Updated

By Larry Fine

(Reuters) - A power shift that could darken the prospects of the World Series champion St. Louis Cardinals and threaten the Texas Rangers' American League dominance is one of many changes in store for the 2012 Major League Baseball (MLB) season.

The Seattle Mariners and Oakland Athletics launch the six-month regular-season marathon with a two-game series starting March 28 in Japan's Tokyo Dome before the campaign gets underway in MLB's home parks starting on April 4.

The climax of the season will have a new layer of suspense with the addition of a second wildcard berth for the playoffs in both the American League (AL) and National League (NL), bringing the number of postseason teams to 10 from eight last year.

That should provide hope for more teams at the finish, and will produce one-game showdowns for advancement to the best-of-five divisional playoffs. It will also add urgency for teams to win a division in order to avoid the one-and-done scenario.

On the diamond, the biggest changes came with a pair of blockbuster free-agent signings.

Albert Pujols, a three-time NL Most Valuable Player (MVP), joined the Los Angeles Angels while the Detroit Tigers landed Prince Fielder in a hijacking of two of the NL's top sluggers.

Pujols, who hit .299 with 37 homers and 99 runs batted in last season for the Cardinals, signed a massive 10-year, $254 million deal with the Angels in December, six weeks after winning the World Series in St. Louis.

Fielder, who hit 38 homers and drove in 120 runs for the NL Central-winning Milwaukee Brewers, took $214 million for nine years to take his booming bat to the AL Central-champion Tigers, where his father Cecil Fielder once launched massive home runs.

The weakening of the Cardinals, who tried to offset the loss of Pujols by adding former Mets outfielder Carlos Beltran, and the Brewers flung the door open for the Cincinnati Reds to join the Central race after a big trade that added starter Mat Latos.

Besides putting Pujols into the middle of their lineup, the Angels also signed free agent starter C.J. Wilson, formerly of the division-rival Rangers, setting the stage for a titanic struggle between the two clubs in the AL West.

Texas countered by spending $111 million to acquire young Japanese pitching ace Yu Darvish, paying his Japanese team $51 million for his rights before agreeing to a six-year $60 million deal in January.

NEW MANAGERS

Five new managers will be operating from major league dugouts.

A Red Sox shakeup brings former New York Mets manager Bobby Valentine to Boston's dugout, where he is expected to breathe fire into another hot rivalry with the New York Yankees in the 100th anniversary season at Fenway Park.

Mike Matheny takes over from the retired Tony La Russa in St. Louis, Dale Sveum will steer the Chicago Cubs and Robin Ventura will direct the Chicago White Sox.

Major change was in order for the Miami Marlins.

The Marlins, hoping to put a lively product under the retractable dome of new Marlins Park, hired tempestuous skipper Ozzie Guillen and aims to make an NL East splash after adding Jose Reyes, the electrifying shortstop formerly of the Mets.

NL East success may be hard to come by given the formidable pitching of the Philadelphia Phillies. But Roy Halladay, Cole Hamels, and Cliff Lee may not have as many runs to work with as injuries were piling up on the veteran squad.

Slugging first baseman Ryan Howard will miss the early part of the season recovering from a torn Achilles tendon, while second baseman Chase Utley has been sidelined in spring training due to persistent pain in both knees.

But the signing of free-agent closer Jonathan Papelbon should help the reigning NL East champion Phils.

The AL East once again looks like the toughest division in the major leagues, with an improving Toronto Blue Jays team wondering what they have to do to move up in the standings.

The Yankees fortified their pitching by signing free agent Hiroki Kuroda, trading for hard-throwing Michael Pineda and welcoming back Andy Pettitte after a year off in retirement.

Boston hopes to rebound from a September swoon that allowed the Tampa Bay Rays to overtake them for a playoff berth with comeback years from Clay Buchholz and Kevin Youkilis, who were both hampered by injuries last season.

The Rays, as usual, will rely on young starting pitching. This season hard-throwing left-hander Matt Moore is pegged by many to be a leading candidate for rookie of the year honors.

Detroit looks to be easily the best in the AL Central with their potent lineup supporting a pitching staff led by Cy Young and MVP winner Justin Verlander, who posted a 24-5 record with a 2.40 earned run average and 250 strikeouts in 2011.

(Reporting by Larry Fine in New York; Editing by Frank Pingue)