MOSCOW, April 7 (Reuters) - Russian investigators have charged Wall Street Journal reporter Evan Gershkovich with espionage, the Interfax news agency reported, citing an unidentified source. Russia's Federal Security Service, the main successor to the Soviet-era KGB, said on March 30 that it had detained Gershkovich in the Urals city of Yekaterinburg and had opened an espionage case against the 31-year-old for collecting what it said were state secrets about the military industrial complex. See here for a complete list of exchanges and delays