Activity and inactivity of moth sex chromosomes in somatic and meiotic cells

Moths and butterflies (Lepidoptera) are the most species-rich group of animals with female heterogamety, females mostly having a WZ, males a ZZ sex chromosomeconstitution. We studied chromatin conformation, activity, and inactivity of the sex chromosomesin the flour moth Ephestia kuehniellaand the silkworm Bombyx mori, using immunostaining with anti-H3K9me2/3, anti- RNA polymerase II, and fluoro-uridine (FU) labelling of nascent transcripts, with conventional widefield fluorescence microscopy and ‘spatial structured illumination microscopy’ (3D-SIM). The Z chromosomeis euchromatic in somatic cellsand throughout meiosis. It is transcriptionally active in somatic cellsand in the postpachaytene stage of meiosis. The W chromosomein contrast is heterochromatic in somatic cellsas well as in meiotic cells at pachytene, but euchromatic and transcriptionally active like all other chromosomesat postpachytene. As the W chromosomesare apparently devoid of protein-coding genes, their transcripts must be non-coding. We found no indication of ‘meiotic sex chromosomeinactivation’ (MSCI) in the two species.