Kathleeen Gaffney, a key member of a team that invests $82 billion at Loomis Sayles & Co. in Boston, has quit to work for cross-town rival Eaton Vance Corp., according to people familiar with her resignation.

Gaffney was part of a team working for Dan Fuss, one of the nation’s most respected bond managers; The group is responsible for about half of all the fixed income money under management at the firm, including the $21.9 billion Loomis Sayles Bond fund.