The structure of the Xyp sex chromosome complex in male meiosis of two beetles: Tenebrio molitor (Tenebrionidae) and Chrysolina graminis (Chrysomelidae)

Abstract.

Sex chromosome association was studied in male meiosis of Tenebrio molitor, a darkling beetle, using chromosome spreads prepared from testes. Staining with a DNA-specific fluorescent dye showed that the sex chromosomes formed a cone-shaped element at pachytene. The two chromosomes were closely associated. Concomitantly with chromatin condensation at late prophase I, a slender unstained gap developed between the large X and the tiny y chromosome, indicating that the sex chromosome pair is achiasmatic. The gap persisted in metaphase I spermatocytes. The electron microscopic analysis of ultrathin serial sections through metaphase I spermatocytes also revealed one asymmetric bivalent per metaphase plate. This bivalent contained a block of electron-dense material located between a large and a small mass of chromatin. It is plausible to assume that the asymmetric body represented the XY pair. The electron-dense mass corresponded in location to the chromosomal portion not stained by the DNA-specific dye and is believed to consist of non-chromatin material. In order to determine whether electron-dense material is routinely found in the achiasmatric sex chromosome pairs in male meiosis of beetles, primary spermatocytes of a leaf beetle, Chrosolina graminis, were studied as well. As in T. molitor, a homogeneously textured mass was detected in one bivalent, the sex chromosome pair, but in contrast to this species transparent vacuoles were scattered throughout the material in C. graminis.