An upcoming British and Irish Lions tour always adds a bit of extra spice to the Six Nations.

If the tour goes ahead as planned and is not derailed by the pandemic, about 40 British and Irish players will be heading to South Africa in five months' time.

But who will be on the plane? Each week of the Six Nations, BBC Sport will be looking at who might have played their way into, and out of, Gatland's squad in our Lions Watch feature.

Before the tournament begins, here are four key areas of competition to keep an eye on over the next couple of months.

Absent props give others chance to shine

Mako Vunipola - who started all three Lions Tests in 2017 - has been ruled out of England's Six Nations opener against Scotland on Saturday because of a calf injury.

Team-mate Joe Marler has withdrawn from the squad for personal reasons and the pair's absence could provide an opportunity for other props to grab Gatland's attention.

Leicester's Ellis Genge will surely be England head coach Eddie Jones' first-choice loose-head and a string of starts would give the 25-year-old the chance to show he has what it takes to dominate a strong Springbok scrum as well as cause havoc in the loose.

Back in April 2020, legendary Lions coach Sir Ian McGeechan said he would choose Scotland's Rory Sutherland as his starting loose-head against South Africa.

Sir Ian was one of many hailing Sutherland's return after a life-altering injury and the 26-year-old lived up to the hype with some eye-catching displays in the 2020 Six Nations.

There is a former tourist in the mix too, with Ireland's Cian Healy - part of Gatland's 2013 squad - looking to make his mark again at the age of 33.

Young centres make late charge

Gatland has no shortage of experience at centre, with England captain Owen Farrell and 2017 Lions player of the series Jonathan Davies both on hand to help.

But with so many exciting young centres buzzing about this Six Nations, the Lions head coach could be tempted to make a surprise inclusion.

With England's Manu Tuilagi a doubt for the tournament because of an Achilles injury, Ollie Lawrence and Paolo Odogwu are jostling for the limelight.

After making his international debut in the autumn Worcester centre Lawrence, 21, now has three England caps to his name and has made a quiet but promising start to his Test career.

An England call-up for Wasps' 23-year-old centre Odogwu - who can also play on the wing - comes during a breakthrough Premiership season in which he has used his explosive combination of pace, footwork and power to make more clean breaks than anyone else.

The England pair will face opposition from north of the border, though.

Cameron Redpath - son of former Scotland captain Bryan - is set to make his Scotland debut, and there has already been plenty of talk about the 21-year-old thanks to his performances for Bath.

Harlequins and England scrum-half Danny Care describes him as a "serious player", adding: "He's one of those guys you can see that, once he gets his foot in the door in a 12 shirt internationally, it's going to be very hard to get it off him."

Now or never for Jones?

A Lions tour without Wales legend Alun Wyn Jones would not seem quite right - he has been on every one since 2009 after all.

There are encouraging signs that the Wales captain will return to full fitness soon after sustaining a knee injury in December.

But the 35-year-old lock will not welcome the uncertainty surrounding the tour because of the coronavirus pandemic, with postponing it for a year one option being considered.

Former Lion Ugo Monye said on BBC Radio 5 Live's Rugby Union Weekly podcast: "If the Lions tour gets postponed to 2022, I think Alun Wyn Jones could be a casualty."

Jones said he felt it would be "better in the long run" if the tour were to go ahead this year, providing it can do so safely.

Whatever happens, the second-row scenery is challenging with England's Maro Itoje and Ireland's James Ryan two stellar options.

But with England's Joe Launchbury out injured, there are still decisions to be made for Gatland and Scotland's Jonny Gray is poised to turn the New Zealander's head.

The 26-year-old proved his worth in big games in Exeter's double-winning season and, if he can continue to do the same with Scotland, may follow in older brother Richie's footsteps by earning a Lions call-up.

Wainwright bidding for back-row spot

Competition is just as high in the back row but one man who could work his way up the pecking order over the next couple of months is Wales' Aaron Wainwright.

Versatility is always useful on a long tour and it seems the 23-year-old can offer that. Having played most of his career at flanker, Wainwright is now seen as a number eight by Wales head coach Wayne Pivac.

There is a wealth of talent for Gatland to choose from in the back row - England stars Tom Curry, Sam Underhill and Billy Vunipola are just three heavyweight contenders.

But it would not be the first time Wainwright has received an unexpected call-up from the former Wales boss. The back row was brought into the Wales squad for the 2018 summer tour just three years after taking up rugby.

Since then Wainwright has established himself on the international stage, with a particular highlight being a man-of-the-match display in Wales' 2019 World Cup quarter-final.

"I think he's such a dynamic ball-carrier," Monye said. "He's really tall and rangy and a good line-out option. I really rate him."