Continuing his longstanding ties with quantitative-investment guru Peter Muller, Chalkstream Capital chief investment officer Andrew Tsai is stepping down to work at PDT Partners.The two men formed Chalkstream in 2003 as a family office for Muller, who at the time led the now-independent PDT as a unit of Morgan Stanley. Soon after, Tsai helped New York-based Chalkstream evolve into a commingled fund operator pursuing a range of investments....