milan, where she went to pray.

Traditionally, art historians say Leonardo started panting the Mona Lisa in 1503, when he was back from that Milan stay. But Vinceti has said he may have started in the late 1490s in Milan.

Vinceti, a media-savvy writer and art investigator, made his name when he said he had located Caravaggio's long-lost bones last year. He combines state-of-the-art, CSI-like techniques with old-fashioned library research.

Analyzing high-definition scanned images of the Mona Lisa, Vinceti claimed in recent weeks to have found the letters "S" and "L" in the model's eyes, and the number "72" under the arched bridge in the backdrop of the painting.

'All Leonardo subjects look like each other because he represents an abstract ideal of beauty. Therefore they all have this dual characteristic of masculine and feminine'—Pietro Marani

He attaches several symbolic meanings to these letters: the "S" pointed him to Salai and the Sforza dynasty that ruled Milan, while the "L" is a reference to the artist himself and Lisa Gherardini.

Marani, the Leonardo expert, said at least three historical documents prove that Gherardini was the original model. He said there are no known paintings of Salai, though he conceded it was entirely possible that the young apprentice might serve as a model for other Leonardo works such as St. John the Baptist.

But he warned against reading too much into possible similarities between subjects.

"All Leonardo subjects look like each other because he represents an abstract ideal of beauty. Therefore they all have this dual characteristic of masculine and feminine," said Marani, an art professor at Milan's Politecnico university.

"The work began as the portrait of Lisa Gherardini, but over the years in Leonardo's hands it slowly turned into something else: an idealized portrait, not a specific one," Marani said. "That's also why you have this fascinating face that transcends time and transcends a specific person, and why all these theories keep piling up."