The Philadelphia Stock Exchange and six Wall Street firms that became its investors last year are being sued by a shareholder, alleging that the transaction diluted his ownership interest without adequate compensation.

The lawsuit, filed under seal in Delaware’s Court of Chancery on Tuesday because of confidential documents provided by the exchange, also named as defendants Citigroup Inc., Merrill Lynch & Co., Credit Suisse Group, Morgan Stanley & Co., UBS AG and Citadel Derivatives Group LLC.