Doping casts big shadow on trials

Allegations still swirling around Salazar and Rupp

Doping allegations against US coach Alberto Salazar and his top runner, 2012 London Olympic 10,000-meter runner-up Galen Rupp, loom large as the US Track and Field Championships commence on Thursday.

Athletes will be competing for spots on the US squad that will compete at the world championships in August in Beijing, with Tyson Gay and Justin Gatlin - two sprint stars who have served doping bans - battling to position themselves to challenge defending champion Usain Bolt of Jamaica for 100 and 200m world titles in China in two months.