Cytogenetics of eight Neotropical species of Chauliognathus Henzt, 1830: implications on the ancestral karyotype in Cantharidae (Coleoptera)

Why Are X Autosome Rearrangements so Frequent in Beetles? A Study of 50 Cases

Amongst the 460 karyotypes of Polyphagan Coleoptera that we studied, 50 (10.8%) were carriers of an X autosome rearrangement. In addition to mitotic metaphase analysis, the correct diagnosis was performed on meiotic cells, principally at the pachytene stage. The percentages of these inter-chromosomal rearrangements, principally fusions, varied in relation to the total diploid number of chromosomes: high (51%) below 19, null at 19, low (2.7%) at 20 (the ancestral and modal number), and slightly increasing from 7.1% to 16.7% from 22 to above 30. The involvement of the X in chromosome fusions appears to be more than seven-fold higher than expected for the average of the autosomes. Examples of karyotypes with X autosome rearrangements are shown, including insertion of the whole X in the autosome (ins(A;X)), which has never been reported before in animals. End-to-end fusions (Robertsonian translocations, terminal rearrangements, and pseudo-dicentrics) are the most frequent types of X autosome rearrangements. As in the 34 species with a 19,X formula, there was no trace of the Y chromosome in the 50 karyotypes with an X autosome rearrangement, which demonstrates the dispensability of this chromosome. In most instances, C-banded heterochromatin was present at the X autosome junction, which suggests that it insulates the gonosome from the autosome portions, whose genes are subjected to different levels of expression. Finally, it is proposed that the very preferential involvement of the X in inter-chromosome rearrangements is explained by: (1) the frequent acrocentric morphology of the X, thus the terminal position of constitutive heterochromatin, which can insulate the attached gonosomal and autosomal components; (2) the dispensability of the Y chromosome, which considerably minimizes the deleterious consequences of the heterozygous status in male meiosis, (3) following the rapid loss of the useless Y chromosome, the correct segregation of the X autosome-autosome trivalent, which ipso facto is ensured by a chiasma in its autosomal portion.

Cytogenetic characterization of Eurysternus caribaeus (Coleoptera: Scarabaeidae): evidence of sex-autosome fusion and diploid number reduction prior to species dispersion

Mitotic and meiotic chromosomes of several populations of Eurysternus caribaeus (Coleoptera: Scarabaeidae) were analysed through conventional staining, C-banding, base-specific fluorochromes, silver nitrate staining and fluorescent in situ hybridization (FISH). All specimens showed 2n = 8 in their karyotypes, with a neo-XY sex system (Y is a submetacentric and X a metacentric) and three pairs of submetacentric autosomes. The analysis of constitutive heterochromatin (CH) revealed small blocks located in the centromeric region of all chromosomes which do not present positive staining under the fluorochromes CMA3 and DAPI. Silver nitrate staining revealed that the nucleolar organizer region (NORs) is associated with the sex chromosomes. The FISH technique revealed that rDNA sites in the X and Y are different in size. Data from different populations indicate that the diploid number reduction (2n = 8) observed in E. caribaeus is established and presumably has preceded the dispersion of this species. Moreover, this reduction occasioned the translocation of rDNA sites to the sex chromosomes, X and Y, an uncommon pattern in Scarabaeidae that was observed for the first time by the FISH in this work.

A chromosomal investigation of some British Cantharidae (Coleoptera)

The karyotypes of 20 species of Cantharidae occurring in the British Isles are described and illustrated, 19 of which are hitherto unrecorded. These are Cantharis cryptica Ashe, Cantharis fusca Linnaeus, Cantharis lateralis Linnaeus, Cantharis livida Linnaeus, Cantharis nigra (Degeer), Cantharis nigricans (O. F. Müller), Cantharis pallida Goeze, Cantharis pellucida Fabricius, Cantharis rufa Linnaeus, Cantharis rustica Fallén, Cantharis thoracica (Olivier), Rhagonycha fulva (Scopoli), Rhagonycha lignosa (O. F. Müller), Rhagonycha limbata Thomson, Rhagonycha lutea (O. F. Müller), Rhagonycha testacea (Linnaeus), Silis ruficollis (Fabricius), Malthinus seriepunctatus Kiesenwetter, Malthodes dispar (Germar) and Malthodes minimus (Linnaeus). The main findings were as follows: (1) Cantharis, Rhagonycha and Silis have karyotypes of six pairs of autosomes plus sex chromosomes, which are X0 (male), XX (female). (2) Malthinus and Malthodes differ from the other genera in having only five pairs of autosomes. (3) The chromosome complement of Silis differs from those of the other genera studied in having a variable number of B-chromosomes in individuals from the same population. (4) The karyotypes of the Rhagonycha species show a degree of uniformity in the relative chromosome lengths, centromere positions and C-banding patterns. (5) The karyotypes of the Cantharis species are much more distinct from each other where these characteristics are concerned. (6) Maturation of the gonads takes place in the late larval stage. (7) The segregation of the X chromosome of Cantharis rufa during meiosis is pre-reductional.

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.