In general Ions are always present in gases due to cosmic rays and many other factors. so when we apply a potential difference across the discharge tube the ions get accelerated due to electric field. if this potential difference is large enough then these iron gets enough energy to ionize the molecule on collision. Thus a large number of Ions are produced and conduction starts. Generally an electron gets detached from a molecule to form a positive ion and at a low pressure these electrons move through a large distance and gets attracted to another molecule and thus froms the negative ion so we can say during the conduction electrons, positive ions and negative ions takes part.