Reverse heteropyknosis between euchromatin and heterochromatin of the X chromosome in meiotic prophase of the cockroach Blaberus discoidalis

The X chromosome and all autosomes of the cockroach Blaberus discoidalis (2n = 37, XO) contain large segments of constitutive heterochromatin (C-bands). From spermatogonial metaphases, the C-bands are found to be located in the middle of most chromosomes, including the X. The C-bands show bright fluorescence when stained with Hoechst 33258. In pachytene, autosomal heterochromatin can be easily identified in acetic orcein squash preparations as condensed segments. In the same preparations the X chromosome exhibits a "closed" appearance with a lightly stained middle loop and two heavily stained terminal segments which lie side-by-side as if they are paired. In C-banded preparations, an opposite reaction is found, i.e., the loop is heavily stained while the tips are lightly stained. In Hoechst 33258 preparations, the loop is brightly fluorescent while the tips are less brightly stained. Thus, in pachytene of conventional orcein preparations the heteropyknotic behavior between euchromatin and heterochromatin of the X chromosome is the reverse of the usual staining pattern; instead of condensed heterochromatin and decondensed euchromatin, euchromatin is condensed whereas heterochromatin is decondensed. The "paired" euchromatic tips may suggest autologous homology between the original X and the original Y which might have been translocated onto the X in the course of karyological evolution.