Query: When did election of borough heads begin?
Context: For administrative purposes, the Federal District is divided into 16 "delegaciones" or boroughs. While not fully equivalent to a municipality, the 16 boroughs have gained significant autonomy, and since 2000 their heads of government are elected directly by plurality (they were previously appointed by the head of government of the Federal District). Given that Mexico City is organized entirely as a Federal District, most of the city services are provided or organized by the Government of the Federal District and not by the boroughs themselves, while in the constituent states these services would be provided by the municipalities.