The dikaryon is a nuclear feature which is unique to some fungi. The green alga Derbesia has been long considered an exception, until the heterokaryotic hypothesis was challenged by later studies. Compatible cell-types can fuse cytoplasms (plasmogamy). When this occurs, the two nuclei of two cells pair off and cohabit without fusing (karyogamy). This can be maintained for all the cells of the hyphae by synchronously dividing so that pairs are passed to newer cells. In the Ascomycota this attribute is most often found in the ascogenous hyphae and ascocarp while the bulk of the mycelia remains monokaryotic. In the Basidiomycota this is the dominant phase with most Basidiomycota monokaryons weakly growing and short-lived.