The system of nucleolar controlled sex-chromosome segregation which characterises Xyp species of hide beetles is also present in the one species (haemorrhoidalis) with an XY system. This, coupled with the fact that the karyotype in the
XY species is asymmetrical, whereas species with smaller y-chromosomes show greater symmetry, suggests that "erosion" of the y may have involved translocation of the material of the y onto the autosomes rather than simple loss. Finally, supernumerary y chromosomes present in laboratory cultures of two species (maculatus and/frischii) demonstrate the efficiency of the sex nucleolus as a mechanism for securing segregation (Summary see p. 384).