LOS ANGELES—Jamie Spears, father of Britney Spears, has asked for an investigation of the pop superstar’s claims that she had been mistreated under a legal conservatorship by being denied the ability to make her own medical decisions. In a court filing late on Tuesday, attorneys for Jamie Spears said he was “greatly saddened to hear of his daughter’s difficulties and suffering” and “believes there must be an investigation into those claims.” Britney Spears has been under a conservatorship since she suffered a mental health breakdown in 2008. The “Stronger” singer told a Los Angeles judge last week that she had been forced to take the drug lithium against her will and was prohibited from marrying or removing a birth control device, and that she wanted the “abusive” conservatorship to end. Jamie Spears is a joint conservator of his daughter’s finances and also was in charge of her personal affairs until …