Tenebrio obscurus satellite DNA is resistant to cleavage by restriction endonucleases in situ

Isolation of a Pericentromeric Satellite DNA Family in Chnootriba argus (Henosepilachna argus) with an Unusual Short Repeat Unit (TTAAAA) for Beetles

Ladybird beetles (Coccinellidae) are one of the largest groups of beetles. Among them, some species are of economic interest since they can act as a biological control for some agricultural pests whereas other species are phytophagous and can damage crops. Chnootriba argus (Coccinellidae, Epilachnini) has large heterochromatic pericentromeric blocks on all chromosomes, including both sexual chromosomes. Classical digestion of total genomic DNA using restriction endonucleases failed to find the satellite DNA located on these heterochromatic regions. Cloning of C0t-1 DNA resulted in the isolation of a repetitive DNA with a repeat unit of six base pairs, TTAAAA. The amount of TTAAAA repeat in the C. argus genome was about 20%. Fluorescence in situ hybridization (FISH) analysis and digestion of chromosomes with the endonuclease Tru9I revealed that this repetitive DNA could be considered as the putative pericentromeric satellite DNA (satDNA) in this species. The presence of this satellite DNA was tested in other species of the tribe Epilachnini and it is also present in Epilachna paenulata. In both species, the TTAAAA repeat seems to be the main satellite DNA and it is located on the pericentromeric region on all chromosomes. The size of this satDNA, which has only six base pairs is unusual in Coleoptera satellite DNAs, where satDNAs usually have repeat units of a much larger size. Southern hybridization and FISH proved that this satDNA is conserved in some Epilachnini species but not in others. This result is in concordance with the controversial phylogenetic relationships among the genera of the tribe Epilachnini, where the limits between genera are unclear.

Different behaviour of C-banded peri-centromeric heterochromatin between sex chromosomes and autosomes in Polyphagan beetles

Heterochromatin variation was studied after C-banding of male karyotypes with a XY sex formula from 224 species belonging to most of the main families of Coleoptera. The karyotypes were classified in relation with the ratio heterochromatin/euchromatin total amounts and the amounts of heterochromatin on autosomes and gonosomes were compared. The C-banded karyotypes of 19 species, representing characteristic profiles are presented. This analysis shows that there is a strong tendency for the homogenization of the size of the peri-centromeric C-banded heterochromatin on autosomes. The amount of heterochromatin on the X roughly follows the variations of autosomes. At contrast, the C-banded heterochromatin of the Y, most frequently absent or very small and rarely amplified, looks quite independent from that of other chromosomes. We conclude that the Xs and autosomes, but not the Y, possibly share some, but not all mechanisms of heterochromatin amplification/reduction. The theoretical models of heterochromatin expansion are discussed in the light of these data.

Cytogenetic analysis of Lagria villosa (Coleoptera, Tenebrionidae): emphasis on the mechanism of association of the Xy(p) sex chromosomes

The Xy(p) sex determination mechanism is the system most frequent and ancestral to Coleoptera. Moreover, the presence of argyrophilous material associated with the sex bivalent is described as being responsible for the maintenance and association of these chromosomes. There are no karyotype data available regarding the genus Lagria and no consensus in the literature regarding the argyrophilous material present in the lumen of sex bivalent. Therefore, the aim of this work was to investigate the mechanism of sex chromosome bivalent association in Lagria villosa by analyzing the argyrophilous nature of the material present in the Xy(p) lumen. It was also intended to characterize L. villosa cytogenetically. The analysis of meiotic cells showed 2n = 18 = 16+Xy(p) for males and 2n = 18 = 16+XX in females and the meiotic formula was 2n = 8(II)+Xy(p). The C-banding showed blocks of pericentromeric heterochromatin in all chromosomes except in the y(p) chromosome. In these regions, the use of fluorochromes revealed the presence of heterochromatin containing GC rich DNA sequences. The study of synaptonemal complex showed a gradual increase in the electron-density of the axial elements of the sex chromosomes and their association with strongly electron-dense material. The pepsin pretreatment revealed that the material impregnated by silver is protein.

