The karyotype of the giant beetle Dynastes hercules hercules is composed of only 16 autosomes and large sex chromosomes. Meiotic studies in the males showed that a large part of the sex chromosomes undergo synapsis at pachynema similarly to autosomes, demonstrating that both derived from an autosome-gonosome translocation. Therefore, karyotype formula is 18,neoXY. The heterochromatisation of the neoX short arm at pachynema indicates that it corresponds to the ancestral X. It carries the nucleolar organizer region (NOR) in its proximal part, which is undercondensed, especially in male mitotic and meiotic cells. In female mitotic cells, both NOR staining and undercondensation were more difficult to observe in the neoX short arms. In somatic interphase nuclei, NOR expression strongly varies with the sex. Two separated compact groups of silver dots were observed in female nuclei, while a single dispersed and large group of silver deposit exists in the males. Both the lower condensation and the higher NOR expression of the single neoX of the males, compared to each of the two neoXs of the females, is interpreted to be a consequence of dosage compensation, a mechanism not yet described in Coleoptera. In mammals as well as in Coleoptera, the carriers of gonosome-autosome translocations not exhibiting deleterious phenotypes show constitutive heterochromatin at the autosome-gonosome junction. Thus, heterochromatin may play an important universal role by clearly separating chromosome segments with different regulations of gene expression, such as inactivation or dosage compensation of the X chromosome on the one side and a conventional autosomal structure on the other side.
Copyright 2007 S. Karger AG, Basel.