By Matthew Waller

MIDLAND, Texas (Reuters) - The former leader of a breakaway Mormon sect charged with being married to more than one woman at the same time went on trial in Texas on Wednesday, putting a spotlight on the rarely prosecuted crime of bigamy.

A jury of five women and seven men was chosen to hear testimony in the trial of Wendell Loy Nielsen, 71, a former president of the polygamist Fundamentalist Church of Jesus Christ of Latter Day Saints who is charged with three counts of bigamy.

The sect, which teaches that for a man to be among the select in heaven he must have at least three wives, is estimated to have 10,000 followers in North America.

Nielsen, wearing a blue sweater vest and shirt, looked on from the front of the courtroom during jury selection.

Patrick Metze, director of the Criminal Defense Clinic at the Texas Tech University School of Law, said he did not know of a bigamy case going to trial in recent memory.

"It isn't a crime committed and prosecuted very often," added George Dix, a criminal law professor with the University of Texas.

Most bigamy cases involve people from other countries married in the United States who said they did not know that their marriage to a person in another country was still valid, or when a divorce did not go through before another marriage took place.

Nielsen's situation was also unusual because questions remain over whether the marriages performed by the sect he once headed were valid under Texas law.

Prosecutors charge that Nielsen married two women on the same day in February 2006. According to a report filed in 2010, prosecutors believe Nielsen "married" 34 women in addition to his legal wife, although he is only being charged with three counts of bigamy.

The sect Nielsen headed has practiced polygamy ever since breaking away from the mainstream Church of Jesus Christ of Latter-Day Saints - also known as the Mormon church - in the late 1800s. The Mormon church has condemned the sect.

TWELVE MEN INDICTED

Nielsen was one of 12 men indicted for crimes including child sexual assault, bigamy and performing an illegal marriage after an April 2008 law enforcement raid on the sect's Yearning for Zion Ranch in Texas to check on accusations of sexual assault.

Ten of the men have been convicted, including church leader Warren Jeffs, who was found guilty last year of sexually assaulting two underage girls he wed as spiritual brides, the younger of whom was just 12 years old.

Jeffs is in prison in protective custody in Palestine, Texas, for life plus 20 years. But he still exerts influence in the sect through his brothers, sends out prophetic messages to public officials and has taken out advertisements in newspapers across the country.

Nielsen was the president of the sect's corporation in Utah until he stepped down so Jeffs, who was still the supreme leader, could assume the presidency early in 2011.

This is the first bigamy case of the 12 men to go to trial. Two have pleaded no contest to bigamy charges before and received seven and eight years in prison.

Bigamy is a third-degree felony punishable by two to 10 years in prison and up to a $10,000 fine.

Opening statements in the trial will begin on Thursday. Presiding Judge Robert Moore said the trial could last until March 30.

(Editing by Cynthia Johnston and Will Dunham)