Motley Fool contributor Leo Sun's recent Agents of S.H.I.E.L.D. article produced some very interesting replies in the comments section. Many readers questioned why the show has yet to acknowledge the existence of mutants, wondered how Agents of S.H.I.E.L.D. missed such an obvious opportunity to introduce the Sub-Mariner, and talked about the future inclusion of a Spider-Woman story line. Unfortunately for Marvel fans, the tangled web that is Marvel's movie rights makes these and other crossover possibilities unlikely.

The Wolverine. Source: Marvel.com.

20th Century Fox
20th Century Fox -- now owned by 21st Century Fox (NASDAQ: FOX) (NASDAQ: FOXA) -- has been one of the most prominent names in superhero movies since 2000's X-Men essentially birthed the modern comic-book movie renaissance.

Today, Fox's X-Men film series comprises six movies that have collectively grossed more than $2.3 billion at the global box office. That is $2.3 billion and counting, as May 2014's X-Men: Days of Future Past and 2016's X-Men: Apocalypse are on the horizon, as well as a sequel to The Wolverine and an untitled X-Force movie, both in the early development stages.

Fox's other Marvel movie franchises have not been able to replicate that same level of success, however. 2003's Daredevil was met with mostly negative reactions from critics and fans; more so for the terrible 2005 Elektra spinoff movie. Today, the Daredevil and all related character rights have reverted back to Marvel Studios.

Fox found slightly more box-office success with Fantastic Four in 2005 and Fantastic Four: Rise of the Silver Surfer in 2007, although both films received decidedly negative reviews from critics and fans alike. The Fantastic Four movie rights still remain with Fox, which plans to reboot the franchise sometime in 2015.

The Amazing Spider-Man Source: Marvel.com

Columbia Pictures
A division of the Japanese conglomerate Sony (NYSE: SNE), Columbia Pictures counts its Spider-Man trilogy as one of the most successful movie franchises of all time. The three films have together grossed $2.5 billion at the box office -- $200 million more than the combined six X-Men films. Spider-Man, Spider-Man 2, and Spider-Man 3 are today the 38th, 45th, and 29th highest-grossing films of all time, respectively.

With the completion of director Sam Raimi's trilogy in 2007, Sony and Columbia rebooted the franchise just five short years later as The Amazing Spider-Man. The financial and critical success of this movie resulted in the green-lighting of at least three planned sequels, the first of which will be released in May 2014. The Spider-Man movie rights also include other characters core to the Spider-Man universe, such as The Sinister Six and Venom -- the focus of two untitled spinoff movies currently in development.

Marvel Studios
The bulk of the remaining Marvel movie rights resides with Walt Disney (NYSE: DIS), which acquired the entirety of Marvel Comics in 2009. These rights include Iron Man, Captain America, Thor, Hulk, other Avengers characters such as Ant-Man -- his Marvel Cinematic Universe debut will be in 2015 -- and the ABC Studios television series Agents of S.H.I.E.L.D.

Over the years, Disney and Marvel Studios have been slowly reacquiring the movie rights to other Marvel Comics intellectual properties. The previously mentioned Daredevil is an example. Marvel has also reacquired the rights for several other characters: Hulk from Comcast's Universal Studios, The Punisher from a Lions Gate subsidiary, Ghost Rider from Sony, and Blade from Time Warner. It is unlikely that Disney will ever be able to get the rights back to cash cows like X-Men and Spider-Man, however. As long as Fox and Sony release a movie based on those characters every few years, these rights will never revert back to Disney.

Why we can't have nice things
For those comic fans clamoring for an Avengers and X-Men crossover movie or a Defenders film -- a superhero team featuring Hulk, the Silver Surfer, and the Sub-Mariner -- the rights issue will likely make these potential movies a practical impossibility. Negotiating an Avengers and X-Men crossover movie between Disney and Fox would be difficult enough, much less the negotiations needed to produce a film starring Disney's Hulk, Fox's Silver Surfer, and the Sub-Mariner -- the Sub-Mariner movie rights are currently held by Comcast's Universal Studios.

This is unfortunately true of live-action TV as well. While 20th Century Fox's X-Men rights are limited to live-action movies, Marvel cannot produce a live-action TV series based on the X-Men without Fox's consent. This contract provision was the subject of multiple lawsuits involving the 2001-2004 TV series Mutant X; a Marvel TV series about non-X-Men mutants with powers. The lawsuit was settled confidentially in 2003, so we the general public doesn't know the exact settlement terms. Considering that there has yet to be another live-action X-Men or Marvel mutant-related TV show since Mutant X, it is fairly safe to assume that Fox's consent is still required.

Scarlet Witch and Quicksilver. Source: Marvel.com.

Foolish bottom line
It can be frustrating for Marvel fans to know that their favorite characters in the comics are walled off in their own separate movie universes. While a recent announcement proclaimed that Quicksilver and Scarlet Witch -- X-Men characters -- will appear in Disney's 2015 Avengers: Age of Ultron sequel, this is likely a uniquely special case given the two characters' close association with The Avengers dating back to the 1960s. I would not expect friendly cooperation between these competing movie studios anytime in the near future. 

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Fool contributor Matthew Luke owns shares of Walt Disney. The Motley Fool recommends Walt Disney. The Motley Fool owns shares of Walt Disney. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.