Fitch Ratings completed a review of ratings of jumbo mortgage-backed securities last week, resulting in downgrades of 6 percent of outstanding jumbo MBS. The downgrades were concentrated on pre-2005 MBS. “Adverse selection and structural features vulnerable to tail-risk have increased negative rating pressure for seasoned jumbo MBS,” Fitch said. The rating service noted that 14 percent of jumbo MBS remains on watch for downgrade and a determination on the ratings is expected by the end ... [Includes two briefs]