Rodney McKay and Samantha Carter may have found a way to tame their rivalry three years ago: by each getting to be the smartest person in their own galaxy. But that’s all about to change when Colonel Carter moves to Stargate Atlantis this season … and takes over as Rodney’s boss.

“Carter” actress Amanda Tapping joins the cast of Atlantis for its fourth season, premiering tonight at 10 p.m. Eastern/Pacific on SCI FI Channel in the U.S.

“There are a couple of scenes right off the top that are very funny between the two of them — and I mean laugh out loud funny, for me when I read them,” Tapping said in a SCI FI Channel press event Wednesday. “And then she backs off. She really does allow McKay his chance to shine and she doesn’t want to usurp his power. And she does the same with Sheppard. She really backs off a lot in terms of, you know, that sibling type of rivalry. She knows that she can’t really go there, especially publicly, with McKay. There’s the odd barb because she can’t help herself … and because he’s McKay!”

The feud was first ignited in the fifth season of Stargate SG-1, when McKay was introduced as an unbelievably arrogant scientist who is deathly allergic to citrus, and who regarded Sam as a “dumb blonde.” The two got along a little better when he returned in Season Six’s “Redemption” … which even ended with a peck on the cheek for Rodney.

Rodney gets a fond farewell in SG-1 Season Six’s “Redemption, Part 2.”

The two also butted heads — in a more controlled environment — when SG-1 visited the Pegasus Galaxy in last year’s “The Pegasus Project.” And when McKay found himself trapped alone in a sinking Puddle Jumper in “Grace Under Pressure,” who should he and his head injury conjure up but a fantasy of Carter in her bathing suit.

With such strong personalities and loads of smarts, don’t expect the sibling rivalry to simmer on the back-burner forever. “There’s an episode coming up towards the end where the relationship comes back again,” Tapping said. “That sort of sibling snarkiness comes back. There’s a different amount of respect that she has for McKay now because he’s in a different position, and she has to respect where he is. So it’s a more respectful relationship in a lot of ways. Not without its humor, though.”

And hopefully not without the constant threat of citrus.

Stargate Atlantis premieres tonight at 10 p.m. Eastern/Pacific on SCI FI! Learn more about what is to come in GateWorld’s Season Four episode guide.