Toronto: 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 dishonoured 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 Rona, 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 court heard.

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.

The trial is adjourned until January 9.