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.

Cytogenetic Analysis of <b><i>Lagria villosa</i></b> (Coleoptera, Tenebrionidae): Emphasis on the Mechanism of Association of the Xy_p Sex Chromosomes

“…In the case of Coleoptera, as a means to elucidate the nature of the association between the Xy p sex chromosomes [Galián et al, 1995;Petitpierre, 1996;Wolf, 1997], few (approximately 1%) were analyzed in recent decades with higher resolution techniques. These analyses could show differences between the systems and their modes of association even when all are Xy p .…”

Fine structure of the kinetochores in six species of the Coleoptera

“…The comparison of metaphase spermatogonia and primary and secondary spermatocytes of T. molitor showed that kinetochore structure was uniform: kinetochore MTs were consistently in direct contact with the chromatin. The appearance of bivalents differed in that the chromatin was homogeneously textured throughout in some chromosomes, while a zone of lesser electron density was observed in the equatorial region of other bivalents (Wolf 1997; this study). The differences are interpreted to indicate substages of metaphase I.…”

“…This corresponded to the adhesion site between the homologous chromosomes and may indicate that the cell was in late metaphase I prior to anaphase segregation. The degree of chromatin condensation did not affect the appearance of the kinetochores (compare Wolf 1997). Chromosome structure in secondary spermatocytes was similar to that in spermatogonia, in that the rodshaped chromosomes showed MT contacts in central regions and kinetochore material was not detectable in ultrathin sections through conventionally fixed tissue (Fig.…”

“…Scale bar = 5 pm. chromosomes of T. molitor are characterized by the presence of large pericentromeric heterochromatic regions (Weith 1985;Juan and Petitpierre 1989;Wolf 1997), which may be obscuring the primary constrictions normally expected in monokinetic chromosomes (Wolf 1996b). If the giant heterochromatic segments are active in anchoring kinetochore MTs throughout their entire length, the large kinetochore size is accounted for.…”