On all my extended bike tours, road or mountain, I have always brought a fleece shirt.  It is a midweight fleece, probably a Polartec 100 or something.  On cool nights it helps keep me warm, but is also breathable enough that I can wear it while riding on cool days.  However, I just threw it on a scale and found it to be 12 ounces, which seems to be a lot for one item of clothing.

Are there other options that will keep me warm, while not weighing quite so much?  Down?  Wool?  etc? 

My next trip will be in Alaska/Yukon in May/June, so nighttime temps might be around freezing.  I also have a coolmax T, long sleeve coolmax shirt (thin), and a thin HellyHansen rain coat.