Jealousy is one of the central themes of Othello, and the play shows it being planted: act by act, Iago leads a trusting man to kill his innocent wife over a suspicion that he himself provokes. The play also deals with honor and reputation, with prejudice, and with the distance between appearing and being.
Jealousy has a declared origin. In Act II, Scene 1, Iago says in a soliloquy that, if everything fails, he wants at least to plunge the Moor into a jealousy so intense that he loses his wits. In Act III, Scene 3, he gives the thing its name to Othello, a monster "with green eyes, that mocks the flesh", and in Scene 4 Emilia says: "Jealousy comes from jealousy."
Reputation appears in Act II, Scene 3, when Cassio, dismissed, laments: "Reputation, reputation, reputation! O, I have lost my reputation!" Prejudice appears in Act I, Scene 1, when Iago shouts to Brabantio that a "black ram" is covering his "white ewe", and returns in Act III, Scene 3, when Othello, alone, thinks his wife may have left him perhaps "because I am black".
The distance between appearing and being appears in Iago's confession to Roderigo, "I am not what I am", and in the "honest" that Othello and others attribute to him.