"The employee, who is head of the Computer Incidents Investigations Team (CIIT), is under investigation for a period predating his employment at Kaspersky Lab. We do not possess details of the investigation."
The company says that the CIIT won't be affected by the arrest. As Ars Technica notes, Stoyanov was a major in the Russian Ministry of Interior from 2000 to 2006 prior to joining Kaspersky in 2012.
Article 275 is a broad ruling that defines high treason as "espionage, disclosure of state secrets, or any other assistance rendered to a foreign organization, or their representatives in hostile activities to the detriment of the external security of the Russian Federation committed by a citizen of the Russian Federation." Punishment includes a "deprivation of liberty for a term of 12 to 20 years with confiscation of property."
One of Forbes' Russian infosec sources says that simply giving the FBI information on a botnet could be considered treason. What's interesting are his accomplishments while at the Lab. Stoyanov's work apparently helped jail plenty of Russian cyber criminals, including a trio who extorted UK betting shops for $4 million, and another group of thugs who stole $45 million from Russian banks.
Precisely what the outcome of the arrest will be, or any details coming from it could be difficult to come by. Forbes' source further explains that violations of Article 275 are handled by a secret military tribunal.
Update: Now, Russian newspapers are reporting that a senior official in the Russia's cyberintelligence department has also been arrested in Moscow on charges of treason. American officials say he oversaw last year's election hacking.