Ashlee Simpson files for divorce from Pete Wentz

Ashlee Simpson has filed for divorce from Pete Wentz after less than three years of marriage.

Simpson-Wentz, 26, and Wentz, 31, signed documents Monday and filed them Tuesday in Los Angeles Superior Court, citing irreconcilable differences. There was no third party in the case.

The spIit between the singer-actress and the musician was "very amicable," sources tell People.

"They've just grown apart. They've been growing apart for quite some time."

But TMZ is saying that Jessica Simpson's little sis couldn't handle the former Fall Out Boy bassist's "erratic behavior."

The couple married in May 2008 after dating for about two years. They have a 2-year-old son, Bronx Mowgli Wentz, of whom Simpson-Wentz is seeking primary physical custody with visitation for her soon-to-be-ex.

"We remain friends and deeply committed and loving parents to our son Bronx, whose happiness and well-being remains our No. 1 priority," the couple said through a representative. "We ask that everyone honor our privacy as we navigate this next phase of our lives."

"dont forget 2 notice-when i look into the lil dudes eyes & smile, the way he loves. i realize @ashsimpsonwentz is a good moma + so much more," Wentz tweeted Monday.

Simpson-Wentz is also seeking spousal support from Wentz, who is touring with his new band, Black Cards. She plans to drop "Wentz" from her last name.

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Photo: Singer Ashlee Simpson and musician Pete Wentz on Dec. 15, 2010, in Hollywood. Credit: Christopher Polk / Getty Images for JS