Cytogenetics of the darkling beetles Zophobas aff. confusus and Nyctobates gigas (Coleoptera, Tenebrionidae)

Males of Zophobas aff. confusus and Nyctobates gigas (Tenebrionidae) collected in the State of Pernambuco, Brazil, were studied through conventional staining, C-banding, silver nitrate impregnation (AgNO₃), and the base specific fluorochromes CMA₃ and DAPI. Z. aff. confusus was found to have 2n = 20 (9+Xyp) while N. gigas exhibited 2n = 18 (8+neoXY). Large pericentromeric blocks of constitutive heterochromatin (CH) were detected throughout the autosomal complement of the two species, except in one autosomal pair of N. gigas in which no heterochromatic block was observed. The sex chromosomes of both species were almost totally heterochromatic. Double staining with CMA₃/DA (distamycin) and DAPI/DA marked CH in Z. aff. confusus. However, DAPI staining was more intense. N. gigas was found to possess blocks of CH-positive CMA₃ and homogeneous DAPI. AgNO₃ staining also revealed differences between the two species. In Z. confusus an NOR was observed in the sexual bivalent Xyp and N. gigas was found to have an autosomal NOR.

Heterochromatin of the scarab beetle, Bubas bison (Coleoptera: Scarabaeidae) II. Evidence for AT-rich compartmentalization and a high amount of rDNA copies

An unexpected result arising from a previous characterization of the scarab beetle Bubas bison (Coleoptera: Scarabaeidae) heterochromatin was its unusual homogeneous reaction to different staining methods. In particular, silver stainability of heterochromatic ends of all chromosomes prevented identification of the number of rDNA transcriptionally active regions. Data formerly obtained using silver impregnation (Ag-NOR), C- G- and DAPI banding are here improved and completed by application of CMA(3) staining and rDNA FISH with the aim to investigate heterochromatin base composition and locate rDNA regions with respect to NOR-associated heterochromatin. Our results show that B. bison has a high amount of heterochromatin (almost 50%) and that--as revealed by rDNA FISH--major rRNA genes are spread over the heterochromatic telomeric regions of eight chromosomes, thus suggesting that only a portion, although consistent, of total heterochromatin is associated with ribosomal clusters. Moreover, DAPI-positive (AT-specific) and CMA(3)-negative (GC-specific) reactions of heterochromatic DNA confirm its AT-rich composition. Finally, possible explanations for the bright DAPI-fluorescence of both heterochromatin and rDNA sequences are discussed.

Uniform distribution of satellite DNA variants on the chromosomes of tenebrionid species Alphitobius diaperinus and Tenebrio molitor

The chromosomes of tenebrionid species Alphitobius diaperinus contain large blocks of pericentromerically located constitutive heterochromatin, as revealed by C-banding procedure. As previously reported, satellite DNA of this species is composed of two related monomeric units organized in three satellite subfamilies. In order to analyze the chromosomal location of the satellite DNA and the distribution of monomeric variants within it, and compare it with the distribution of monomer variants in Tenebrio molitor satellite DNA, the methods of in situ hybridization and restriction enzyme/nick translation were performed. Fluorescent in situ hybridization with the entire satellite DNA reveals the pericentromerically located signals on all chromosomes of the complement, coinciding with heterochromatic blocks. Results of fluorescent in situ hybridization with particular monomeric variants and of in situ restriction enzyme/nick translation show that monomeric variants are homogeneously dispersed within the entire satellite DNA. The spreading of satellite monomeric variants of the related species T. molitor within the pericentromeric heterochromatin of the entire complement, is demonstrated using the method of in situ restriction enzyme/nick translation. Although the complexity of organization of satellite DNAs is quite distinct in these two species, obtained results suggest similar efficiency of mechanisms of spreading and homogenization resulting in random chromosomal distribution of their satellite variants.

C-bands on chromosomes of 32 beetle species (Coleoptera: Elateridae, Cantharidae, Oedemeridae, Cerambycidae, Anthicidae, Chrysomelidae, Attelabidae and Curculionidae)

C-banding patterns of 32 beetle species from the families Elateridae, Cantharidae, Oedemeridae, Cerambycidae, Anthicidae, Chrysomelidae, Attelabidae and Curculionidae were studied using the C-banding technique. Mitotic and meiotic chromosomes were previously described for 14 species. From among 18 species that had never been cytogenetically studied, we determined the diploid and haploid chromosome numbers and the sex determination system for 12 beetles. The karyotype for 6 species is not described because of a lack of mitotic and meiotic metaphases. Results confirm that most of the beetle species possess a small amount of heterochromatin and C-positive segments are weakly visible in pachytene stages and weakly or imperceptible in mitotic and meiotic metaphases. In some species with a large amount of heterochromatin, C-bands were observed in the centromeric region in all autosomes and the X chromosome. The Y chromosome does not show C-bands with the exception of Oedemera viridis in which it possesses a small band of heterochromatin.

