The Mulberry Tree
Throughout Middlesex, Eugenides uses the symbol of the mulberry tree to relate Cal to his family history and the silk thread that figuratively binds him to his ancestors. On page 22, Desdemona watches as the silkworms “clung to bundled mulberry twigs” in her cocoonery. Later, at the beginning of The Silk Road chapter, the narrator Cal tells the story of Princess Si Ling-chi who was sitting under a mulberry tree in 2640 BC when a cocoon fell into her tea. This allegedly lead to the discovery that the cocoons could be unraveled to produce miles of silk thread. Fortunately, Cal does his own symbolic analysis for us and states, “Like her I unravel my story, and the longer the thread, the less there is left to tell” (63). Interestingly, this simile seems the opposite of the structure of the story Eugenidies is telling. Each chapter starts with two ends of the story: Cal’s present situation and the mutated gene’s history in 1922 Asia Minor. As we read, we slowly gather up more and more of the story, getting closer to where the ends meet. From Cal’s point of view, the story is unraveling, but the reader ends up with the finely wrapped cocoon.
If we fast forward to chapter Middlesex, we find that a mulberry tree is growing against the front of the house into which the Stephanides family moves. Though the family has moved up in status, the ties to the Old Country and the livelihood made on silk are not broken. In the very last paragraphs of Book Three, Callie finds herself packed up and about to be whisked off to the doctor in New York a week after the discovery at the hospital. Sitting with her suitcase and picking mulberries off of the tree to keep her mind off things, she says:
The mulberries had ripened in the last week. They were fat and juicy. The berries stained my hands. Outside, the sidewalk was splotched purple, as was the grass itself, and the rocks in the flower beds. The sound in my parents’ bedroom was my mother weeping. 
Like the berries, the truth about Callie’s genes has just about burst. Accordingly, Callie has been forever stained by her parents’ and grandparents’ incestuous relations. From Desdemona’s mulberry trees on Mount Olive to Callie’s mulberry tree at Middlesex, the gene had been waiting for this moment, smuggled from generation to generation like silk while finally ripening with Callie’s entrance into puberty. In conclusion, Callie defines this “section” of her life with this fact:
The most widely raised type of silkworm, the larva of the Bombyx mori, no longer exists anywhere in a natural state. As my encyclopedia poignantly puts it: “The legs of the larvae have degenerated, and the adults do not fly.” 
What could this mean? Since the silkworms were bred by humans to be their silk producers, their natural state of legged larvae and flying adults became unnecessary and the species evolved. Perhaps the statement refers to Calliope’s natural male state and his evolution by way of family nurture. They raised Cal as a girl and he became crippled under society’s expectations. For example, he had to pretend to menstruate, had to undress alone in the locker room, and had to go through the motions with Jerome.
In any case, I liked that this statement was left to the reader’s interpretation, since often Cal interprets on his own. Other ideas?