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.