In general, a sports bra is fine. You don't need to wear a layer of clothing between you & your jersey.
When the weather is cold, you might want to wear a thin baselayer under your jersey, either a technical fabric or very fine wool. Check what TE has in their catalog for baselayers to see what I mean.
Ed. to add: There are also baselayers designed to wick sweat away when it's hot out, but I'm not familiar with those because I don't often ride in extreme heat.