2013 USL Pro champion Orlando City may be on its way to MLS, but its current third-tier league is building quickly. (Scott A. Miller/AP)

On Wednesday afternoon, the United Soccer Leagues will announce the addition of a USL Pro club in Tulsa that will kick off in 2015.

For those keeping track (and it’s likely that an increasing number are) it means the third-division circuit -- which fielded just six teams in 2010 – will have at least 15 a little more than a year from now. And there are more on the way.

The catalyst for that rapid growth was last January’s landmark agreement between USL Pro and MLS, which will result in the demise of the latter’s reserve league at the end of 2014. In its place, MLS clubs will be able to either affiliate with an independent USL outfit and send players down for seasoning, or enter and run their own team in USL Pro.

The initiative will do more than provide MLS clubs with a place to develop younger talent (see Dom Dwyer, Sporting Kansas City/Orlando City). It also has solidified the USL’s professional product after more than 20 years of unpredictability and upheaval. Whether it’s because of the potential for a path to MLS like the one taken by Orlando or the marketing and technical boost offered by affiliation, owning a USL Pro team has become significantly more attractive. One source told SI.com that the league entry fee already has doubled in the past year to $500,000.

“Our league doesn’t lack for ambition,” USL president Tim Holt said during a recent interview. “There are certainly many markets in our league, from a size standpoint, where Major League Soccer isn’t really an option. So in a broader sense, [USL] management and owners will work to continue building the strongest possible league that we can below, and in support of, Major League Soccer … We don’t have the ambition to compete with them one day. We recognize we play a very important role on the pro soccer landscape.”

The new nature of that role has generated a lot of activity, and even a bit of turbulence. As Tulsa prepares to come aboard and as the final season of the MLS reserve league approaches, here’s where things stand on the active expansion and affiliation fronts:

EXPANSION

Sacramento Republic and Oklahoma City Energy are scheduled to join USL Pro next year, while VSI Tampa Bay and the Antigua Barracuda have exited. The L.A. Galaxy almost certainly will become the first MLS organization to field a standalone USL Pro team, which would lift USL Pro to 14 clubs in 2014. The Galaxy likely will unveil its plans early next month.

Sacramento is in good shape. Head coach Preki Radosavljevic has been on the job since July and the club already has sold more than 3,100 season tickets. It will construct and play in an 8,000-seat, soccer-specific stadium at the Cal Expo fairgrounds while it searches for the site for a permanent facility meeting MLS standards. The Republic has every intention of following Orlando and needs to move forward on a stadium project before attracting the investment required to make the jump.

Oklahoma City isn’t as far along, but it appears the Energy won’t be going it alone. The club is talking with recently retired Sporting KC goalkeeper Jimmy Nielsen about becoming its first coach, which would establish strong ties with the new MLS champion.

Then, in 2015, “The floodgates will open,” a source said.

Colorado Springs, a likely affiliate of the Colorado Rapids, already has been confirmed. FC Dallas is expected to enter the league as well. Club technical director Fernando Clavijo said Tuesday that FCD was considering starting next year, but, “we decided that we are going to wait, review things, and hope for 2015 to make things happen.”

Word is that Real Salt Lake also may field a USL Pro team in 2015. The club has had significant success with its Arizona youth program, which won the U.S. Soccer Development Academy under-16 championship this year, and now is looking to branch out to Southern California. A source told SI.com that RSL owner Dell Loy Hansen is looking to launch a USL Pro outfit in San Diego County, where he keeps a home.

Premier Development League (fourth tier) club FC Tucson also is considering a jump to USL Pro in 2015.

USL’s goal is to make Pro a national league focusing on regional rivalries and scheduling.

AFFILIATION

There were four partnerships this year: Orlando-SKC, Richmond Kickers-D.C. United, Rochester Rhinos-New England Revolution and Harrisburg City Islanders-Philadelphia Union. Those will continue in 2014, when they’ll be joined by the Pittsburgh Riverhounds and Houston Dynamo.

Sacramento and the Portland Timbers also are hoping to announce an agreement soon, but the process has been delayed by the San Jose Earthquakes. The Republic initially sought an affiliation with the Quakes, who play some two hours to the south, but were rebuffed. The next logical choice was Portland, especially considering the fact that Republic founder and president Warren Smith was part of the group that sold the Timbers to Merritt Paulson in 2007. The two clubs forged a deal that called not only for Portland players to head south, but at least one coach as well.

Citing territorial concerns, the Earthquakes have lobbied MLS to block the Sacramento-Portland partnership. Neither a short-term Timbers marketing presence in Northern California, nor the prospect an eventual MLS club in the state capital, is appealing. A ruling is expected soon, and it’s unlikely San Jose will get its way (although some sort of co-affiliation remains an option).

In the Midwest, the Columbus Crew and Dayton Dutch Lions are expected to announce an agreement in early 2014. The Lions entered USL Pro in 2011 and suffered through two miserable seasons before making the playoffs last year.