Reported by Tomoharu Ugawa on the researchers mailing list (see http://sourceforge.net/mailarchive/message.php?msg_id=30656088 for original post).
Short summary: The class org.mmtk.utility.options.PretenureThresholdFraction imports the constant DEFAULT_PRETENURE_THRESHOLD_FRACTION from org.mmtk.plan.generational.Gen . This forces the Gen class into the bootimage for non-generational collectors. More importantly, it results in the creation of a nursery for non-generational collectors because the nursery is created in the static initializer of Gen. The nursery can be observed by using the -X:gc:verbose=3 option on a non-generational collector (no class file or jar necessary).