* Gloucester centre replaces Tuilgai for Calcutta Cup match
* Marler in at prop as England seek to extend 30-year unbeaten home run (Adds details and quotes)
By Mitch Phillips
BAGSHOT, England, Jan 31 (Reuters) - Billy Twelvetrees will make his debut when England play Scotland in their opening Six Nations game at Twickenham on Saturday after coach Stuart Lancaster rewarded him for a flying first half of the season for Gloucester.
The 24-year-old centre has only just been promoted to the senior squad but goes straight into the starting team in place of Manu Tuilagi, who has an ankle injury.
Twelvetrees, who moved from Leicester last year in a bid to improve his England prospects, will start at inside centre, with tough-tackling Brad Barritt switching to 13.
With Alex Corbisiero ruled out for the tournament, Joe Marler has edged Mako Vunipola for the loosehead prop position in the only other change from the starting team that beat New Zealand 38-21 two months ago.
Toby Flood, who missed that game through injury, returns to the bench as Owen Farrell retains the flyhalf berth.
Tom Youngs held on to the hooker's jersey, despite the availability of former regular Dylan Hartley, who is named among the replacements after missing the whole Autumn series through injury.
Twelvetrees's inclusion takes the new cap tally to 19 since Lancaster began his stint in charge as interim coach for the corresponding fixture at Murrayfield a year ago.
He blooded seven new players in that match as England ground out a 13-6 victory en route to a second-place Six Nations finish that earned him the job on a permanent basis.
Still very much building towards the 2015 World Cup, which England host, the coach has stayed true to his word that he would reward players in form.
"I don't see a lack of international experience as a barrier to international selection," he said this week.
Twelvetrees is a powerful all-round centre who can run hard but also has some lateral movement and a sharp pass developed from years as a flyhalf. He also has a good kicking game, something absent from the England midfield for a long time.
Otherwise, Lancaster has kept faith with the team who hammered the All Blacks in one of England's most impressive performances for years.
"We are happy to stick with the team that did so well against New Zealand, especially in light of the fact that they have all been in good form for their clubs since then," he said in a statement.
"There were some very tight calls, with some experienced players coming back into contention, and I am sure the bench will have a significant impact on the game.
"We can't wait to get back to Twickenham and hopefully experience the same atmosphere as the autumn when the crowd was superb."
Fans expecting another flowing England display might have to limit their expectations, however.
Though Scotland have not won at Twickenham for 30 years and have claimed only two victories in their last 15 Six Nations matches, they have pushed their oldest rivals very closely in the last few meetings and with wind and rain forecast they will enjoy turning the contest into a scrappy battle. (Editing by Clare Fallon)