On a summer morning in 2009, in canal locks east of Toronto, police made a grisly discovery: In a submerged Nissan car were the bodies of three teenage sisters and a 52-year-old woman.

A joyride gone tragically wrong, claimed the father, Mohammad Shafia, 58, who reported the disappearance.

An 'honour killing', prosecutors allege. A murder trial is under way, heating up a national debate about how to better absorb immigrants into the Canadian cultural mainstream.

The prosecution accuses Afghan-born Shafia, his wife, and their 20-year-old son of killing the daughters because they dishonored the family by defying its disciplinarian rules on dress, dating, socialising and going online.

The older victim was Shafia's first wife, Rona Amir Mohammad, who was living with him and his second wife, Tooba Mohammad Yahya, 41, in Montreal.

It was a polygamous relationship, the court has been told, and if revealed, could have resulted in their deportation.

The parents and son, Hamed, have pleaded not guilty to four counts of murder.

The family had left Afghanistan in 1992 and lived in Pakistan, Australia and Dubai before settling in Canada in 2007. Shafia, a wealthy businessman, married Yahya because his first wife could not have children.

The second marriage produced seven children.

The months leading up to the deaths were not happy ones in the Shafia household, the court has heard.

Zainab, the oldest at 19, was forbidden to attend school for a year because she had a young Pakistani-Canadian boyfriend, and she fled to a shelter, terrified of her father, the court was told.

The jury heard testimony that Zainab's sisters, Sahar, 17, and Geeti, 13, were hounded and trailed by their brothers because the parents suspected them of dating boys; that Sahar repeatedly said her father would kill her if he found out she had a boyfriend; that she had bruises on her arms; that Mohammad, the first wife who was helping to raise the children, also was brutally treated.

Zainab ran away from home for a couple of weeks and her sisters contacted authorities, saying they wanted to be removed from the home because of violence and their father's strict parenting, the prosecution said.

Prosecutor Laurie Lacelle presented wire taps and cell phone records from the Shafia family in court.

In one phone conversation, the father says his daughters 'betrayed us immensely'.

Fazil Javad, Shafia's brother-in-law, said Shafia tried to enlist him in a plan to drown Zainab.

"Even if they hoist me up to the gallows, nothing is more dear to me than my honour. There is nothing more valuable than our honour," Lacelle quoted Shafia as saying in an intercept transcript.

Taking the stand and speaking in his native Dari through an interpreter, Shafia portrayed himself as a loving father with his daughters' best interests at heart. He repeated his contention that the famil members were returning from a Niagara Falls holiday, were in two cars, and were overnighting at a motel when Zainab took one of the cars.

The daughters met an accidental but 'rightful' death for their disobedience, he said.

"You believe there's no value in life without honour, don't you?" asked Lacelle in cross-examination.

"My honour is important to me," Shafia replied.

"But you can't regain your honour with murder, respected lady, you must know that.

"I'm a strict Muslim, but I'm not a killer."

Other relatives — two of the children and a brother-in-law of Shafia — testified in support of the joyride scenario and portrayed the family as loving and caring.

The trial then adjourned for the holidays and will resume on January 9.

Canada takes in 250,000 immigrants a year, more per capita than anywhere save Australia, and in recent years a number of so-called honor killings have prompted debate about absorbing immigrants into the mainstream and dealing with culture clashes between immigrant parents and their children.

Even before the trial, Rona Ambrose, the women's affairs minister, had said the federal government was considering making such killings a separate category in the criminal code.

Her office has not replied to recent questions about whether the change is going through, and the debate continues about the larger issues the Shafia case has raised about assimilating immigrants.

More than 80 Canadian Muslim organisations, imams and community leaders have signed a call for action against 'the reality of domestic violence within our own communities, compounded by abhorrent and yet persistent pre-Islamic practices rooted in the misguided notion of restoring family honour'.

On the other hand, statistically, nonimmigrant Canadians have a higher rate of murdering spouses and children, in some instances, also over family dishonor.

Jeffrey Reitz, a sociology professor at the University of Toronto who specialises in immigration issues, warns against using the term honour killings and equating it with any specific culture.

"If you label it an honour killing, the tendency is to say, 'Oh, what a terrible culture that is,' and the problem (of domestic violence) stems across cultural groups," he said.

The United Nations reports 5,000 females a year are victims of honour killings around the world. In Canada, social worker Aruna Papp says she has counted 15 cases since 2002, while psychiatrist Amin Muhammad, commissioned to write a report for the government about honour killings in Canada, predicts there will be more as immigrant communities grow, bringing in some newcomers with militant cultural beliefs.

"Immigrants who come here can't bring their own mindsets with them. They can't practice their own cultural ideologies if they go against the grain," he said.

The government must do more, and offer services that are more visible and accessible, especially to non-English-speakers, he said.

Tarek Fatah, the Pakistani-born founder of the Muslim Canadian Congress, is a fierce opponent of Islamic militancy. He says it is shocking that honour killings are happening in Canada, calling them 'a slap in the face of our fundamental value of what it is to be a human being'.

Papp, the social worker who wrote a report on honour killings for the Frontier Centre for Public Policy, a privately funded conservative think tank, worries that domestic violence rooted in family honour has spread to second-generation families.

She argues for tougher background checks on would-be immigrants, as well as teaching immigrants Canadian rights and values.

Papp, who is of Indian descent, speaks from experience.

"I came here when I was 21, with a third-grade education. I had children when I was young. I didn't know how to properly parent," she said.

"I did and said things I didn't know at the time were wrong, things my parents did and said to me growing up that were acceptable within the Indian culture. It's a learning process. Parents, especially immigrant parents, need to be taught parenting skills and what's acceptable behavior here."