Meiotic behaviour of a new complex X-Y-autosome translocation and amplified heterochromatin in Jumnos ruckeri (Saunders) (Coleoptera, Scarabaeidae, Cetoniinae) N. Macaisne, A. M. Dutrillaux & B. Dutrillaux* UMR 5202 CNRS/MNHN, Organisation, Structure et Evolution de la Biodiversite´, Muse´um National d_Histoire Naturelle, CP 39, 16, rue Buffon, Paris, 75005, France; Tel: +33-1-40795343; E-mail: bdutrill@mnhn.fr * Correspondence Received 4 September 2006. Received in revised form and accepted for publication by Paul Popescu 3 November 2006 Key words: autosome, gonosome, heterochromatin, Jumnos ruckeri, Scarabaeidae, translocation Abstract Species belonging to the Cetoniinae subfamily studied so far possess 20 chromosomes, including a small X and a punctiform Y: 20,Xyp in the males. In a series of species from the Goliathini tribe under study we found a very unusual karyotype, with 12 autosomes and large sex chromosomes (14,neoXY) in Jumnos ruckieri from Thailand. Applying various techniques including pachytene bivalent spreading, we showed that 40% (mitotic and meiotic prophases) to 60% (metaphases) of the karyotype length was composed of heterochromatin. Both sex chromosomes were NOR carriers. At pachynema they underwent a complete synapsis of their distal regions, indicating their autosomal origin. At contrast, their very uneven central regions remained separated, but associated with nucleolus material. This association persisted until diakinesis, forming a pseudo-chiasma between the neoX and the neoY, which were always in end-to-end association. Compared to free autosomes the autosomal parts of the neo-sex chromosomes had a significant lack of interstitial chiasmata, indicating a possible lack of recombination at their proximal regions. As in the cases of X-autosome translocations in mammals, autosomal and gonosomal parts of the neo-sex chromosomes were insulated by heterochromatin, which may be a necessary condition to avoid deleterious position effects, whatever the mechanisms of gene dosage compensation. Introduction Among the thousands of coleopterans cytogenetically studied (Smith & Virkki 1978, Serrano & Yadav 1984, Petitpierre 1988, Serrano et al. 1994), the most frequent karyotypic formulas are 20,XY in the male and 20, XX in the female. The relative lengths of the X and Y chromosomes represent about 5% and 1% of the hap- loid karyotype, respectively. Such formulas, observed in species belonging to many different families, are assumed to represent, or to be very close to, the ancestral status. By mere coincidence, gonosome relative sizes appear to be fairly similar to those of mammals, in which deviant sex formulas are rare. In mammals, dosage compensation between XX and XY formulas is princi- pally related to the inactivation of one X in the XX formulas. The X chromosome is highly conserved and the Y chromosome varies by its heterochromatic com- ponent, at least at the cytogenetic definition. In our own species, autosomeYgonosome translocations are almost always deleterious. They induce sterility in males. In females the alternate inactivation of the paternal and maternal X chromosomes is altered and the inactivation may spread on the attached autosome in somatic cells, inducing a mosaic functional monosomy associated with malformations and mental retardation, reducing their Chromosome Research (2006) 14:909Y918 # Springer 2006 DOI: 10.1007/s10577-006-1098-6