Spermatogenesis inTenebrio molitor(Tenebrionidae, Coleoptera): A Fine Structure and Anti-tubulin Immunofluorescence Study

The microtubular system and posttranslationally modified tubulin during spermatogenesis in a parasitic nematode with amoeboid and aflagellate spermatozoa

Using transmission electron microscopy and immunologic approaches with various antibodies against general tubulin and posttranslationally modified tubulin, we investigated microtubule organization during spermatogenesis in Heligmosomoides polygyrus, a species in which a conspicuous but transient microtubular system exists in several forms: a cytoplasmic network in the spermatocyte, the meiotic spindle, a perinuclear network and a longitudinal bundle of microtubules in the spermatid. This pattern differs from most nematodes including Caenorhabditis elegans, in which spermatids have not microtubules. In the spermatozoon of H. polygyrus, immunocytochemistry does not detect tubulin, but electron microscopy reveals two centrioles with a unique structure of 10 singlets. In male germ cells, microtubules are probably involved in cell shaping and positioning of organelles but not in cell motility. In all transient tubulin structures described in spermatocytes and spermatids of H. polygyrus, detyrosination, tyrosination, and polyglutamylation were detected, but acetylation and polyglycylation were not. The presence/absence of these posttranslational modifications is apparently not stage dependent. This is the first study of posttranslationally modified tubulin in nematode spermatogenesis.

Fine structure of the kinetochores in six species of the Coleoptera

Kinetochore structure was examined in a total of 6 species from 5 different families of the Coleoptera using transmission electron microscopy of ultrathin serial sections. Metaphase spermatogonia and primary and secondary spermatocytes were studied in Tenebrio molitor (Tenebrionidae) to determine whether kinetochore structure varies depending on the cell type. In all three cell types, the kinetochore microtubules (MTs) were in direct contact with the chromosomal surface, and kinetochore plates were not detectable. In the other species, only metaphase I spermatocytes were examined. As in T. molitor, distinct kinetochore plates were also absent in Adelocera murina (Elateridae), Agapanthia villosoviridescens (Cerambycidae), and Coccinella septempunctata (Coccinellidae). However, bivalents in male meiosis of two representatives of the Chrysomelidae, Agelastica alni and Chrysolina graminis, showed roughly spherical kinetochores at their poleward surfaces. Microtubules were in contact with this material. Thus, although the present survey covers only a small number of species, it is clear that at least two kinetochore types occur in the Coleoptera. The cytological findings are discussed in the context of chromosome number and genome size variability in the Coleopteran families studied. It is suggested that properties of the kinetochores could play a role in karyotype evolution in the Coleoptera.

Preparation of insect chromosomes for immunolabeling

We describe a method for isolating chromosomes from testes of the desert locust, Schistocerca gregaria, and their subsequent incubation with antibodies directed against chromosomal proteins. The procedure involves hypotonic pretreatment of the germ cells, centrifugation onto coverslips in a cytocentrifuge and immunolabeling, while still unfixed, using a chromatin-stabilizing buffer. In the present case, an antibody specific for the acetylated isoforms of histone H4 was tested. After the antibody treatment, the preparations are fixed using formaldehyde, stained with a DNA-specific fluorescent dye and mounted. Analysis of the preparations revealed good preservation of chromosome structure in prophase spermatogonia and late prophase I spermatocytes. Fully condensed chromosomes were not observed and are probably lost during preparation. The bright fluorescence of the autosomes indicates that the reaction between the antibody against acetylated histone H4 and its chromosomal antigen is not impeded. In contrast, the X univalent remained unstained with the exception of a small terminal band. Thus, cytospin preparations of locust germ cells allow high resolution immunolabeling with antibodies against chromosome-associated proteins.

Microtubule organization and the distribution of gamma-tubulin in spermatogenesis of a beetle, Tenebrio molitor (Tenebrionidae, Coleoptera, Insecta)

The present study focuses on the restructuring of the microtubule (MT) cytoskeleton and microtubule-organizing centres (MTOCs) throughout spermatogenesis of a darkling beetle, Tenebrio molitor (Tenebrionidae, Coleoptera, Insecta). To this end, serial ultrathin sections through male germ cells were studied using transmission electron microscopy. Additionally, spindles and young spermatids were isolated from testes under MT-stabilizing conditions and doubly labeled with antibodies against beta- and gamma-tubulin. The latter is a tubulin isoform detected in MTOCs of a wide variety of species. The observations suggest that microtubules may be nucleated from sites with and without high gamma-tubulin content and that these sites do not necessarily possess canonical centrosomes. In a prominent cytoplasmic MT system of primary spermatocytes in prophase, microtubule nucleation apparently occurs in the absence of immunologically detectable gamma-tubulin. At the poles of meiotic spindles, MTs are directly inserted into gamma-tubulin-containing material and this connection is considered responsible for their nucleation. The interzone spindle MTs of telophase cells contain gamma-tubulin and this may confer stability to them. Finally, manchette MTs of spermatids originate in the vicinity of the acrosome precursor but are not inserted into this body. The acrosome precursor is surrounded by a membrane and is clearly detected by the antibody against gamma-tubulin.