Cytogenetic analysis of Epicauta atomaria (Meloidae) and Palembus dermestoides (Tenebrionidae) with Xyp sex determination system using standard staining, C-bands, NOR and synaptonemal complex microspreading techniques

The mitotic and meiotic chromosomes of the beetles Epicauta atomaria (Meloidae) and Palembus dermestoides (Tenebrionidae) were analysed using standard staining, C-banding and silver impregnation techniques. We determine the diploid and haploid chromosome numbers, the sex determination system and describe the chromosomal morphology, the C-banding pattern and the chromosome(s) bearing NORs (nucleolar organizer regions). Both species shown 2n = 20 chromosomes, the chromosomal meioformula 9 + Xyp, and regular chromosome segregation during anaphases I and II. The chromosomes of E. atomaria are basically metacentric or submetacentric and P. dermestoides chromosomes are submetacentric or subtelocentric. In both beetles the constitutive heterochromatin is located in the pericentromeric region in all autosomes and in the Xp chromosome; additional C-bands were observed in telomeric region of the short arm in some autosomes in P. dermestoides. The yp chromosome did not show typical C-bands in these species. As for the synaptonemal complex, the nucleolar material is associated to the 7th bivalent in E. atomaria and 3rd and 7th bivalents in P. dermestoides. Strong silver impregnated material was observed in association with Xyp in light and electron microscopy preparations in these species and this material was interpreted to be related to nucleolar material.

High heterochromatin content in somatic chromosomes of two unrelated species of Diplopoda (Myriapoda)

For the first time, a conventional analysis of C-banded karyotypes was carried out in two distantly related diplopod species; this revealed an impressive percentage of heterochromatin in both genomes. In Acanthopetalum sicanum (Order Callipodida) (2n = 12), heterochromatin constitutes about 60% of the total DNA in females and 56% in males, whereas in Enologus oxypygum (Order Julida) (2n = 22) it is about 67% in both sexes. Heterochromatin of the two species was found to be similar in base composition (AT rich) and heterochromatin distribution, indicating that it has accumulated in a species-specific manner. Sex-determining mechanisms of the XY type were detected in both A. sicanum and E. oxypygum. In A. sicanum, the Y presented the lowest heterochromatic content of all chromosomes in the karyotype, whereas the X presented the highest.

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.

Chromosomal localization of a highly repeated EcoRI DNA fragment in Megoura viciae (Homoptera, Aphididae) by nick translation and fluorescence in situ hybridization

To investigate the genome of the aphid Megoura viciae at molecular level, we have studied total DNA by agarose gel electrophoresis after cleavage with different restriction endonucleases. EcoRI digestion produced a highly repeated DNA fragment, about 600 pb long. The contribution of this EcoRI element to the total genome of M. viciae was estimated at about 6% by means of densitometric scanning of agarose gel photographs. The chromosomal localization of this fragment, investigated by fluorescent in situ hybridization (FISH), constantly showed one large and two narrower fluorescent bands located on the X chromosome, all corresponding to C-positive heterochromatic areas. These results are in full accordance with the data obtained by in situ nick translation experiments carried out after EcoRI digestion, and clearly demonstrate that a substantial amount of M. viciae heterochromatin consists of EcoRI fragments which are mainly located on the X chromosome. Using the EcoRI restriction fragment as a molecular probe may be a practical tool for the investigation of taxonomic and evolutionary relationships in this group of insects.

Characterization of two abundant satellite DNAs from the mealworm Tenebrio obscurus

Two highly abundant satellite DNAs comprise 36% of the Tenebrio obscurus (Tenebrionidae, Coleoptera) genome. They are designated as satellite I and satellite II with the monomer length of 344 and 142 base pairs (bp), respectively. Both satellites differ in their nucleotide (nt) sequences, but the frequency of point mutations, well-conserved length of monomer variants, stretches of shared mutations characteristic for the process of gene conversion, and distribution of both satellites in regions of centromeric heterochromatin of all chromosomes indicate that the same evolutionary processes act on both of them with the same, or similar, rate. While satellite I shares no sequence similarity with any other known nt sequence, satellite II is 79.7% homologous with the highly abundant satellite from closely related Tenebrio molitor. Difference in the frequency of point mutations and absence of shared mutations indicating gene conversion strongly suggest that in these two closely related species mutational processes affecting satellite DNAs seem to be changed. Retarded electrophoretic mobility, due to sequence-induced curvature of DNA helix axis, was observed for T. obscurus satellite II, but not for satellite I. Although evolutionary processes act with different rates in T. obscurus and T. molitor satellites the monomer length and sequence-induced curvature are well preserved in both 142-bp satellites, as well as in, at the nt sequence level completely divergent, Palorus ratzeburgii (Tenebrionidae) satellite, indicating potential importance of these parameters in their evolution.