On "Doctor Who," every companion character comes with an expiration date. Some may last longer than others, but none of them last forever. Karen Gillan's Amy Pond holds the record for the modern "Who" era, at 31 episodes, two Christmas specials and a cameo in Matt Smith's last episode. Billie Piper's Rose Tyler made 35 total appearances, but at least four of those were cameos, and her final appearance wasn't as Rose. Freeman Agyeman and Catherine Tate lasted just one season each.
Jenna Coleman joined the show as Clara Oswald with the 2012 Christmas special, "The Snowmen," after a mysterious earlier appearance in the Season 7 premiere, "Asylum of the Daleks." She starred in the second half of Season 7, the 50th Anniversary special "The Day of the Doctor," the 2013 Christmas special "The Time of the Doctor" and stars in all of Season 8 alongside the new Doctor, Peter Capaldi.
The Daily Mirror is reporting from inside production sources that Coleman has decided that one more season is all Clara has in her. It's unknown how her character will exit the show, but she's expected to depart sometime during the 2014 Christmas special.
"The conversations about Jenna's exit have started, and a plan is in place which is being ironed out," the source told the Mirror. "It was very much a mutual decision," they added.
Companions have a higher turnover rate than the Doctor himself; there have only been 12(ish) Doctors, but almost 60 companions have joined him on his adventures over the years. Often, when a new actor takes on the title role, the last Doctor's companion is kept on the show as a sort of anchor for the audience. Fans believe this to be an intentional way of keeping the main character's transition from being too jarring. Billie Piper, for example, stayed for a whole season after watching Christopher Eccleston regenerate into David Tennant's 10th Doctor.
It's not terribly surprising news for Coleman, who's on record as saying that she was quite upset by Matt Smith's departure. She and Smith were very close in age, and had a flirtatious friendship on the show. Coleman's character was famously created to be a plucky, clever, fast-talking match for Smith's 11th Doctor, who demonstrated those same qualities.
Capaldi is literally twice Coleman's age, so any potential for romance has already been struck down by showrunner Steven Moffat. Clara's relationship with the far more serious 12th Doctor will no doubt be very different to what she's used to, so her departure could make way for a companion who's designed as a more ideal fit for Capaldi.
The Mirror's news has not yet been confirmed by the BBC or BBC America. Season 8 of "Doctor Who" premieres to much fanfare on Saturday, August 23, 2014.